Note: Descriptions are shown in the official language in which they were submitted.
9 ~j 2
BACKGROUND OF THE INVENTION
This invention relates generally to security or
subscription television systems and more particularly, it
relates .to a method and system for encoding and decoding
of standard television signals thereby-enabling reception ;
thereof in an intelligible manner only by authorized
.subscribers.
Generally, there are known in the prior art
various types o~ secure subscription television sytems in
which televisio~ signals are transmitted in a coded form
for use only in subscribers' receivers having proper decod-
ing means. In these systems,.the coding is accomplished by
modifying th~ sound and/or video portions of the television
siynals rendering them uninte71igi~1e or unpleasant to non-
subscribers or to subscribers who had not paid a fee to .
the broadcaster.
. In these proposed secure subscription television
sys~ems, upon decodi.n~ the modified television signals, it
was generally required that the precisQ modific2tion signals
. must be removed or any missing signals must be sener~ted
and added to the modified television signals to D~oduce a
restored.video as similar as possible to it~ original ~uality. .
Problems existed in these techniques due to the facL tha~
the quality of the picture was generally 5ubjec~ to d~gradation
ar~d/or that dificulties were encountered in maintaining criti-
cal phase and other signal relationships in restoration,
9 ~ ~
The efectiveness of the secure subscription
television systems is measured.by the degree of success
which it has in deterring unauthorized viewers frcm
watching the modified television signals transmitted and
in preventing the duplication o the decoding means.
: Ithas also been experienced that in general the
decoding technique utillzed in the prior art systems CQUld
be easily duplicated by many resourceful viewers.and thus
defeat the security thereof. It would,. therefore~ be
desirable to provide.a method and sys~em for encoding and
. decoding television signals in which maximum security is
achieved and which will effectively deter u~authorized
viewers in attempting to defeat the security. Moreover,
it is needed to provide a system such that the theft
., .15 thereof will be of little use or value.
'~ , .
: . In addition, the prior secure subscription
. , .
television systems have a d.isadvantage in that none of
them possess a positive and coni:inuous control means for.
~ controlling automatically ~he decoder at the v.~xious sub-
- 20 scriber stations whereby any or all of the decod2Ls become
`selectively disabled or "locked out" if it does no~ receive
, , .
~`; the appropriate control signals from the broadcaster period-
ically before a pre-determined interval of time.has elapsedO
Thus, it would be desirable to provide a method and system
for encodïng and decoding of standard television signals in
9 ~ 2
which the decoder at the subscriber's station are de?endent
c~ntinuously upon control signals transmitted by the broad
caster. In the absence of the periodic control sign21s, the
decoder will automatically and rapidly become disabled
. rendering it essentially useless withou~ the necessity o. .
.- physically traveling to the subscriber's location.
It would also be suitable to provide in connection
with a subscription television system means by which the
subscribers could select the programs desired in a short
time in advance of ~elecasting in a simple and easy ~anner.
To this end, telephone communication circuitry can be
provided so.that the ~ubscribers can request their programs
to the broadcaster via a telephone interface which will
transmit the subscriber's request to the broadca5ter autom~-
~ 15 tically wit~out any great effort on his part o~her than by
: simply depressing a button at the subscriber's siteO The
. ' telephone interface would automatically dial the broadcaster'stelephone number, transmit ~he subscriber's ~nique account
code to a computer at the broadcaster's site, and thus make-
possible ~he program selection by the subscriber at æ~y time
prior to the broadcast~ Further, this would allow .he s~ tem
to be very reliable and substantially maint~enance-l_ee 2~ter
the initial installation since all ~he control signals are
done via over-the-air transmission and requests for ser~ice
are done electrically on the tel~hone interface. ~here is
eliminated the need to physically travel to the su~scriber~s
location during normal use so as to supply or retriev~ coin.s~
cards, or tapes and the li~e for billing purposes as
encountered in some prior art systems.
9 ~ ~
SUMMARY_ OF THE INVENTION
Accordingly, it i~ an over-all objective of the
present invention to provide a new and novel method and
apparatus for encoding and decoding standard television
signals which possess very high security and deter un-
authorized viewers.
Another object of this invention is-to provide an
improved television transmitter for encoding standard
television signals which restores the scrambled video and audio
signals withou~ degradation in picture and sound qualities.
Still another object of this invention is to pro-
vide a television transmitter for encoding standard television
signals wherein a plurality of unique pulse-coded control
signals consisting of 32-bit binary pulse trains are transmitted
separately to identify individual authorized subscribers and
to provide the information needed for unscrambling of the
~ideo and audio signals in the same s,equence as used for
scxambling.
Still another object o* this invention is to
provide a television transmitter for encoding standard tele-
vision signals wherein the audio scrambling lS accomplished
by conversion of analog audio signals to coded digital
audio signals.
Still another object of this invention is to provide
a television transmitter for encoding standard television
signals wherein control means continuously enable decoding
means at the various subscriber stations whereby any or all
of the decoder means become selectively disabled or "locked
out" if it does not receive the appropriate control signals .
-- 6 -- .
:~ 1 5~9~
from the broadcaster perlodically before a pre-determined
interval of time has elapsed.
Yet still another object of this invention is to
provide a television transmitter for encoding standard
television signals wherein telephone communication circuitry
allows the subscribers to request their programs to the
broadcaster via a telephone interface.
Yet still another object of this invention is to
provide a television transmitter for encoding standard
television signals having means for transmitting aural
barker signals simultaneously with scrambled video signals
and means for receiving the barker signals regardless of
whether the subscriber is authorized to receive the un-
scrambled video signals so as to promote usage of the sub-
scription television system.
In accordance with the present invention, there is
provided a subscription television transmitter for generating
television signals having a pxogram video portion and a program
aural portion in a non-standard format to enable reception of
both program video and prcgram portions in an intelligible
manner only by authorized subscribers; said television transmitter
comprising:
a) encryption code signals generating means î
b) at least one carrier signal generator means
adapted to convey by cable or over-the-air transmission the
program video and program aural portions and the encryption
codes signal from the transmitter to authorized subscribers;
c) means for digitizing the program audio signal;
'
1 ~5952
d) means responsive to the encryption code signal
for e~crypting the digitized program audio signal from the
digitizing means;
e) means for combining ~he encryption codes signal,
~he digitized and encrypted audio program signal and a ~ideo
program signal, with the carrier signals whereby the encryption
codes signal/ the digitized and encrypted audio signal and
the video signal can be individually separated at a receiver.
Briefly, the presen~ invention provides a method
and system for encoding and decoding of standard television
signals thereby enahling reception thereof in an intelligibla
manner only by authorîzed subscribers. The audio scrambling
is accomplish~d by conversion of analog audio signals to
coded digital audio signals. A plural.ity of unique pulse-
coded control signals consisting of 32-bit binary pulse
trains are transmitted separately to identify individual
authorized subscribers and to provida the information needed
to unscramble the scrambled audio and video signals. When
there is a comparison b~tween one of t:he pulse-coded control
signals wi~h a unique address code associated with a particular
subscriber, unscrambling of the video and auaiol~ ~ -~~~-~`~~ - ~ ~~~~
... .. _ ___ ... . , . .... __ . . _ . _~ ~ .
signals occurs in the same sequence as used for scrambling
to provide restored video and audio signals wi~hou~ degrada~
tion in picture and sound,qualities. This scra~bling technique
is done without affecting or altering the normal specification
for composite video, color and aural transmission or'for
the broadcaster's transmitter utilized in the normal
telecasting.
It will be a~preciated from the foregoing that
.
the present invention provides a new and novel method and
... .
; 10 apparatus for encoding and decoding standard television
signals in subscription television systems. In particular,
since the invention utilizes control signals consisting.of
32-bit binary pulse trains to identify the various authori'zed:
subscribers each having a different code combination, it
provides a very high security system thereby preventing
unauthorized viewers from unscr.ambling of the video sig-
nals. Moreover, the audio signals are also scrambled
to increase' the security of the system and to deter most ' .
unauthorize~ viewers by convertiny the analog audio signals
to coded digital audio signals. Additionally, the present
invention includes control means for continuously enabling
decoder means at the various subscriber stations ~hereby any
or all of ~hese decoder means bPcome selectively disabled
or "locked out" if it does not receive appropriate control
2S signals frpm the broadcaster periodically before a pre~
determined interval of time has elapsed.
. . ' ' " .
BRIEF DESCRIPTION OF THE_DR~INGS
These and other objects, features and advantages
of the invention, as well as the invention itself, will
become more apparent to those skilled in the art from the
following detailed description when read in conjunction
wi~h the accompanied drawings in which like reference
numerals indicate like or corresponding parts ~hroughout
the several views wherein: . .
Pigu~e 1 lS a simplified, over-all block diagram
of a subscription television system in accordance with.the
presenk invention;
Figure 2 is a block diagram of a television
transmi~ter at the broadcaster site illustrating the means
for encoding or scrambling the standard television signals~
embodying the present invention;
- ~igure 3 is a block cliagram of a receiver at the
subscriber site for decoding or unscrambling the encoaed
television signals received from the transmitter shown i.n
Figure 2, according to the present invention;
Figure 4 is a block diagram of telephone
communication circuitry for interconnection between .he
transmitter and receiver, employing the novel methods
of the present invention;.
Figure S is a simplified block diagram depicting
- circuitry suitable for use as the adder networks 80, 136 Q~
Pigures 2 and 3;
--10--
~5~
Figure 6(a) is a time-amplitude graph of a
conventional, normal scanning line in a television signal;
Figure 6(b) is a graph similar to that sho~.m in
Figure 6(a), in which some video signal portions havea been.
randomly inverted, according to the present invention;
Figure 7 is a spectral distribution of the signais
transmitted in the present invention;
Figure 8(a) is an example of a control signal
consisting of 32 bits, according to the present invention;
Fisure 8~b) is an example of a digitized aural
signal consisting of 11 bits, employed in the present
invention;
.
Figure 9 is a schemat.ic diagram showing circuitry
suitable for use as the shift register 122 of Figure 3;
lS. Figure 10 is a more detailed schematic block
diagram showing circuitry suitable for use in certain of
the blocks of Figure 3;
Figure ll(a) is a schematic diagram illustrating
circuitry which may be employed in certain of the other
blocks of Figure 3;
Pigure ll~b) is a schematic diagram illu5 ~rating
in more detail the circuitry which may be used as the
video switching amplifier 88,
Figure 12 is a schematic diagram showing circuitry
suitable for use as the shift register 152 of ~igure 3;
- Figure 13 is a block dlagram depicting the
circuitry suitable for use as the blocks 146, lS0 shown in
Figure 3;
~ ~55~2
.~igure 14 is a schematic diagram illustrating
circuitry which may he employed in certain of the blocks
of Figure 4;
Figure 15 is a schematic diagram illustrating
suitable circui.try for use as the gating circuit 1~6.of
~igure 4;
Figure 16 is a schematic diagram showing circuitry
sui,table for use as the switches 156 of Figure 4,
Figure 17 is a schematic'diagra.m illustrating
cix~uitry which may be employed,to control the lights 180
and 186 of Figure 4; and
'. ' Figure 18 is a schematic block diagram ill~lstrating
circuitry.which may be employed as the master clocks 66
and 130 shown in Figures 2 and 3.
- DET~ILED DESCRIPTION OF, T~E PREFEP~RED_E~IBODIMEN~ ,
Referring now in detail to the drawings, ~igure l
: illustra~es a simplified, over-,all block diagram o~ a ~ub-
scription television system according to the present inven~ion.
: It should be understood that this system can be utili2ed in
2Q various forms of television transmission and recepticn~ in- ''
. . .
~- . cluding over-the-air or cable television. Like refe-ence numerals
have been employed thrc~ughout the variouc drawinss to cesignate
like parts~ '
. .In Figure l, the subscriptlon television s~stem
'-~ 25 designated'generally b~ reference numeral 10 consist~ng of
a transmitter section 12, a receiver section 14, and telephone
, communication circuitry 16 located at the receiver sectlon
.
': -
1 ~59~2
14 for communication with the transmitter section 12. A .
central computer 18 is used to store informa~ion signals
identifying the various authorized subscribers and sends
out these information signals to a transmitter 20 for
transmission over-the air via antenna 22. A synchronizing
- (sync) genera,tor 24 is connected to the computer 18 for
synchronizing th~ information signals sent from the computer
to the horizontal scanning rate used by a television program
source such as television camera 26. In order to prevent
' 10 unau~hori~ed viewers from receiving the telecast, the
signals from the television camera 26 are processed by a
code converter 28 along with the same information signals
from the computer utilized to identify the authorized sub-
scribers, which modifies the signals in such a way to be
unintelligible to unauthorized subscribers. These signals
are then sent over-the air via a transmitter 30 2nd antenna
.~ 32. Analog audio signals from a sound source such as
microphone 34 can also be processed or modi~ied in th~: converter 28 by sending th.e audio signals through an analog~
to-digital converter 31 to produce digitized audio signa
: The scrambled audio signals from the code converter 28 are
: also transmitted via the transmitter 20 and ~ntenna 22.
.The scrambled television signals, the scra~bled
audio signals and the information signals are received by
antenna 36 These signals are then delivered to an ~ tuner
3~ coupled to an intermediate frequenc~ (IF) amplifier and
'
: -13~.
'
115~9~2
dete~tor 4Q which detects the scrambled television signals
and to an ~P tuner 42 coupled to an IF amplifier and detector
44 which detects the scrambled audio signals and the in~or-
mation signals. The scrambled television signals are sent.'
to a code converted 52 for re-processing~ The information
. signals from,the detectvr 44 are sent to a comparison circuit
46 to determine if one of'the information signals matches
- , the uni~ue address associated with a particular subscriber
. stored in read-only-memory 4~O These same information signals
serve as control signals and are delivered to a code lock- .
out circuit 50 as is the output of the comparison circuit
46. When there is a match,~the co~parison circuit 46
activates the circuit 50~to allow.the control signals'to
pass thxough to the code converter 52 which unscrambles the
television signals from the detector 40 and the scrambled
audio signals ~rom the detector 44. The unscramblèd television
signals are sent to a modulator 54 ~or converting the television
signals to a frequency corresponding an unused numbered
channel on a conventional.television xeceiver 56. The
~: 20 digitized audio si.gnals from the code converter 52 are also
sent to the modulator 54 for transmission on the receiver
56 via a digital-to-analog converter 58 for co~verting the
digitized,audio signals to t}le oriyina,l analo~ audio sig-
nals.
Each subscriber can select the programs he wishes
to view by simply depressing switches or buttons (not shown)
-14- .
~ ~5~2
associated with the telephone communication circuitry 16
. located at the receiver section 14. The selections are
transmitted to the computer 18 directly by telephone lines
62. It should-be apparent to those skilled in the art that
the present subscription television system can be easily
~onverted to conventional cable television systems by simpiy
replaciny the antennas 22, 32 and 36 with an appropriate,
interconnected coaxial cable (not shown)O
; The .~ransmitter section 12 shown in Figure 1 will
now be described in greater detail with reference ~o Figure.
2. The central computer 18 is utilized t~. generate randomly
a sequence of codes, each one representing a particular
account number or address o~ an indlvidual subscriber. ~ach
of the codes is a pulse train of 32 bits, each or the bits
being either high ~"1" state) or low ~"0" state), exce?t
~ that the first bit is always made to be low and the last bit
; is always made to be high so as to facilitate the detection.
and synchronization b~ the circuitry in the receiver section
14~ Thus, the number of different subscxiber codes available
20 is 23 or 1,073,741,824 since this is the number of possible
combinations of thirty bits each of which can be either high
or low.
.These codes are selected by the computer 18 such
that each of the eligible subscriber's code is transmitted
in a pre-determined sequence and is then repeated on a
continuous cycle thereafter. Each of the 32-bit codes are
, . ~ . ' ' ;
g ~ 2
addresses of the individual subscribers located in the
broadcaster~s coverage area~ When these transmitted codes
are reprocessed at the receiver station 14, it produces the
.xequired information that it utilized to determine the code
used to encode or scramble both the aural and visual portions
- of the broadcasted television program.
The broadcast of these 32-bit codes or enoded
signals is synchronized to the horizontal scanning rate
produced,by the sync generator 24 as is conventionally
used by the television camera ?6 in ~he standard television.
studio. The sync generator 24 provides a.pulse pattern
at the rate of 15, 750 time-s per s,econd (15,734 for color).
which corresponds to 525 horizontal scan.lines in the visual'
raster of a conventional television picture traced by an
electron heam of varyins int,ensity from the top to the ,
bottom of the picture in lf30 o:E a second. The horizontal
sync pulse or signal output of the generator 24 is,'connected
via lead llne 60 to the inputs of the televislon,camera 26
and a monostable circuit (one-shot) 64. The monostable
ZQ circuit allows adjustment in the pulse width of the hori20ntal
sync signal, and its output controls a master clock 56
having pre-set frequency of 16~128 ~IHz (16.111 ~IHz for
color) and provides data request signals to the computer 18.
; The output pulses from the master clock 66 are phase-locked
to the horizontal scanning rate. The vertical sync pulse or
signal output o the generator 24 is connected via lead lin~
9 ~ 2
.
68 to an inpu~ of the television camera 26 and to monostable
circuit 70 in which the width of the vertical sync pulses
are adjusted and are used to provide lnterrupt request
signals to the computer 18.
The computer sends out randomly the various 32 - ..
bit binary subscriber codes on line 72 in a parallel manner
to a parallel-to-serial converter 74. Since a dif~erent - -
32-bit code is sent out by the computer 18 at the rate of
the horiz.ontal scanning frequency o 15,750 times per second.
as provided by the sync generator 24, this means that 94~000
inaividua:L subscribers can be selectively controlled to
unscramble the transmitted signals.every minute. The
converter 74 transorms the codes from parallel into a seria~
sequence for modulation of ~he transmitter 20 via a ~reqi~ency
: 15 shift ke~.ing (FSK) FM modul~tor 76. These same 32-bit codes~
of the various subscribers are also made available on line
~. . .
78 and are processed into corresponding 4-bit codes or
;~ words.by an adder network 80. These 4-bit words .contain
~ .
the informat.ion to be utilized in determining the encoding
. or scra~bling pattern of the video and sound portions of
the broadcasted signals. The details of the adder network
80 will be discussed more fully hereinafterO
.The coded signals from the output of the adder
network 80 are coupled to datainput of a D-type gating
flip-flop for synchronization with the horizontal scanning
.
: ,
-17- . -
5 ~
rate'which is connected to the clock input of the ~-~ype
flip-flop via lead line 84. The output of the flip-flop
82 contains the encoded pattern which scrambles the visual
and aural portion of the televised signal via an active
li~e gate,86 and a gated'video switching amplifier 88 and
an additional adder network 100, respectively. The
switching amplifier 88 consists actually of three separate
amplifiers, one being a non-inver,ting amplifier 90, a
second being an inverting amplifier 92-and a thixd ampli-
fier 94 determining whether the output of the non-inverting
amplifier 90 or the inverting amplifier 92 is fed to its
, input. (See Figure ll(b)). ~Since the active line gate 86
drives the'switching amplifier 88, the output of the switch-.
ing amplifier will be dependent upon the binary stateof the
, active line gate 86, which is either in the "1" (high) or
"O" (low) states. The line gat:e 86 is actuated only during
the portion of the televised si.gnal that contains the actual
visual or picture portion and i.s de-activated during the
synchronization intervals. When the output of the line gate
2~ 86 is in the high state, the video portion of the televise~
signal is sent through the non-inverting amplifier 90 and -
the output of the switching amplifier 88 is not inverted.
~owever, when the output of the line gate 86 is in the low
state the video portion of the televised signal is inverted.
2~ Since the output of the adder network ~0 passed through the
active line gate 86 switcles ~etween the two binary states
1 ~5~9~2
on a'pseudo-random basis~this causes some of the video
signal portions at the output of the switching amplifier
' 88 to be positive and some'to be negative, which creates
a mosaic quality in the picture. This scrambled picture
is very unpleasant and completely unintelligible to the
- unauthorized viewers. The output of the switching amplifier
88 having the scrambled picture is coupled to an AM
modulator 96 for amplification and modulation. The output
of the modula~or 96 is then sent to the ~ransmitter 30
for broadcasting the'scrambled p'icture via the antenna 32.
: . . . . .
As is well-known, the origi'nal visual information
and the chrominance informa~ion originate in the television
camera 26 and a chromatic (color) sub carrier generator or
phase modulatox 98 respectiv~ly. As previously mentioned,
15~ ~he horizontal and vertical synchronation signals sent to
the central computer 18 are also used to synchronize the
horizontal and vertical scanning rate of the television
camera 26. A typical audiosource originating the micro-
phone 34 at the broadcaster's site or a similar audio
2Q source such as a tape recorder, film chain, phonograph
record and the li~e is also encoded or scrambled so as to
increase the security of the system by first converting the
original aural signals (analog form) to digital pulse
trains each consisting of 11 bits via the analog-to-digita
.
:
--19--
1~l5~9~2
converter 31n Each of the bits are eithex high ("1l' state)
or low ("0" stateJ, but the first bits are always made to
be low and the last bits are always made to be high so as to
facilitate the detection and synchronization by th~ circuitry
.5- in the receiver section 14. Then, these pulse trains o~
the digitized aural signals are added binarily with the
4.-bit binary outputs of the D-type flip-flop 82 resulting
from the addèr network 80 via the additional adder network
100. These resultant ll-bit binaries are converted to -
serial pulse trains by a shift register or parallel-to-
: serial converter 102. These serial pulse trains are coupled
to a frequency shift keying~M modulator 104 for amplifica-
tion and modulation before being sent to the transmitter 20
and transmitting antenna 22 for broadcast.
; lS . In additiont a second audio source 106 can beprocessed in an unscrambled form by an FM modulator 108
- for transmission ~ia the transmitter 30 and the transmittin~
antenna 32. The second audio source 106 is refe~red to
as a "barkçr" source and canbe heard by all of the television
: 20 . receivers. It is utilized for encouragîng the viewers to
use the programming of the subscription television system
: . and i5 ~vailable for announcement and to promote mar~eting
of the subscription tele~ision programs to potential pur-
chasexs or other suitable use.
A control terminal 110 is connected to the
computer lB for controlling manually the enabling and
- -20-
.
~2
disabling of ~arious subscribers. A modem 112 is coupled
also to the computer 18 for transferring the program .'
requests from the various subscribers sent on the tele-
phone line 62 as will be explained in detail in connect;.on,
S with the telephone communication,circuitry 16~-
,. The receiver section 14 shown in Figure 1 o~ the
drawings will now be discussed more fully with reference'to
Figure 3. Eac,h subscriber to the subscription television
system is provided with a,housing or box-type enclosure (not
shown) containing all of the receiver-cixcuitry in the
receiver section 14 including the telephone communication
circuitry 16 for-operative connection to'his conventional
television receiver 56. The enclosure is typically place~
adjacent or on top of the subsc:ri~er's television receiver~
The enclosure is interconnectecl between the subscri~er;s
receiving antenna terminal connections and the television
receiver 56. Power is supplied to the enclosure via a
:' ' . 120 VAC power input terminal located on the encl~sure.
This e~cl.osure is further'connected to a telephone termin~.
:~ 20 outlet (not shown) conven'tionally supplied ~y a local
telephone utility company for'communication with the broad-
caster to request service of'the programs desired vi2 the '.
telephone communication circuity 16, as will be discussed
more,fully later~
All of the transmitted signals ~rom the transmitting
antennas 22 and 32 and all of the other conventionai, ~nscram~led
.
-21- . -
television channel signals are received as incoming.signals
on the receiving antenna 36. It should be recalled that the
transmitting antenna 22 is sending out two separate sets of
digitally excited frequency shift key.ing data signals,
S namely, the digital control signals each having a pulse
train of 32-bits and the digital audio signals each having
a pulse train of ll-bits. The other transmitting an~enna
32 is sending out the encoded or scrambled video signal.s,
the normal chrominance signals, and the "barker" audio
signals.
The incoming signals are now processed by s?litting
them into three paths at an RF splitter 114. One path drives
the RF tuner 38 coupled to the IF amplifier and detector 40
~hich extracts all of the scra~led video, normal chrominance,
and "bar~er" audio signals. A second path is processed by
the RF tuner 42 coupled to the }:F amplifier and detector 44
for removing all of the digital audio signals and digital
control signals. A third path via lead line 116.is util~.zed
to receive the other unscrambled channel signals when the
subscription television system i5 not in use.
It will be apparent that in svsLems dealins with
scrambling and unscrambling of signals -the methods used in
unscrambling are functiona~ly complementary to the vnes
used in the scrambling process, and that the unscræ~bling
and scrambling processes have to be com~let~ly syncnroni~ed.
Thus, many of the circuit componen~s that will be discussed
.
-22-
.
`
9 5 ~
hereinafter in the receiver section 1~ are simiiar in
operation to those previously mentioned in the transmitt-
ing section 12.
- The scrambled video signals from the output of
the detector 40 is connected to a gated video switching
-- amplifier 118 similar to the switching amplifier 88 for
unscrambling and restoration of the picture back to a normal
image. However, this can occur-only if proper decoding
signals controlling an active line gate 120 similar to the
line gate 86 will cause inverting or non-invertiny of the
video portions of the televised signals on a line for line
basis in exact correspondence to the scrambling by the
active line gate 86. The reprocessing of the control
signals to generate the decoding signals will now be
explained in detail.
An output containing the control signals from the
detector 44 is sent to a serial-to-parallel converter or
shift register 122 and to a pulse detector 124. Since the
first bits of the incoming pulse train con~rol signals are
always beginning with a low level, the pulse detec~or 124
`~ i5 utilized to sense the shift from a high level to a low
- le~Jel. When the ~hift i5 detected, the output of the
detector 124 drives a monostable circuit 126 (one-shot)
to synchronize the receiver circuitry with the transmitter
horizontal scanning rate of 1,57~Hz and also to permit
adjustment o~ the pulse width from the detector.
' ' ' ' . .' .
-23-
~ ~ .
~ ~5~952
Once the shift is detected, the monostable
- circuit 126 initiates the counking of a divide-by-eisht
counter 128 (- 8). The clock rate of the counter 128
is provided by a master clock 130 similar to the clock 66
. having a pre-set frequency of 16.128 MHz which is actuaily
16 times greater than the pulse rate of the incoming
control signals and is phàse-locked to the horizontal
scanning rate o~ the transmitter section 12~ Thus, after
ei~ht clock pulses are counted, the middle of the first bit
or pulse of a 32-bit incoming control pulse train will be .-
at the input of the shit register 122O At.this point,
a transfer ~ulse will be generated to shift the first bit
into the shift register 122 for storageO
The output of the counter 128 is coupled to a
- 15 divide-by-16 counter I32 ~. 16~, which is in turn connected
to a divide-by-32 counter 134 (. 32). Subsequ2nt to the
storage of the 1rst bit, the counter 128 is stopped and
the counter 132 is activated to count so that 16 pulses
later~ the shift register-122 will have at its input the
middle portion of the second bit of the 32-bit pulse train.
Once again, a transfer pulse will be generated to sni't now
the second bit into the shift register 122 for storaseO The
first bit previously stored will be shifted sexially do-~n the
shift register. This process is repeated thirty more times
. and as a result will cause all 32-bits to be stored in the
. -24-
~ 1~5952
shift register 122. When the thirty-secondth pulse-is
stored, all of the counters 128, 132 and 134 are reset
and made ready for the next control signal. Thus, the serial
control pulse trains each of a full 32-bits will become
stored in the shift regist~r and is made available as
parallel sets of.32 data ~its (see Figure 9).
The 32-bits o the shift register 122 are taken
out to an adder network 136 which .converts the 32-bits
down to a 4-bit unscrambling code. The adder network 136
is identical in operation to the adder network 80 in the
`~ transmitter section 12 and produces the 4-bit unscrambling
code identical to the 4-bit.scrambling code used to
scrarnble the video and aural portîon of the originally.
:~ transmitted signals. A simplified block diagram of the
adder networks 80 and 136 are illustrated in Figure S and
is compxised of seven fllll binary adder circuits 138 inter-
connected as.shown. However, it should be understood that
the 32-bit input lines can be interchanged by their bit
locations on a pre-determined basis to provide increased
security in the system. . Thus, this produces the 4-bit
codes at the output of the adder networ~ 13G in a ps.udo-
. random scrambling pattern.
A D-type clocking flip-~lop 1~0 is connected ~o
. the output of the adder network 136 for holding the ~-bit
pattern for the duration of the next horizontal scanning line
.
- .
-25-
.
in order to decode that scanni~g line and all of the audio
signals received during tha~ same interval. The flip-flop .
140 drives the active line gate 120 for controlling the
switching amplifier 118 to switch its inputs between the
inverted and non-inverted inputs in the same sequence
, as the encoding operation by the switching amplifier 88
In this mannert the hor.izontal scanning lines at the
transmitter section 12 are reproduced at the receiver
section 14 so that the received video signals will become
restored to the original unscrambled picture.
However, this restoration process. is permitted
to occur only if one of the~plurality of 32-bit control
signals generated on a pseudo-random basis by the central
. computer 18 and transm.itted via the antenna 22 is matched
perfectly (bit for bit) with. a unique 32-bit code assigned
to a particular subscriber's te:levision receiver 56 ~hich
is stored in the read-only-memory (ROM) 48. The transmitted
:`~ incoming control si~nals at the output of the dectector 44
are.compared by a comDarator 142 with the unique contents
of the ROM 48. The content of the ROM 48 is synchronized
with the incoming control signais.by the counter 134.
If the comparison is unsuccessful, the 4-bit
code in the D-tyPe 1ip-flop 140 .is prevented to be clockea
to the active line sate 120 controlling the switching
ampliier 118 to unscramble the video signals. On the
other hand, if there is a successful match between one of
-26- .
5~52
the control signals and the ROM 48 o the particular sub- -
scriber, then a flip-flop 14~ is set and gates a decode
enable flip-flop 146 which permits the output signals of the
monostable circuit 126 to be sent as clock pulses and set ~
at the horizontal scanning rate to the D-type flip-flop
. 140. Unless the clock pulses of the monostabie circuit
126 via lead line 148 are passed through the decode-enable
:: flip-flop 146, the D-type flip-flop 140 will not be actl~ated.
Upon a comparison, the output of the flip-flop
:~ 10 144 also resets a three-minute timeout ti~er 150 to zero.~en the timer 150 reaches the three-minute mark, it will
cause the decode-enable flip-flop 146 to be reset thereby
stopping the decoding process as the clock pulses to ~he
D-type flip-flop 140 will be bloc~ed by the flip-flop
-- lS 146. Thus, continuous decoding is possible and unin.errupted,
restored video and aural signals are available to the
~:~ . subscriber only if the comparator 142 causes~the flip-~lop
144 to reset ~he three-minute timer 150 at leas~ once every
three minutes.
20 . A second output containing the digitized aural
signals from the detector 44 is sent to a serial-to-
. . parallel converter or shift register 152 to convert the
signals to parallel digital binary form; A subtractor
: circuit 154 combines the 4 bit unscrambling codes from the
D-type flip-flop 140 with the encoded digitized aural signals.
-27-
9~2
,.
The subtxactor circuit 154 operates similar to the adder
networks 80 and 136, except that it is actually using
2's complement addition in order to realize unscrambled
digiti2ed aural signals corresponding to the ones at
the output of the analog-to-digital converter 31 in the
- transmitter section 12. The output of the subtractor
circuit is connected to the digital-to-analog converter
58 to produce analog signals identical in form to the
..
original audio signals generated by the source 34 at
the broadcaster~s site. Thus, it can be seen that decod-
ing of the aural sign21s is also dependent upon the D type
flip-flop 1~0.
The restored video signals at the output of .
the switching amplifier 118 and the restored audio signals
1$ at the output of the digital to-analog converter 58 are
fed into the modulator 54 which converts these signals
to a desire~d locally unused V'dF channel (2-13~ as the
secured channel. Typically, channel 3 or 6 can be a
suitable choice. These signals are combined with all of
the other unscrambled televis1on channels recei~ed by
the antenna 36 via RF combiner ci~cuit 1550 Tnis allows
- view of all the channels on the television receiver 56
- including the secured channel, and selection of the particular
channel is achieved merely by turning a channel selector
dial (not shown) on the television receiver 56.
'
.
-28-
i ~59~2
.
The telephone communication circuitry 16 shown
ln Figure 1 of the drawings will now be described in greater
detail with reference to Figure 4O E~ch subscriber can
selec~ the particular televised program he desires to
recieve by depressing one of a plurality of buttons or
switches 15~. It should be understood that any desire~
number of buttons or switches can be utilized and connected
to pexform various functions. For example, some buttons
can be used to select the particular programs to be viewed
a~ a desired time while others can be used to cancel or
correct errors in the program request. Once a button
associated with a particular program is depressed, this
selection is transmitted on the telephone lines 62 to a
local telephone company's central office ~not shown). In
a large metropolitan area, the central office would then
communicate with another central office which, in turn,
.
relays the program requests to the computer 18 at the
broadcaster's site via the telephone lines 62 and the
modem 112 (modulator-demodulator3. This communication
process will now be discussed more fully.
When a subscriber wishes to receive a particular
program, he simply depresses the appropriate button 156
; which activates a sequence control network lsa to energize
a hook-switch relay 160 allowing an"off-hook" or "on-line"
condition with the local central office. ~hen a dial tone
is placed on the telephone lines 62 by the central o fice,
-29-
~ ~5~
dial ~one detector or filter 162 causes the sequence,control
network 158 to transmit dial address stored i~ binary form
in a rea~-only-memory (~OM) 164. This ROM 164 is similar
to the ROM 48 in the receivPr section 1~ and in fact, can
, be the same one. Thexe are two methods of dial addressing
which are availahle. First, a multi-frequency tone generator
166 can be used to transmit a designated telephone number
employed by the broadcaster to receive automatic telephone
,requests~ The tone generator 166 is known generally by
the trademark "Touch-Tone". Alternatively, a programma~le
divide-by-n counter 163 can be provided to interxupt current
flow in the,relay 160 to diàl the ~esi~nated telephone numhex.
This latter method is known as the dial pulse method o~
dial addressing wherein the number of contact closure interrup-
, 15 tions are varied as with a corresponding number on a rotary
dlal. A row/column decode 170 converts the dial address from
the ROM 164 either into row and column numbered pairs for the
tone generator 166 or into 2's complements for loading into
the pr~grammable divide-by-n counter 16~ for pulse method
dialing via an interface buffer 172 and the hook switch relay 160.
Once the dialing process is completed, the se~uence
- control network 158 is arrested and awaits a carrier tone
genera`ted by the modem circuitry 112 at the broadcaster's
site,. When ~ilter 174 detects the presence of the carrier
~5 tone, it causes the sequence control networ~ 1S8 to address
the ROM iG4 storing tne particular subscriber's code consisting
' ,
-30-
:~ ~ 5~2
of 32~bits and to send them out in a parallel sequ~ence of four
eight-bit words. A gating circuit 176 is coupled to the
output of the ROM 164 and groups the four words for activat-
ing a voltage-to fre~uency converter 178 to generate fre-
quency shi~t keying modem,tones which contain the sub-
scriber's unique-address or account code. Subsequent thereto,
the subscriber's request code indicating which of th~ buttons
156 he has depressed is transmitted via the gating circuit
176 and the converter 178. , '
lQ After the request code has been transmitted, the
sequence control network 158 stops the gating circuit
176 and then awaits for an acknowledgment tone from the
modem circuitry 11~ at the transmitter section 12. When
the acknowledgment tone is detected by acknowledgment ton~
filter 175, the sequence control network 158 is discon~ect~d
from the telephone lines 62 by the de-energization o~ the relay
160 and lights an acknowledgment light 180 for 15 seco~ds.
~hile only'one acknowledgment light is illustrated, t should
' be clear that any number could be provided to indicaLe which
~0 button corre,spondi~g to a particular request was depressed.
However, if no ~ial tone, carrier tone, or acknowledcm~nt tone
is detected or if any part of the dialing sequence is ~ied
up beyond a pre~determined time intervalr a time-out timer
182 having its input clock pulses set at the horizontal
~5 scanning rate via lead line 18~ interconnected wi~h 1~2d line
184' (Fig. 3~ will reset the sequence control networ~ 158
and activates a fault light 18~ for 15 seconds.
. ' . I
-31-
~ ~S~52
An isolation transformer 188 is utilized to pro-
tect the telephone communication circuitry 16 and isolates
it from telephone circuits in the local central office.
A holding current coil 190 draws current rom ~he telephone
circuits when the hook-switch relay 160 is closed for causing
:. the "off-hook" condition to be detected at the local telephone
ccmpany's central office which will respond with a-diai
tone~ It should also be apparent to those skille~ in the
art that.the account code and request code of the various
-10 subscri~ers may be placed on storage devices (not shown?
such as disk drives and magn~tic tapes for use by ~he
central computer 18 to gene~ate the account codes of the
various authorized subscribers thereby permitting the
subscri~ers to view the transmitted scrambled video
:: .
. 15 and/or audio signals.
F.igure 6(a) illustrates the time-amplitude
relationship of a conventional, normal scanning line whlch
includes the composite video and synchronization siynals~
. In Figure 6(b), ~here is shown an example of the trar.smitted
~: 20 scrambled video signal of the present invention wherein
certain video portions of the horizontal scan line have
been randomly inverted~
Figure 7 shows a spectral distribution in terms
o relative maximum radiated field strength of t~e signals
transmitted in the present invention verses the frequ~ncy
relative to the lowest channel. In par~icular, the graphical
~: .
9 ~ ~
representation of E`igure 7 illustrates in the left-hand
half the 6 MHz frequency allocation of a standard televlsion
channel as deined by the F. C. C. As is well kno~n, the
picture or visual carrier signal, the chromatic su~carrier
frequency, and the center of the barker aura~ frequency are
located at approximately 1.25 MHz 9 ~ . 83 ~æ, and 5~7~ PlHz,
respectively, above the lower frequency end of the telç~ision
channel. The right-hand half of Figure 7 depicts the~
control signal center fre~uency and the digital 2ural fre-
quency which are located respectively at approximately
9 MHz and 11.25 MHz.
~igure ~la) shows an example o one of the plurality
of unique pulse-ooded control signals consisting o~ a 32-bit
binary pulse train which are transmitted separately ~y the
broadcaster to identify individual authorized subscribers and
to provide the information needed for unscrambling o the
video and audio signals in the same sequence as used for
scrambling. Figure 8(b~ is an example of the scra~bled
or coded audio signals, i.e., digitized audio consis.ing of
a ll-bit pulse train for increasing the security o~ ~-he
system.
~hile the various blocks sho.~n in Figures 1 .nrough
4 may take on various forms, suitable circuitr~ thereror are
illustrated in Figures 9 throu~h 18. Althou~h ~hese schematic
diagrams are believed to be self-explanatory to ~i~i.-ie skilled
.
:
1 ~5~2 -
in the art in view of the foregoing.discussion, a brief.
description of the operatLon of each figure is believed to
be in order. .-
The synchronization oX the entire system is
controlled by the master clock 66 of Figure 2, which is
shown in more detail on Figure 18. The master clock 66
consists of a voltage-controlled oscillator 192 having a
capacitor 194 and an input ~oltage + V. The output of
the voltage-controlled oscillator is fed into an inverter
196 whose output is set at a frequency of 1~.128 MHz. :
, . .
The output of the inverter 196 is fed to all places re-
quiring this frequency output and to a counter 198 (~ by
1024). The frequency at the output of the ~ounter 198
designated by the lead line 200 is 15,750 Hz which is
. 15 delivered to one input of a.phase-lock circuit 202. The
other input of the phase-lock circuit 202 designated by
lead line 204 is the horizontal sync p~llse ~rom the
. generator 24 ~Figure 2)~ Th~s, the voltage output of ~he
. phase-lock circuit 202 on lead line 206 ~aries via le~d line
205 until voltage-controlled osclllator 192 is phase-locked
to the horizontal scanning rate.
.~ Referring now to Figure 9 of the drawings, there ; ~is shown .in more detail the circuitry of the shi~t register : `
122 of Fiyure 3. The output containing the control signals
~rom the detector 44 on lead line 208 ~Figure 3) is eonnected
, . . .
to the lead line 208'. The shift reg.ister is com?osed of-a
~ ' ' " ' '
.
plurality of J-~ flip-flops 210 connected serially and a
plurality of inverters 21~. The output of the flip-flops
210 labeled 2-231 are connected to the corresponding
inputs of the full adders 138 on Figure S. In order to
shift the full 32-bits of the control pulse trains into
the ~lip flops 210, the transfer pulse is delivered o~ the
lead line 214 which is connected to the lead line 21~' in
Figure 10. The control signals on the line 208'.are also
sent via line 216 which is connected to the lead line
216 ' on Figure lQ for reception by the pulse detector
124. The output of the pulse detector on its lead line 218
is utilizecl to reset the counters 128, 132 and 134. These
counters are formed by tèn J K flip-flops 220. Tne clock
input to the counter 128 is.on lead line 222 which is connected
: 15 to the 16.128.MHz output of Figure 18.
As will be recalled, the clock frequency is
actually 16 times g~eater than t.he pulse rate of the incoming
control signals. Therefore, a~tler eight clock pulses are counted
the middle of the first bit of the 32-bit incoming pulse .train will be at the input of the flip-flop 210 desig~a,ed
by lead line 2240 However, in practice it has be~n encountered
that only four clock pulses need to be counted so as .o be
in the middle of the ~irst bit. The reason i5 beca~se of
propagation. delays and other inherent delays associated
with electronic circuitry. Thus~ a NAND gate 226 is inter-
connected so as to genexate a transfex pulse on l.S 1 2d line
,
'
-35-
~ 1~5~3~2
214' after the fourth cloc~ pulse and each sixteen pulses
thereafter~ In this manner, the entire 32-bits of ~he serial
control pulse train is transferred into the flip-flops 210
of Figure 9.
Prior to the transfer of each bit, a comparision
of such bit is being made with one corresponding bit o~
the unique 32-bit code stored in the read-only-memory 48
byt~e comparator 142. The output of the comparator is
coupled to the flip-flop 144 whosa outpu~ i5 connected- to
an input of NAND gat~ 228. When there is a co~parison,
the output of NAND gate 228 on its lead line 230 gate~ the
decode~and-enable flip-flop~146, which is shown in more de-
tail on Fig~re 13.
. The last J-K flip-flop 232 and the NAND ~ates 234
in the counter 134 are used to reset all of the counters 128,
132 and 134 after the last pulse or thirty-secondth pulse of
the 32~bit control pulse train. Then, the ~ounters are
ready to sample and compare the next control pulse train.
In Figure ll(a), there is shown in more de~ail the
. circuitry of the blocks 120~ ana 140 in Figure 3 of
the drawings. The NAND gate 236 has its one input designated
_; by lead line 238 connected to the output of the decode-an~- -
enable fl.ip-flop 145 designated by lead line 238l t~igure 13).
: The other input to the NAND gate 236 designated by lead line
~5 240 is coupled to the clock rate o~ 15,750 Hz.. The output
of the NAND gate 2 36 is connected to one input o~ the active~
:
-36-
. .
1 ~5~9~2
;
line gate 120 and to the`clock input of the D-type flip-
flops 242. The other inputs designated by lead lines 244
through 250 are connected to the output of the adder net-
work 136 shown in Figure 5 and desi~nated by 2 through
23. The output of the D-type flip-flop 140 is connected
to the subcontractor circuit 154. The other input-to the
subtractor circuit is the digitized aural signals from
the output of.the shift register lS2 on the lead lines 252.
The output of the subtractor circuit 154-is coupled to the
input of. the digital-to~analog converter 58 whose ~utput
on lead line 254 is connected to one input of the.modulator
54 .
T~e other input to the active-line gate 120 on its.
lead line 256 is from the 16.128 MHz output of the master clock
on Figure 18. The active line gate includes ten 3-K flip- .
flops 258 which are connected in a serial manner~ As pre-
viously discussed, the active-line gate 120 will keep the
video switching amplifier 118 in the non-inverting condition,-
except during the active portion of the horizontal scanning
line. The output of the active line gate from N~lD sate 260 is
connected to amplifier 117 bf the video switching am?lifi~r
. 118. The other input to the amplifier 118 on its lead line
262 is from the output of the IF amplifier and detector 40
containing the scrambled video signals. The output.of- the
amplifier 118 designated by lead li.ne 264 is connected to
the other input of the modulator 54.
'
~ ` 115S9~
In Figure 12, there is shown in more deta~l
the circuitry of the shift register 152 of Figure 3. The
output containing the digitized aural signals from the
detector 44 on lead line 266 (~igure 3~ is connected to .
the lead line 266'~ The shift register is composed of
. a plurality of J-K flip-flops 268 connected serially.
The output of the flip-flops 268 labeled 2-2l are
connected to the inputs of the subtractor circuit 154 on
the lead lines 252 (Figure 11). In order to shift the full
ll-bits of the digitized aural s.ignals into the flip-flops
268, the transfer pulse is deli~ered on the lead line ~70
connected to the output of ~NAND gate 2i2. The shift register
includes counters 274, 276, and 278. The coun~er 274 is
a divide by-eight (. by 8) countex having its clock input
coupled to the aural clock rate;output in Figure 13 via
lead line 280. In actual practice, it can be seen that the
NAND gate 272 is interconnected so as to generate a .ransfer
pulse after the fourth clock pulse and each sixteen pulses .
thereafterO This is because ~nly four clock pulses are
20 - needed to be counted due to propogation delays in electronic
circuitry hefore the middle of the first bit of the ll-bit
digitized aural 5ignals is at the input of the flip-~lop 268
designated by lead llne 2820 In this manner, the entire ll-bits
. of the aural signal is txansferred into the flip-flops 268
25All of the counters 274, 276 and 278 are ~ormed
from a plur~l~ty of J-~ fllp-~lCOS 84- The counter 276 is
. ' ' . , ' . ,'' .
.
9 5 2
connected to the counter 274 so as to form a divide-by-.
sixteen (. by 16) counter. The counter 278 is inter- '
connected with the NA~D gate 286 so as to ~orm a di~-ide-
by-ll counter so that all of the counters are reset by the
NAND gate 286 after the last pulse or llth-bit of the.
. digitized aural signal'has been conducted~ .
Figure 13, there is shown a divide-by-N counter
288 having the various clock rate outputs. The input to
the counter 288 designated by lead line''290 is from th~ ,
output of the counter 19 8 on its lead line 290' in Figure
18. The three-minute output from the counter 288.is uSed
to reset the decode-and~able flip-flop 146 after thxee
minutes of time has elapsed to stop further decoding by th.e
D type flip-flop 140 unless'there is a comparison between .
one,o~ the control signals with the unique address of the
read-only-memory 48 prior to that time. The 30-second output
from the counter 238 is connected to the lead line 184
tied to the inpu~ of the time-out timer 182 in Figure 4.
The output labeled "clock-rate 2 " is connected to the lead
. line 294 in Pigure 14, and the output labeled '~clock-rate 213l'
is connected to the lead line 296 in ~igure 14.
_ In Figures 14-17, there is shown in more detail
suitable.circuitr~ which may be used for the telephone
'communication circuitry shown in Fiqure 4 of the drawings~
In Fiyure 16, there is shown seven button5 or switches 298
for generating the program request code signals at the outputs
'
-39-
3~
of the NAN~ gates 300O The output of the NAND gate 302
- designated by lead line 304 is connected to the lead
304' on Figure 14 to initiate the telephone dial up
sequenceO The lead line 306 is used to control the iele-
S phone re~uest code which is connected to lead line 3~.6'
on Figure 14,. and ~he lead line 308 is used to re~et
the telephone request code which is connected ~o lead iine
308' also on Figure 14O
.In ~igure 15~ there is shown in details o~ the
gating circuit 176 composed of a plurality of J-K ~lip- .
flops 310 and NAND gates 312 and 314. The input to the
NAND gates 312 are from the xead-only-memory 48 and the
program request code si~nals from the NAND gate 300.
rrhe ~ransfer of data into the gating circuit is con-
15 . trolled by the lead line 316.whi.ch is connected to the
lead line 316' on Figure 14. The output of the gating
circuit is designated by lead line 318 which is connect2d
to the input of the voltage-to-frequency converter 178~
In Figure 14. ~ere is shown ~he detailg of the
: 20 sequence control network 158 and the programmable coun~er
168. The sequence control network includes 3-K ~lip-flops
~-~ 324. Each of the outputs of the flip-flops is connected
to the read~only-memory 48 and the NAND ga~e 322 O! tne
counter 168.
In Pigure 17, there is shown a typical ligh~
driver circuit for lighting the light 180 to indicate a
fault and to ligh~ the light 185 to indicate receip~ o~ the
acknowledgement tone. Illumination of either lig~t 180 or
186 is for fift en seconds as controlled by lead line 181
tlS sec. output) which is connected to lead line 181'
on Figure 13~ While there has only been one suh circuit
~ 1~59~i~
shown, it should be undexstood that seven are used in this
example to correspond to the number of swîtches 298.
It will be understood from the foregoing
description that the present invention significantly advances
the state of the art of cod~ng and decoding of standard
television signals which allows the reception thereof in
an intelligible manner only by authorized subscribers~
In particular, the scrambling of th video signals in the
invention is effected by inversion of th~,video signals
of some horizontal scan lines on a pseudo-random basis to
-pr~duce a picture having some video signals inverted and
others not inverted. The scrambling of the audio signais
is effected by conversion o~ analog audio signals to coded
digital audio signals. Telephone communications circuitry can
also be provided so that the subscribers can request their
programs to the broadcaster via a telephone interface.
While there has been illustrated and described
- what is at present to be a pre~erred embodiment o,' ~e
present invention, it wil~ be understood by those skilled
in the art that various changes and modifications ma;~ be made
and equivalence may be substituted for elements thereo~
,~ without departins from the true scope of the inventi~n.
In addition, many modiications may bP madel,to adap~ a
particular situation or material to the teachings OI the
invention without departing from the central scope t~ereof.
Thereforc, it is intended that ~is invention not be l~'mited
1.
-41-
9 ~ 2
~o the particular embodiment disclosed as the best mode
contemplated for carrying out this invention, but tha~ the
invention will include all embodiments falling ~ithin the
s_ope or th~ app-nded c ~i~s.
'
'
.
,~ '
; ' . ' ' '
~, - .
., ~ .
~
.
-42-
