Note: Descriptions are shown in the official language in which they were submitted.
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1 49,662
AUDI~ SCRAMBLER UTILIZING AN AUXII,IARY
C~ANNEL FOR SYNCH~ONIZING THE DESCRAMBLER
BACKGROUND OF THE INVENTION
Field of the Invention:
The invention relates generally to audio trans-
mission systems and more specifically to an audio trans-
mission system in which the audio is first converted to adigital signal which is then scrambled and reconverted to
an audio signal for transmission with the descrambler at
the utilization point being synchronized using data trans-
mitted via an auxiliary channel.
Description of the Prior Art:
Many techniques have been utilized in the prior
art to scramble speech. These include bandwidth inversion,
band splitting, rearrangements of segments of the speech
as well as combining the audio with random signals of
various types. In general, these systems either offered a
relatively low level of security or a low quality signal,
or both of these. Many of the low-level security and
noise problems were related to the fact that relati~ely
long segments of speech were utilized for purposes of
scrambling and the synchronization between the scrambler
and descrambler was relatively poor due to the narrow
bandwidth of the synchronizing channel. These problems
are substantially solved by the disclosed invention by
using a broadband auxiliary channel for synchronizing the
descrambler.
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SUMMARY OF THF INV~NTION
The invention comprises a method and apparatus
for scrambling an audio signal. Scrambling is accomplished
by sampling and digitizing the audio to produce digital
numbers to which a random digital code is added, The
digital numbers are then reconverted to an audio signal to
produce a scrambled analog audio signal. This analog
audio signal is then transmitted over a normal audio
channel of limited bandwidth, for e~ample, 15 KHZ.
Synchronization at the receiving station for
purposes of descrambling is accomplished utilizing a
parallel auxiliary channel of high bandwidth such as a TV
video channel. In an exemplary system, a digital number
specifying the starting point of the random number gener-
ator is transmitted during an unused video line of the
vertical blanking interval.
Descrambling is accomplished by separating the
digitial number specifying the starting point of the
random number generator from the TV signal and utilizing
this number to set the s~arting point of a random number
generator. The audio is sampled to generate a series of
digital numbers which are subtracted from the random
number to descramble the digitized audio signal. The
digital numbers are then converted to analog and filtered
to reproduce the original audio signal.
BRIEF DESCRIPTION OF THE DRAWIN~S
Figure 1 is a generalized block diagram of the
invention;
Figure 2 is a block illustrating the scrambling
technique as applied to a TV signal;
Figure 3 is a diagram illustrating the transmis-
sion of the synchronization code during the vertical
blanking interval of the TV video signal;
Figure 4 is a block diagram illustrating another
embodiment of the invention in which a broad band channel
is utilized to synchronize descrambling of an independent
audio channel.
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DESCRIPTION OF THE INVENTION
Figure 1 is a diagram illustrating a ~irs~
embodiment of the invention. The basic scrambler digitizes
the audio signal and combines the digitized audio signal
with digital pseudo-random nurnbers. Synchronization
signals and a digital pseudo-random signal for scrambling
the audio signal are provided by a scrambler synchroniza-
tion generator 10. Synchronizing data from the scrambler
synchronization generator 10 is coupled to the input of a
multiplexer 12 and combined with a data signal to generate
a composite signal comprising the data signal and the
synchronizing code. In general, the synchronization code
contains relatively high frequency components in order to
ade~uately synchronize descrambling of the audio signal.
These high frequency components require the output signal
of the multiplexer 12 to have a relatively high bandwidth.
The composite output signal of the multiplexer 12, which
includes a data signal combined with the synchronization
code, is coupled to a transmitter 14 for transmission to a
remote location. It is contemplated that the composite
signal will be transmitted to a remote location by modu-
lating a radio frequency carrier. Transmission is accom-
plished by coupling the composite signal to modulate an
exemplary transmitter 14.
The pseudo random digital numbers from the
scrambler synch generator 10 are coupled to the input of
an audio scrambler 16. The audio scrambler 16 digitizes
the audio signal and combines the digitized audio signal
with the pseudo-random numbers from the synchronization
generator 10 to produce a scrambled digital signal. The
scrambled digital audio signal is converted to a scrambled
analog audio signal for transmission. Since the synchro-
nizing code is transmitted over an independent broadband
channel, the scrambled analog audio siynal does not include
any information for synchronizing the receiving station
descrambler with the scrambler.
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A conventional radio frequency analog transmit-
ter 18 receives as an input the scrambled analog audio
signal for transmission to the remote location. In most
applications it is contemplated that transmission will be
accomplished by modulating a radio fre~uency carrier ~ith
the scrambled audio signal. However, other transmission
techniques a~e usable.
At the remote location a radio frequency receiver
20 detects the modulated radio frequency signal, recovers
and couples the scrambled analog audio signal to a de-
scrambler 22. A synchronization signal receiver 24 detects
the output signal of the high bandwidth transmitter 14 and
recovers from this signal the synchronization code which
was originally generated by the scrambler and sync gener-
ator lO. This synchronization code is coupled to thedescrambler 22 and utilized to synchronize a pseudo-random
number generator to generate a pseudo-random signal which
is utilized by the audio descrambler 22 to reproduce the
descrambled audio signal. This system can be assembled
using well known circuit techniques and apparatus. De-
scrambling is the inverse of the scrambling previously
described.
The above-described system is most conveniently
applied in situations where the data signal to the multi-
plexer 12 contains some unused time portions which may be
utilized to transmit the synchronizing code. Typical
examples of such a signal is a standard TV signal in which
the unused lines of the vertical blanking interval can be
conveniently used to transmit the synchronizing code.
Another embodiment of the invention, as applied
to a standard TV signal, is illustrated in Figure 2. In
this embodiment the standard TV signal is generated by a
conventional color TV camera 26, for example. The com-
posite video signal, including the video and all the
normal TV synchronization signals, is coupled to a coding
circuit 28 where the random number generator synchronizing
code is inserted in the vertical blanking interval of the
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video signal. Synchronization signals from the camera 26
are coupled to an analog-to-digital converter 30 to syn-
chronize this circuit such that the audio input signal is
sampled at a rate which is synchronized with the TV sync
signals including the color burst. Pseudo-random diyital
numbers are sequentially generated by a random number
generator 32. These pseudo random numbers are also syn-
chronized with the TV sync signals, including the color
burst. Scrambling of the audio signal is accomplished by
coupling the digital output signals from the analog-to-
digital converter 30 and the pseudo-random numbers from
the pseudo-random number generator 32 to the inputs of a
digital adder 34. The output signals of the adder 34 are
coupled as in input to a digital-to-analog converter 36 to
generate at the output of the analog-to-digitai converter
36 the scrambled audio signal. Communication means 38, a
standard TV transmitting system for example, receives as
an input the scrambled audio and video signals for trans-
mission to a remote location. Coding means may also
include conventional apparatus for scrambling the video
signal, if desired.
At the remote location both the audio and video
signals are coupled to a video audio separation circuit of
40. The vi~eo signal, lf scrambled, is descrambled using
conventional techniques and coupled to a display 42 and to
a synchronization separation circuit 44. The display 42
may simply be a conventional TV type display, for example.
Sync separation circuit 44 separates the horizontal and
vertical synchronizing pulses of the TV signal and the
codes used to synchronize the random number generator 32.
The horizontal synchronizing pulses, the vertical synchro-
nizing pulses and the color burst signal are coupled to an
analog-to-digital converter 46 to synchronize this circuit
to sample the scrambled audio signal from the video audio
separating circuit 40 at precisely the same rate that the
audio signal was sampled at the point of origination. The
digital nu~bers generated by the analog-to-digital con-
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random numb~r generator ~ are coupled as inp~ts to an
adder 50. The adder 50 performs an in~erse operation to
the adder 34 to generate at the output of thL~ circuit a
digital, but descrambled audio signal. The descrambled
audio signal is coupled to the input o~ a dlgital-to-
analog converter 52 to generate a descrambled audio signal
which is filtered by a filter circuit 5~ to reproduce the
original audio.
Figure 3 is a diagram illustrating the prior art
technique used for coding the television signal for secur-
ity purposes and for transmitting the synchronizing codes
for the random number generators and may be applied to
scramble the video signal in the system illustrated in
Figure 2. ~ach of the lines of the television signal
beginning with the trailing edge of the horizontal sync
pulse is divided into 910 sample periods which are synchron-
ized w}th the color burst. At each sample period the
video signal is digitized to generate a digital number
which is stored in a memory. During the following line
the stored values corresponding to sample counts 0 through
143 are read from the memory and reconverted to analog
form to regenerate the synchronizing information of the TV
signal. Then the samples ranging from R to 887 are read
followed by those corresponding to samples 145 to R+12.
This results in portions of the video ~ine being inter-
changed for transmission. As more fully described in the
above-referenced patent the value of R is a random number
which is determined by a random number generator. This
results in the transmitted video signal being scrambled.
At the receiver the process is reversed to recreate the
original video signal.
Since the above-described scrambling system
requires a random number generator at both the point of
origination and the point of use, it is necessary to
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transmit synchronizing codes permitting the random code
generator at the point of use to be synchronized ~o the
similar generator at the point of origination. This
synchronization code can be conveniently transmitted
duriny the last line of the vertical blanking interval as
illustrated in line 3 of Figure 3. Line 4 indicates a
typical synchronizing code which is transmitted during the
17th (last) line of the vertical blanking interval. This
technique can be used to transmit the sync codes for the
random number generator illustrated in Figure 3. As is
obvious from the above description the remainder of the
lines of the vertical blanking interval are available for
other uses.
Figure 4 illustrates another embodiment of the
lS invention in which an independent audio signal is scram-
bled and descrambled using synchronizing signals trans-
mitted over an independent high bandwidth channel such as
a TV video channel. More specifically, a standard conven-
tional TV signal is generated by a video signal generator
56. The video signal generator 56 also generates the
conventional TV synchronizing signals which are coupled to
a random number generator 58. The random number generator
58 is in turn coupled to a video combiner 60 to combine
the synchronizing code for the random number generator
with the TV signal. The TV audio signal in this embodi-
ment is generated by the audio section 62 and in this
embodiment is coupled directly to the TV transmitter 64
and transmitted without scrambling. In this exemplary
embodiment, the video signal including the random number
generator sync code and the unscrambled audio signal are
coupled to conventional TV transmitting apparatus 64 for
transmitting to a remote location.
An independent audio source 66 which is not
associated in any way with the TV signal is generated by
an audio source 66. This audio signal and random numbers
from the random number generator 58 are coupled to the
inputs of an audio scrambler circuit 68 of the type pre-
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viously described. The scrambled audio signal of theaudio scrambler 68 is transmitted to a remote location by
an audio transmitting apparatus 70. At the remote loca-
tion the television signal is received by a sync receiving
circuit 72 and decoded to recover the synchroniziny code
and the TV sync pulses to synchronize the descrambling
apparatus 74 as described in the previous embodiment. The
second input to the descrambler 74 is the scrambled audio
signal. The descrambler 74 descrambles the audio signal
using techniques previously described with reference to
other embodiments. This embodiment permits an audio
scrambler to be synchronized by information transmitted
over the non-related high bandwidth signal.
Depending on the specific techniques used above
it may be necessary to phase-equalize the audio signal in
order to prevent distortions due to sampling. Additional-
ly, it may be necessary to introduce delays in either the
audio transmission path or in the high bandwidth path in
order to e~ualize the transmission delays in the two
transmission paths.
All of the systems described above can be con-
structed utilizing readily available components and known
circuit techniques. Therefore, no detailed schematic
diagrams or list of the apparatus necessary has been
included.
