Note: Descriptions are shown in the official language in which they were submitted.
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1 This invention relates to television decoding devices
and particularly to a simply constructed reliably operable
cable television converter including means for decoding a sine
wave scrambled television signal.
Another purpose is a cable television converter of the
type described in which the decoding means includes an AGC de-
tector in a closed loop with amplifying means in the converter.
Another purpose is a decoding means of the type
described utili~ing the residual scrambling signal, after
1~ decoding, as a means for generating a decoding signal.
~ nother purpose is a sine wave scrambling system in
which the scrambling sine wave applied to the television signal
is suppressed during at least a portion of the vertical interval
of the television signal.
Another purpose is a sine wave scrambling system in
which the scrambling sine wave is applied at a greater level to
! the audio carrier than to the video carrier.
To this end, in one of its aspects, the invention
provides a TV converter having multichannel input capabilities
and a single channel output, oscillator means for converting
input TV signals to signals usable in a TV receiver, means
for decoding a generally constant frequency sine wave scrambled
~V signal in which the audio carrier has a greater scrambling
level than the scrambling level on the video carrier and in
which the scrambling sine wave is suppressed during at least
a part of the TV signal vertical interval, and including RF
amplifier means having one input of the scrambled TV signal,
a second input of a generally constant fre~uency unscrambling
sine wave, and an output of the unscrambled TV signal varied by
a residual of said sine wave, a detector connected to the output
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1 of said amplifier means for detecting the residual sine wave,
audio carrier filter means connected between said RF amplifier
and detector, an amplifier connected to said detector for raising
the level of the detected residual sine wave, with the output of
said amplifier being connected to said RF amplifier means for
providing the unscrambling sine wave, and an AFC circuit con-
nected between said RF amplifier output and said oscillator
means.
The invention is illustrated diagrammatically in the
following drawings wherein:
Figure 1 is a block diagram of a cable television
converter, including means for unscrambling a coded signal,
Figure 2 is a block diagram of a modified decoding
means,
Figure 3 is a diagrammatic illustration of certain wave
forms utilized in the coding and decoding system shown and
described,
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1 Figure 4 is a wave form diagram showing suppression of
the scrambling wave during a portion of the vertical interval,
and
Figure 5 is a diagrammatic illustration of the means
~or applying the scrambling signal at the head end of a cable
system.
The present invention relates to a television decoder
or unscrambler which is usable in subscription or pay television.
In particular, the televised signal is scrambled at the head end
or transmitting station by the application of a sine wave as
additional modulation to the composite television signal. For
example, a scrambling sine wave having a frequency of approximately
15.75 khz is applied directly to the television signal. The
sine wave is phase locked to the horizontal sync as shown in
Figures 3A and 3B. The sine wave modulation has the effect
of suppressing the horizontal sync and enhancing the video
between horizontal sync pulses. The scrambling sine wave may
vary the level of the television signal by approximately 6 DB.
Loughlin 3,081,376 shows a means for applying such a scrambling
sine wave to a composite television signal. Court 3,729,576
also discloses a sine wave scrambling subscription TV system.
Looking specifically at Figure 5, a video input signal
is indicated at 100 and an audio input signal is indicated at
102. Both the video and audio inputs pass to an IF modulator 104
which has a video IF output signal 106 and an audio IF output
signal 108. A scrambling sine wave generator is indicated
at 110 and has one gain control output signal 112 controlling a
video IF signal amplifier 114. A second gain control output
J ~ signal 116 passes to an audio IF signal amplifier~ 118. The
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gain control signal 116 will be at a higher level than that of
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1 the gain control signal 112, with the result that the audio IF
signal will have a higher level of scrambling modulation than the
video IF signal. The audio and video IF signals are combined in
a mixer 120 which will pass the audio and video IF signals to a
channel frequency up converter 122, the output of which is
the composite audio and video carrier scrambled as described
for transmission over the cable system.
In Figure 1, a cable TV converter of the ~eneral type
shown in Mandell Patent 3,333,198 includes cable input
texminal 10 connected to a first mixer 12. A variable oscillator
14 is connected to the mixer 12 with the output from the mixer
passing through a filter 16, an amplifier 18, to a second filter
20. The filter 20 is connected to a second mixer 22 having an
oscillator 24, with the output of the second mixer 22 being
connected to a filter 26. The components thus far described
are conventional and generally shown in the Mandell patent. The
input terminal 10 receives a wide spectrum of cable channels, as
many as 13 to 26 are conventionally used in CATV systems. The var-
iable oscillator 14 is used to control the particular channel that
is selected, with oscillator 24 converting the selected channel to
a predetermined VHF channel, normally not locally used as a broad-
cast channel, but one which can be received on a co~entional TV re-
~-~ cqiver. Thus, all channels on the cable can be shown on a particu-
lar TV receiver,on a normally unused channel, for example channel 3.
The output from filter 26 is fed to an FR amplifier
28. Between the amplifier 28 and the filter 26 is a trap 30
which normally is used to remove the picture signal of the
next higher channel, for example channel 4. An attenuator 36
is connected to amplifier 28 with the output from the attenuator
being connected directly to the television receiver. If the
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1 particular channel chosen by operation of oscillator 14 is a
non-subscription ch~nnel, thus one with no scrambling super-
imposed, the television signal will be readily viewable at the
TV receiver on channel 3.
The output from amplifier 28 also passes through
a channel 3 filter 32, with the output from filter 32 passing
to a second RF amplifier 34. Automatic frequency control is
provided by an AFC discriminator 38 which is connected to the
output of amplifier 34 and connected by line 40 back to the
variable oscillator 14.
The decoding portion of the circuit includes a channel
3 audio carrier filter 42 connected to the output of amplifier
34, with the output of filter 42 being connected to an RF
amplifier 44. A second channel 3 audio carrier filter 46 is
connected to amplifier 44 with the output from filter 46 being
connected to an AGC detector 48. A 15.74 khz filter 50 is con- -
nected between the AGC detector 48 and an AGC amplifier 52.
~ In operation, the composite TV signal varied in ampli-
:~ tude by the applied sine wave, assuming oscillator 14 is set
to the subscription channel, will be present at the input of
amplifier 28. Filters 42 and 46 provide that only the audio
carrier frequency, and related sidebands, are present at the input
of AGC detector 48. Since the scrambling signal is present as
amplitude modulation on the audio carrier, the scrambling signal
will be present at the output of detector 48. Also present is
; a DC level which is dependent on the signal input level. The
output of detector 48 is applied, via filter 50, and amplifier
52, as a gain control signal to amplifier 28. This gain control
signal is in anti-phase to any changes of audio carrier output
level, from amplifier 28, therefore, any changes in audio carrier
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1 output level, from amplifier 28, will be reduced by a factor
equal to the loop gain between the output of amplifier 28 and the
gain control input of amplifie~ 28. Thus, the audio carrier
output from amplifier 28 contains only a small residual scrambling
signal. The video output from amplifier 28 will be simultaneously
unscrambled by the gain control signal input. Because the
video carrier has a lower level of scrambling signal applied,
it will be practically free of scrambling, whereas, there will
be a residual of scrambling remaining in the audio output.
In Figure 2, the same decoding principle is applied
without a conventional CATV converter. In this case, ganged
switches 54 and 56 either connect the cable input directly to the
receiver, by line 57, or to the decoding apparatus as
described. The decoding apparatus may include a bandpass filter
58 which will pass the particular channel chosen as the subscrip-
tion channel. The output of filter 58 is connected to an amplifier
60 and then to a second filter 62. -A mixer 64 has one input from
filter 62 and a second input from an oscillator 66. For example,
if the subscription channel is a letter channel, for example
channel C, oscillator Ç6 will be set such that the output from
the mixer is a normally unused channel in the TV set, for example
channel 3, as described above. In this connection, quite
obviously the invention should not be limited to channel 3,
but this is a conventionally uséd CATV output channel. The
output from mixer 64 goes to a filter 68. Filter 68 is connected
to RF amplifier 28, with the remaining portions of the decoding
circuit being identical with those described in connection
with Figure 1.
In operation, the circuit of Figure 2, as far as
decoding the sine wave scrambled TV si~nal, functions identically
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7 as the circuit of Figure 1, The principal di~-ference is that
there is no conversion of all cable channels to a particular
selected normally unused channel in the receiver. Thus, only
the subscription channel is converted to that channel, with the
remaining channels on the cable being passed directly to the
receiver.
In Figure 3, the scrambling signal sine wave is
indicated at the top of the figure, with the television signal
horizontal sync pulses shown directly beneath it. The sine
wave scrambling signal has the same frequency as the TV signal
horizontal sync and has the particular phase relationship shown
in Figure 3. The described scrambling results in suppression
of the horizontal sync pulse and enhancement of the video signal
which tends to result in a loss of the correct horizontal sync
in the television set.
As indicated, the scrambling level of the audio carrier
is higher than that of the video carrier. The unscrambling
" feedback loop will have the effect of attempting to level the
audio signal which will effectively level the entire channel.
~0 However, as describedr there will always be some residual
~ scrambling remaining in the audio output signal and this residual
; forms the basis for the unscrambling signal. Since the audio
carrier is at a higher level than the video scrambling, the end
result is to effectively remove the residual scrambling from the
video output, leaving a small residual scrambling in the audio
output.
Although as much as five percent residual scrambling
can be tolerated in the video output, the present system, with
a higher level of scrambling on the audio carrier will effect-
ively provide a clean video output signal. The small amount of
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1 residual scrambling on the audio output is clearly tolerablein a normal television set.
Although the video carrier will be essentially clean
at the receiver input, it is important to insure against
interference with the operation of the receiver vertical sync
and AGC circuits. During a portion of the vertical interval,
sync pulses occur at twice the normal horizontal sync frequency.
This takes place during the equalizing pulse intervals and the
serrated (vertical sync) puIse interval. See Figure 3C. The
effect of any residual scrambling on the vertical interval is
to produce a higher peak level than occurs at any other time
during a TV frame. Since both the sync and AGC circuits in a
TV set are operated by peak signal levels, this change in peak
level oco~i ~ during a small portion of a frame may confuse
the sync and AGC circuits. The residual scrambling effect
may be eliminated at the head end scrambler by suppressing the
scrambling signal during at least the nine line interval shown
in Figure 3C.
Figure 4 shows the envelope of the scrambling signal
suitable suppressed during a portion of the vertical interval.
Note that the suppression is gradually applied and gradually
removed and is only utilized for a portion of the vertical inter-
val, although in some applications it might be for the entire
period. Preferably, it is only for that portion of the vertical
interval when the equalizing pulses and vertical sync pulses
are transmitted.
Whereas the preferred form of the invention has been
shown and described herein, it should be understood that there
may be many modifications, substitutions and alterations thereto.
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