Pulse generating apparatus

Disclosed is a pulse generating apparatus used in a CATV receiving adapter for descrambling the sync signal in the video signal. The pulse generating apparatus comprises a peak detector for sampling the peak in the detected video signal, a half level detector for evaluating the half level of the detected peak voltage, and a pulse generator which compares the detected video signal with the half level voltage and produces a pulse rising at a time point of coincidence or the two voltage signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a pulse generating apparatus, and more 
particularly, to a pulse generating apparatus for generating pulses used 
to descramble a scrambled video signal in a cable television system for 
example. 
2. Description of the Related Art 
Cable television (CATV) is a toll broadcasting service and only subscribers 
can view its programs. The CATV video signal has its sync signal portion 
scrambled so as to preclude non-subscribers from viewing the programs, and 
subscribers' CATV receiver sets are equipped with a device for 
descrambling the scrambled sync signal. 
A conventional descrambling circuit will be described with reference to 
FIG. 7. The block diagram in FIG. 7 shows only the pulse generating 
apparatus which produces pulses for descrambling the scrambled sync 
signal, and the input signal 1 is a AM audio signal in the television 
signal. In this conventional system, the audio signal is amplitude 
modulated by a signal used for descrambling the video signal, and the 
descrambling signal is detected from the amplitude modulated (AM) audio 
signal. The descrambling signal is added to the video signal so that the 
scrambled sync signal is descrambled, and then the video signal is made 
normally visible by an ordinary TV set. In FIG. 7, symbol 1 denotes an AM 
signal input, 2 is an AM detector, 3 is an AM detector output, 4 is an 
integrator, 5 is a reference voltage, 6 is a threshold voltage, 7 is a 
comparator and rectangular pulse generator which compares the AM detector 
output with the threshold voltage, and 8 is a rectangular pulse output 
which is delivered to the video signal processing portion of the CATV 
receiver to descramble the scrambled video signal. 
The AM signal input 1 is detected by the AM detector 2, and the resulting 
AM detector output 3 is fed to one input of the pulse generator 7. The AM 
detector output 3 is separately integrated by the integrator 4, which 
output is added by the reference voltage 5 to form the threshold voltage 
6, and it is fed to another input of the pulse generator 7. The pulse 
generator 7 produces a rectangular pulse when the voltage of the AM 
detector output 3 is higher than the threshold voltage 6. The integrator 4 
provides an average voltage of the AM detector output 3 to shift the 
threshold voltage 6, thereby compensating the fluctuation of the rise 
timing of rectangular pulses caused by a change in the level of the AM 
signal input 1 due to transmission conditions or the like. 
In the foregoing conventional circuit arrangement, the fluctuation of the 
AM signal peak level is followed by the d.c. level of the integrator 
output. Therefore, the displacement of the pulse rise timing of the 
rectangular pulse can be corrected. However, in case the AM modulation 
depth differs among transmitters due to their accuracy or the like, only 
the peak level of the AM detector output varies differently on each 
transmission channel, with the d.c. output voltage level of the integrator 
4 being unchanged, and therefore the displacement of the pulse rise timing 
cannot be corrected. A significant timing displacement causes the video 
signal portion to receive the rectangular pulse output at an erroneous 
timing, and such inaccurate descrambling operation cannot reproduce 
well-synchronized pictures. The matter will further be described in 
connection with FIG. 8. 
FIG. 8(a) is the case of the standard AM modulation depth. For the standard 
AM modulation depth to have a peak-to-bottom voltage ratio (between the 
portions 1A and 1B) of 6 dB for example, the reference voltage is 
determined so that rectangular pulses are produced at a 50% AM detector 
output. In the figure, symbol 1C denotes the threshold voltage, 1D is the 
AM detector output, and 1E is the rectangular pulse. 
FIG. 8(b) is the case of a greater modulation depth than standard, showing 
that the threshold voltage does not follow the varying AM modulation 
depth, and the rectangular pulses are generated at the AM detector output 
below 50%, resulting in a leading pulse rise timing as compared with the 
case of FIG. 8(a). 
FIG. 8(c) is the case of a smaller modulation depth than standard, showing 
the generation of the rectangular pulses at the AM detector output above 
50%, resulting in a lagging pulse rise timing as compared with the case of 
FIG. 8(a). 
The cases of FIGS. 8(b) and 8(c) fail to achieve the accurate descrambling 
and do not reproduce correct, synchronized pictures. 
SUMMARY OF THE INVENTION 
An object of this invention is to overcome the foregoing prior art 
deficiencies and provide a pulse generating apparatus which generates 
pulses always at a constant timing of rising edge, even if the modulated 
signal input has a fluctuating modulation depth, so that the circuit 
receiving the pulses operates stably. 
The inventive pulse generating apparatus comprises a peak detector which 
detects the peak voltage in a periodical input signal, a half level 
detector which detects the half level of the peak voltage, and a pulse 
generator which generates a rising pulse in response to the coincidence of 
the input signal with the half level. The pulse generator produces pulses 
always at a 50% input signal level as a threshold level, and therefore 
always at a constant timing even if the modulated input signal level or 
its modulation depth fluctuates, whereby a circuit receiving the pulses 
can operate stably. The invariable 50% input threshold level also 
minimizes the influence of noises created by low-voltage and high-voltage 
circuit portions, and this is effective for the stable operation of the 
apparatus in constant timing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of this invention will now be described. It is assumed that 
the CATV video signal has its sync signal scrambled, and a signal for 
descrambling the sync signal is amplitude modulated to the audio signal. 
In FIG. 2, a CATV station 10 has facilities for channels from channel-1 
(11) to channel-82 (12). Among the channels, channel-1 is used exclusively 
for converting an ordinary TV radio signal into a CATV signal. In this 
case, the radio signal is received by an antenna 13 and demodulated by a 
demodulator 14. The video signal and audio signal are modulated into a 
CATV signal by a modulator 15, mixed with signals on other channels by a 
mixer 10A, amplified by an amplifier 16, and distributed through a 
distributer 17 to subscribers. The CATV signal is fed through a 
subscriber's set-top terminal unit 18 to a television receiver set 19, on 
which the picture and sound are reproduced. The channel-1 (11) alloted to 
the ordinary TV programs is not tolled, and therefore the sync signal in 
the video signal is not scrambled. The remaining channels from channel-2 
to channel-82 (12', 12", . . . , 12) are original broadcastings by the 
CATV station 10, and they are offered only to the subscribers. In this 
case, a radio signal from a program producing station is received through 
an antenna 21 and satellite broadcasting receiver 22, and signals are also 
supplied from a video tape recorder 12' and a studio 12". In each 
modulator 23 for these signals, the sync signal in the video signal is 
scrambled using a signal produced by a scrambler 24, and the audio signal 
is modulated in AM mode by the modulator 23 using a signal for reproducing 
the scrambled sync signal. The modulated signal is mixed with signals of 
other channels by the mixer 10A, amplified by the amplifier 16, and 
distributed through the distributer 17 to subscribers. The CATV signal is 
fed through the subscriber's set-top terminal unit 18 to the television 
receiver set 19, on which the picture and sound are reproduced. 
Next, the internal arrangement of the set-top terminal unit 18 will be 
described with reference to FIG. 3. The signal from the distributer 17 is 
selected in a tuner 25 to extract only a channel specified through a 
remote control photosensor 26 or keyboard 27. The video signal with the 
scrambled sync signal is reproduced correctly by a descrambler 28 and fed 
to the television receiver set 19, on which the picture and sound are 
reproduced. In this case, implementation of descrambling the sync signal 
is dependent on as to whether the pertinent channel has been set in a 
memory 29 at the subscription contract. When the channel is allowed by the 
contract, the channel allowance information is set to the memory 29 by way 
of an address receiver 30, decoder 31 and CPU 32, and based on the 
information the tuner 25 is operated through a PLL 33 to select the 
station and, at the same time, the CPU 32 issues a command for reproducing 
the video signal by descrambling the sync signal. Reference number 34 in 
FIG. 3 denotes a power supply circuit. 
The following describes scrambling and descrambling of the sync signal. In 
FIG. 6B, a sync signal K protruding downward as shown in (a) is turned 
upward by scrambling as shown in (b). When the signal is descrambled, the 
sync signal K protruding downward is restored as shown in (c), allowing 
correct reproduction of the sync signal, and consequently well 
synchronized pictures are reproduced. Symbol L in the waveforms (c) 
denotes the output of the AM detector 40, and M denotes the output of the 
comparator and pulse generator 41. 
Next, the descrambler 28 will be described with reference to FIG. 1. The 
scrambled television signal tuned by the tuner 25 is split into two parts 
at the entry of the descrambler 28. One part is delivered to an RF 
amplifier 35 which raises the signal amplitude in response to pulses 
supplied from the outside, while another part is fed through a band-pass 
filter 36 for band confinement to an amplifier 37, by which the signal is 
amplified, and the video signal is detected by a video detector 38. A 
band-pass filter 39 extracts only the 4.5 MHz beat from the output signal 
of the video detector 38, and it is subjected to AM detection by an AM 
detector 40. The AM detector output is split into two parts, one being fed 
to a comparator and pulse generator 41, while the other being fed to a 
peak detector 42, by which the peak voltage of the output waveform from 
the AM detector 40 is detected. A half level detector 43 evaluates the 
half level of the peak voltage, and it is supplied to the comparator and 
pulse generator 41. The pulse generator 41 compares the AM detect output 
with the half level detector output, and produces a pulse when the AM 
detector output voltage is higher. The output pulse has its width adjusted 
by a pulse width adjuster 42a to the duration of a compressed sync signal, 
and it is delivered to the RF amplifier 35. The RF amplifier 35 operates 
to raise the signal amplitude only during the period of pulse application, 
and consequently the scrambled sync signal can be descrambled. 
FIG. 5(a) is the case of the standard AM modulation depth. The threshold 
voltage 44 is half the output of the peak detector 42 as determined by the 
half level detector 43, and therefore rectangular pulses are produced at a 
50% level of the output 45 of the AM detector 40. 
FIG. 5(b) is the case of a greater modulation depth than standard. The 
threshold voltage 44a which represents the half level of the peak voltage 
45a rises with the rise of the peak voltage 45a, producing rectangular 
pulses at a 50% level of the AM detector output, and the rise timing of 
rectangular pulses is identical to the case of the standard modulation 
depth shown in FIG. 5(a). 
FIG. 5(c) is the case of a smaller modulation depth than standard. The 
threshold voltage 44b falls with the fall in the peak voltage 45b, and the 
rise timing of rectangular pulses is identical to the case of the standard 
modulation depth shown in FIG. 5(a). Accordingly, the rise timing of 
rectangular pulses is invariable irrespective of the modulation depth. 
FIG. 6A shows in graph the variation in the rise timing according to the 
conventional pulse generating apparatus as indicated by 16 and that of the 
inventive pulse generating apparatus as indicated by 17. The graph show 
that the inventive pulse generating apparatus retains a constant rise 
timing of rectangular pulses against the variation in the AM modulation 
depth. 
FIG. 4 shows the arrangement of the peak hold circuit (peak detector) 42 
and its periphery. The circuit 42 consists of a diode D and a parallel 
connection of a capacitor C and resistor R1 connected in parallel to the 
diode D. The half level detector 43 consists of resistors R2 and R3 
connected in parallel to the parallel connection of the capacitor C and 
resistor R1, with the node of R2 and R3 being connected to the comparator 
and pulse generator 41. The AM detector 40 consists of a limiter amplifier 
40a and a multiplier 40b. Reference number 46 denotes a current mirror 
circuit which produces a reference voltage for a mute circuit 47 which 
cuts off the pulse output when a non-scrambled channel, e.g., channel-1 in 
this embodiment, is tuned. A constant voltage circuit (voltage stabilizer) 
48 is used to stabilize the voltage at the lower end of the resistor R3, 
and the circuit may be replaced with the ground wiring.