Transient suppression and grey level coincidence in a subscription ...

TRANSIENT SUPPRESSION AND GREY LEVEL COINCIDENCE IN A SUBSCRIPTION TELEVISION SYSTEM

BACKGROUND OF THE INVENTION 5

In subscription television systems such as, for example, the system disclosed in U.S. Pat. No. 3,824,332 (Horowitz), the picture information is inverted in a random manner on a field basis while maintaining the 10 synchronizing signals continuously at the same polarity. The present invention is, of course, not limited to such systems, but may be applied wherever a reversal of polarity of the television signal occurs. It will, however, be described as employed in the system disclosed in the 15 above mentioned patent.

In such a subscription television system, the picture information is, of course, reconstituted at the decoders by reversing the inversion which has taken place at the encorder. In systems of this type, the following difficul- 20 ties have been encountered.

1. The electronic reversing switch producing the inversion introduces spikes or transients in the signal. Such transients provide decoding information to would be pirates. 25

2. The reconstitution process in the decoder as mentioned above requires a switching operation to be performed between two video signals of opposite phase. The introduction of transients in the switching operation can adversely affect the operation of the home 30 television receivers.

3. Mismatches in the DC components of the two signals, namely the reversed and the synchronizing signals, result in annoying flicker on the home television screen. 35

The above mentioned difficulties will be described in greater detail with reference to FIG. 1.

Consider one horizontal line of a conventional video signal as shown in FIG. 1(a) and the same signal with the picture information inverted as in FIG. 1(c). For 40 convenience, these are shown as monochrome signals. The inverting pulse which produces the inversion required for scrambling is shown in FIG. 1(b) and the signal with inverted picture information in FIG. 1(c). If the pulse in FIG. 1(b) does not start and stop exactly 45 coincident with blanking, the resulting signals are shown at FIG. 1(d) and FIG. 1(e). In one case, part of blanking and in the other, part of information is present in the wrong place. Information is therefore available to bypass the entire coding system. Also, because of the 50 abrupt transition required by the inversion in FIG. 1(c), it is difficult to avoid an overshoot in the switching circuitry.

To reconstitute the signal in its proper format, a switching operation is again carried out between the 55 sync portion and the video portion of FIG. 1(c). If the instant of switching is exactly coincident with that of the original coded signal (and at exactly the same rate of change), the signal in FIG. 1(a) will be recovered. However, if the switching occurs either too early (A) or 60 too late (B), huge spikes will result, as shown in FIGS. 1(g) and 1(h). It is, of course, impossible to achieve perfect timing and consequently spikes in the decoded signal are inherent in the system.

The reconstituted signal at the decoder, that is the 65 signal whose picture signal is again nonin verted relative to the synchronizing signal, should have exactly the same DC level as the original, otherwise an annoying

flicker results. Without the system of the present invention, it is very difficult to maintain this DC level over extended periods of time.

SUMMARY OF THE INVENTION

It is an object of the present invention to suppress the transients generated during the switching operations described above and to maintain a fixed relationship between the DC signal levels at the transmitting and the receiving ends.

The present invention resides in a subscription television apparatus and method. The subscription television apparatus comprises means for inverting the television signal to be encoded at first selected time instants and reinverting said television signal at second time instants each following a corresponding one of said first time instants. It further comprises clamping means for clamping the television signal to a predetermined reference level for determined time intervals each including one of said first selected or second time instants. The determined time intervals are sufficiently broad with respect to time to encompass the possible variations in the timing of the first and second time instants.

In a preferred embodiment of the present invention the first and second time instants respectively correspond to the beginning and the end of the picture information interval: that is, the picture information is inverted relative to the synchronizing signals. The combined blanking and grey level interval is slightly wider than the conventional blanking interval so that all the active picture information is definitely inverted if such inversion is to take place at all.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a wave form diagram illustrating the problems encountered with prior art subscription television systems;

FIG. 2 is a wave diagram showing television signals including grey level clamped portions in accordance with the present invention;

FIG. 3 is a block diagram of encoding circuitry including the grey level inserter of the present invention;

FIG. 4 is a decoder block diagram including the clamping circuit of the present invention;

FIG. 5 is a circuit diagram showing the encoder clamping and switching circuits of the present invention;

FIG. 6 is a decoder circuit diagram showing the clamping and switching circuit of the present invention; and

FIG. 7 is a circuit diagram illustrating the clamp pulse and invert pulse generators.

DESCRIPTION OF THE PREFERRED
EMBODIMENTS

A preferred embodiment of the present invention will now be described with reference to the drawing.

The problems resulting from the switching of the video signal portion relative to the synchronizing signal portion of a television signal was discussed above with reference to FIG. 1. Referring now to FIG. 2, it is noted that the time instants at which switching takes place are denoted by A and B, respectively. A is thus an illustration of what is herein referred to as a second time instant while B is herein referred to as a "first time in- 5 stant." The first time instants are those following which the television signal is to be inverted, while the second time instants are those following which the television signal is to be reinverted, that is returned to its original polarity. Reference to FIG. 2(a) shows that a one-and- 10 a-half microsecond clamp signal is introduced, the start of which precedes the switching instants by a small amount. Since the desired switching instant, in a preferred embodiment of the present invention, is the beginning of the blanking interval it will be noted that 15 the grey level clamp signal extends into the active signal portion by a small amount. Since the grey level clamping at the decoder, as will be explained in greater detail below, takes place immediately prior to the switching operation, the grey levels prior and after 20 switching are sufficiently the same that residual spikes are now the result of limitations in circuitry alone. Spike amplitudes of ±1 IRE unit or less can readily be attained. It should further be noted that the grey level reference immediately prior to the picture signal inter- 25 val (followng instant B in FIG. 2) is taken from the same channel as that picture information, that is from the inverted channel if the picture information is to be inverted or from the noninverted channel if no inversion is to take place. The grey level information is thus 30 extracted from the same channel as the active video information immediately following it thereby serving as a reference level for clamping. Similarly, following the switching at point A in FIG. 2, the grey level reference is extracted from the noninverted channel, that is the 35 channel from which the synchronizing signal information is always derived. Again, the grey level pulse can thus serve as a reference level for clamping.

FIG. 3 shows the block diagram of a grey level inserter (clamping circuit) of the present invention as 40 interconnected with the inverting amplifier of, for example, the Horowitz system of U.S. Pat. No. 3,824,332. The standard television signal is applied to the input of the grey level inserter 30 whose output is connected to the input of an inverting amplifier 31 and also directly 45 to a switch 32. The output of inverting amplifier 31 is also connected to an input of switch 32. The output of switch 32 is connected to output circuitry 33. Switch 32 operates under the control of invert pulses. Specifically, the standard television signal is clamped to a grey 50 level as illustrated in FIG. 2 and by circuitry which will be described with reference to FIG. 5. The output of the grey level inserter, which is the standard television signal but having the proper portions clamped at the grey level, is applied directly and in an inverted state to 55 the input of switch 32 which, under control of the invert pulses, connects either the output of inverting amplifier 31 or the output of grey level inserter 30 directly to the output circuitry 33.

Similarly, FIG. 4 shows the block diagram of the 60 decoder circuitry. The encoded television signal is received. The signal, of course, has the grey level insertions generated at the encoder. The signal is applied to a grey level clamp 40 and, through an inverting amplifier 41, to a second grey level clamp 42. The outputs of 65 grey level clamps 40 and 42 are applied to a low offset switch 43 which is substantially identical to switch 32 at the encoder. In the operation of the circuitry, the input

to the low offset switch 43, which is either the inverted or the noninverted television signal including portions clamped to the desired grey level, is selectively switched to the output of switch 43 under the action of invert pulses. The latter cause an operation which is inverse to that of invert pulses at the encoder, thereby furnishing the standard television signal again at the output of switch 43. In should be noted that this signal is inherently correct and, because of the grey level clamps 40 and 42, requires no adjustment or set-up.

It should be noted specifically that if the inverting amplifier has a gain of unity ±2 percent and the combined offset of the clamp plus the switch is ±10mv, the flicker of the resultant picture is below the threshold of visibility. Both specifications are well within the capability of present technology without the use of adjusting potentiometers.

The circuit diagram corresponding to the block diagram of FIG. 3 is shown in FIG. 5.

The composite-television signal is received at terminal 50 and applied to the base of a transistor 51 having an emitter resistor 52. The emitter of transistor 51 is further coupled by a resistor 53 to the collector of a transistor 54 at whose base the grey level pulses are applied. The emitter of transistor 54 is connected to the wiper arm of a potentiometer 55 and is connected to ground potential through a capacitor 56. The terminal of resistor 53 coupled to the collector of transistor 54 is also coupled to the direct input of the noninverting amplifier 57 and by a resistor 58 to the inverting input of inverting amplifier 59. A fixed but adjustable potential is applied to the direct input of inverting amplifier 59 by means of a potentiometer 60. Inverting amplifier 59 further has a feedback resistor 61 coupled from the output of the inverting amplifier to its inverting input. The output of noninverting amplifier 57 is applied to the base of a transistor 62 having an emitter resistor 63. Connected in parallel with the emitter-collector circuit of transistor 62 is the emitter-collector circuit of a transistor 64. Transistors 62 and 64 are herein referred to, respectively, as first and second amplifier elements. The emitters of transistors 62 and 64 are connected to the base of a transistor 65 whose emitter is connected to the switching output terminal 66. A resistor 67 is connected from the emitter of transistor 65 to the positive supply source. Connected in parallel with the emitter-collector circuit of transistor 65 is the emitter-collector circuit of a transistor 68. Transistor 65 and 68 are herein referred to as the third and fourth amplifier elements, respectively. The output of inverting amplifier 59 is applied to the base of a transistor 69. Connected between the emitter of transistor 69 and the negative supply source is a resistor 70. Further, a transistor 71 has its emitter-collector circuit connected in parallel with the emitter-collector circuit of transistor 69. The emitters of transistors 69 and 71 are connected in common to the base of transistor 68. The base of transistor 71 is connected to the voltage divider tap of a voltage divider connected between the positive and negative supply source and comprising resistors 72 and 73. Further connected to the base of transistor 71 is the emitter-collector circuit of a transistor 74. The emitter of transistor 74 is connected through resistor 75 to the negative supply line. The invert pulses are applied to the base of transistor 74 through a capacitor 76. The base of transistor 74 is connected through a resistor 77 to the negative supply and through a resistor 78 to ground potential. Connected to the emitter of transis