Patent GB-992192-A (GB)
Title: Color-television test signal generator
IPC: H04N17/02
Filed: 1963-09-02
Abstract: 992,192. Colour television test apparatus. HAZELTINE CORPORATION. Sept. 2, 1963 [Sept. 24, 1962], No. 34543/63. Heading H4F. To test the performance of colour television receivers horizontal synchronizing pulses, colour bursts and a continuous wave signal of frequency differing from the colour subcarrier frequency by a non-integral multiple of the horizontal synchronizing pulse repetition rate are combined and coupled into the receiver to be tested, causing a beat-note signal, from which performance information can be derived, to be developed in each colour information channel. Apparatus, as shown in Fig. 1, comprises means 10 which supply all the information of a complete colour television signal of the NTSC type except chrominance information, that is, luminance information corresponding to a constant grey field, horizontal synchronizing pulses and colour bursts for synchronizing the colour oscillator of the receiver 18 under test, means 12 which supply a continuous wave signal of frequency equal to the colour burst frequency plus one-half of the horizontal synchronizing pulse repetition rate and means 14 which combine the signals from means 10 and 12 to form a composite signal having the continuous wave signal incorporated as simulated chrominance information during periods between predetermined horizontal synchronizing pulses. In operation as the input signal to the demodulators of the colour television receiver 18 differs from the reference subcarrier frequency (colour subcarrier), beatfrequency outputs are produced whose amplitudes are proportional to the relative demodulator gains and the relative demodulation phase angles may be obtained directly from the relative phase angles of the beat-note signals. This signal with a constant frequency difference is called a " slip signal " and may be thought of as a chrominance signal with its phase linearly advancing 180 degrees during each horizontal line interval so that one-half cycle of beat-note signal is produced during each horizontal line interval. The slip signal after being detected in the receiver's video detector is coupled to the chrominance demodulators, and each of the chrominance difference outputs R-Y, B-Y and b-Y is a sine wave interrupted by blanking and a third line keyout interval which is described later. Fig. 2 shows a colour difference signal output of a colour television receiver on oscilloscope 16 synchronized with a horizontal drive signal and adjusted for a sweep frequency of one-twelfth of the horizontal line frequency. The signal inputs required for operation of the slip generator shown in Fig. 1 are horizontal drive, composite synchronizing signals, composite blanking, burst gate and colour subcarrier. The basic signal source is a 3.587412 mc. crystal 30 which is phase locked with the 3À579545 mc. colour subcarrier signal to produce the constant slip frequency. Horizontal drive and colour subcarrier signals are coupled to wide burst modulator 33 which supplies the desired synchronization, via amplifier 35, to phase detector 36, reactance tube 37 and oscillator 38 which stably sidelocks at the desired sideband of the colour subcarrier. The output of oscillator 38 is fed to a blanking modulator 39 which inserts the composite blanking signal and also blanks out every third line so as to establish a reference base line. Blanking of every third line is accomplished by third line key or multivibrator 41 which receives horizontal drive signals from amplifier 42. Amplifier 50 supplies composite blanking signals including components simulating a constant grey luminance signal. The output of amplifier 50 is combined with the output of blanking modulator 39 and added to the composite synchronizing signals and regular colour burst output from amplifier 55. The colour burst is obtained by modulating the reference subcarrier with a burst gate pulse in colour burst modulator 53. The composite slip signal out of amplifier 40 modulates the output of signal generator 58 to produce a signal suitable for application to the antenna input terminals of the colour television receiver 18 under test. Phase markers are generated in means 20 which successively multiply the horizontal line frequency by six and six to derive 36 marker pulses per horizontal line period i.e. the phase markers are spaced five degrees apart. Output amplifier 63 is controlled by phase marker keyer 64. which causes the phase markers to be present only during the blanked third line reference interval. The peaks of the phase markers have an approximately sawtooth envelope which shows 30 degree intervals. To measure the relative phase of the beat-note output of the three colour difference channels of the receiver, which is a measure of decoding accuracy, the oscilloscope sweep frequency is adjusted to correspond to any odd submultiple of horizontal line frequency, e.g. 1/5, thus producing an oscilloscope display as shown in Fig. 5, with a "crossover " and the reference base line together with the phase markers super imposed (Fig. 5 shows only one horizontal line interval at the B-Y output). The relative phases of the " crossover " of the R-Y and G-Y outputs (Figs. 6 and 7, not shown) can then be measured with respect to the "crossover" at the B-Y output to obtain a measure of the decoding accuracy. To resolve the ambiguity which exists because identical displays may be shown when demodulation axes are 180 degrees apart the B-Y, G-Y and R-Y chrominance outputs are observed successively with the oscilloscope sweep of one-sixth horizontal line frequency (Fig. 8, not shown). The peak-to-peak beat-note outputs at the B-Y, G-Y and R-Y outputs can be measured to obtain a measurement of the receiver colour difference signal amplitude. The amount of envelope detection can be obtained by measuring the displacement of the "crossover" above or below the reference base line. Luminance suppression can be observed and measured for the two lines when the slip signal is keyed in, the luminance signal amplitude being less than the amplitude during the third line when the slip signal is keyed out. Hue control range measurements can also be made by varying the receiver hue control between its extreme positions, each time measuring the phase shift of the crossover which is initially adjusted to coincide with a 30-degree marker in the centre of the horizontal line.