Patent GB-1486200-A (GB)
Title: Apparatus for recording television pictures with audio recorders
IPC: H04N7/12
Filed: 1974-09-26
Abstract: 1486200 Television RCA CORPORATION 26 Sept 1974 [3 Oct 1973] 41866/74 Heading H4F A system for transmitting and receiving single frame television images over telephone wires as disclosed in Specification 1,393,674 having the additional features of signal recording means in the form of a magnetic tape recorder 14, Fig. 1, on which the frequency modulated audio frequency signal representing the picture is recorded together with a pilot tone signal so that on playback, variations in tape speed can be detected and compensated for by monitoring frequency variations in the reproduced pilot tone. The system described incorporates an 8 MHz clock in place of the 4 MHz clock of the prior art Specification and generates a 512 column matrix of picture elements in place of the 266 column matrix. The recording is achieved by addition of a second adder 76, Fig. 2, having a first input receiving the combined signal applied to the telephone line and a second input receiving an 8 kHz tone from a "divide by two" circuit 78 coupled from the synchronizing generator 52. In one construction, with an 8 MHz crystal operating as the clock source 50, the actual counting employed in the generator provides a 7À867 kHz pilot tone to the adder 76. Because the crystal clock controls both generation of pilot tone and sampling of the video, the output of the adder 76 comprises both these in fixed frequency relationship. The frequency separation between the 8 kHz pilot tone and 1À4 kHz-2À4 kHz. F.M. signal provides sufficient separation for detection between the two. The playback or receiving circuit Fig. 3 is modified by the addition of a switch 100, frequency modulated audio frequency picture signal and pilot tone separator 101 and a tape speed detector 102. When switch 100 connects A and B the frequency modulated audio frequency picture signal from the telephone line is applied to the converter circuits for processing and display of the video representative signals. When switch 100 connects B and C the recorded frequency modulated signal of the tape apparatus as derived from the separator 101 is applied to the limiter 80 for its processing. The F.M. and pilot tone separator 101 corresponds to a combination of pilot tone extractor 23 and notch rejection filter 22, of Fig. 1b, responding to the output signal of the tape recorder in driving it into its two F.M. and pilot tone components. The tape speed detector 102, Fig. 3, is only used when the recorded picture is being played back. The 8 MHz master clock 88 in this construction incorporates a crystal oscillator used when receiving telephone signals, and a frequency controlled oscillator tunable by varying a direct current voltage on a varactor diode when playing recorded signals, oscillator selection being effected for instance, by different mode-selection potentials applied via switch contacts 100<SP>1</SP>. The signal from the tape recorder is first bandpass filtered at the 7À867 kHz frequency in the separator 101 to extract the pilot tone. This tone is amplified and divided in frequency by 32 in the tape speed detector 102 to provide one input of a digital frequency/phase detector. The 8 MHz output frequency from the frequency controlled oscillator in clock 88 is also divided in the detector 102 to produce the same nominal frequency signal as the pilot tone, and then divided further to be used as a second input to the digital frequency/phase detector. This detector is specifically selected so as to cause the 8 MHz master oscillator to track the pilot tone frequency. If the tape should speed up, the frequency of the divided down pilot tone will exceed the frequency of the divided down clock altering the output square wave duty cycle so asto raise the value of the D.C. component. If the tape speed should slow down so that the frequency of the divided down clock exceeds the frequency of the divided pilot tone, the D.C. component of the proportional duty cycle square wave will be lower. Such D.C. component is extracted by a filter and then applied to the varactor diode in the master clock 88 to adjust the 8 MHz frequency controlled oscillator to lock to the pilot tone. The time constant of the filter may be selected of the order of 1 second to insensitize the 8 MHz oscillator to tape jitter, while the phase comparisons are done at a divided down frequency, e.g., 250 Hz, rather than at the higher pilot tone frequency in order to average out the higher frequency jitter. In this manner, the sampling circuit operations will be seen to lock up to the average tape speed rather than to the instantaneous tape speed. Variations in tape speed which may exist between the recording of the frequency modulated audio signals and the playback of such tape recorded informations are thus offset so that the geographical distortions that would otherwise tend to be produced are kept low.