Patent GB-1520311-A (GB)
Title: Time base compensator
IPC: G11B20/00
Filed: 1975-04-14
Abstract: 1520311 Television AMPEX CORP 14 April 1975 [25 April 1974] 15276/75 Heading H4F Relates to a signal time-base compensator arranged to eliminate time-base errors present for example in a colour television signal reproduced by a video disc recorder. In Fig. 1 the uncorrected colour television signal is A to D converted 111 in response to a clocking signal over line 118 which is derived from the time-base component included in the television signal. After encoding in converter 111 the digitized signal is time buffered 156 and decoded at D/A converter 113 by a clock signal at a clock time coherent with a reference time-base signal such as a reference colour sub-carrier. The television-signal-related clock signal is derived from the colour bursts by coupling to a recyclable digital store 123 binary word representations of one or more cycles of the signal's colour burst available at output 112 of converter 111. From these stored colour burst samples a continuous colour burst signal is continuously regenerated and employed to digitize the remainder of the television signal. To ensure that this continuous signal remains in phase with the uncorrected television signal, the converter 111 is first clocked, during the sampling of the television signal's burst and storing of the resulting samples, by a clock signal at a clock time coherent with the reference clock signal. Thus the converter 111 is initially clocked during a sampling and storing mode (preferably lasting five colour burst cycles) over line 118 by a clock signal locked inphase to the reference clock signal. Then the converter is clocked by the derived clock signal during a following recycling mode lasting for the remainder of the horizontal line interval. These two modes of generation are rendered effective by operation of switch 126 which initially (i.e. sampling and storing mode) supplies reference clock signals from reference clock source 128 to the converter. In the second or recycling state (as shown) the clock signal is provided by store 123 via a D/A converter 133, a bandpass filter 132 and a signal clock 131. Switch 126 is operated in response to horizontal sync pulses (134, 136) in the uncorrected colour television signal, and the write enable input (WE) of the store 123 is enabled by detection 137 of colour bursts in the uncorrected colour television signal. The store 123, Fig. 2, includes a random access memory 139 and an address generator 141 which is responsive to the reference clocking signal over line 122 and provides address signals for write and read access to the memory. In addition a write clock generator 143, responsive to commands over line 138 from burst detector 137, issues over line 144 write enable signals to input (W) each time a reference clock is received. A counter 145 prevents the memory 139 receiving further binary words after 15 samples (5 colour burst cycles) of colour burst have been received. Following the termination of the sampling and storing mode, the address generator 141 continues to access memory 139 in response to the reference clocking signal over line 122 repeating in sequence the same 15 word locations accessed during the write operation, and thus the stored binary words are continuously read out over line 140. The read function is operational during the storage of new digital information received from converter 111 by the operation of a by-pass switch 151 i.e. the binary words being stored in memory 139 are also passed directly to the output. To avoid errors in the re-timing of the digital signals by the A/D converter 111 during the recycling mode a time buffer 156 is employed having a 1-word serial to 3-word parallel converter at its input and a complementary 3-word parallel to 1-word serial converter at its output Fig. 4 (not shown) with a clock isolator and a random access memory therebetween. The clock isolator is supplied with two clock signals i.e. one in phase with the colour burst of the uncorrected television signal and the other comprising the reference clock signals so that the phase of the output of the isolator is re-timed and synchronized to the reference colour subcarrier phase. The random access memory is present so that large time-base errors may be corrected i.e. those greater than one cycle of the colour burst (time-base component cycle). This random access memory corrects the time-base by increments equal to integral whole numbers of the colour burst cycle in response to determination of the relative time of occurrence of the horizontal sync pulses in the uncorrected television signal and of reference horizontal sync pulses. A monochrome television signal may be corrected by inserting 174, Fig. 1, an artificial burst or pilot signal into the signal during the blanking intervals, and a modification of Fig. 1 for monochrome signal compensation is described with reference to Fig. 5 (not shown) so that artificial bursts are automatically and only added whenever a colour burst is not detected.