Patent GB-1445281-A (GB)
Title: Method and apparatus for sampling a quasi-stationary signal
IPC: H04N7/12
Filed: 1974-02-11
Abstract: 1445281 Television WERNS INTER VVS AB 11 Feb 1974 [14 Feb 1973] 06062/74 Heading H4F A video signal bandwidth compression system in which moving images (such as portraits accompanying a telephone conversation) are transmitted with low definition (e.g. only 80 averaged video samples) but static images (such as documents) are transmitted with increasingly higher definition up to a maximum of 81,920 samples (i.e. 32 Î 32 Î 80) spread over several frame periods, the definition increasing so long as the video signal from successive frames are identical, i.e. the sampling period is successively increased when successive video signal frames are identical so that up to 54À60 sec. are used to transmit one static image whereas only one standard frame period, i.e. 0À04 sec. is used to transmit one moving image comprising only 80 video samples. In Fig. 1 the video information from a time frame 4 within the effective picture area 3 of a television video signal 1 is divided into 80 square time slots 5 each occupying 32 lines and thus each containing 32 x 32 elemental video signal samples per field, i.e. 32 Î 32 Î 2 = 2<SP>11</SP> samples per frame. The scanning retrace time is shown as the hatched area 2. The amplitude values sampled from the video signal are binary coded (5 bits) and during the time slots 5 read into a first set of accumulating storage registers (e.g. two groups each comprising 80 accumulating registers, read-in to one group being effected while read-out is effected from the other group). In addition the same signal is fed to a second set of accumulating registers comprising two groups each of 80 accumulating registers which are used to compare the accumulated (average) amplitude values of the samples in each time slot 5 with the corresponding accumulated values in the successive frames to thereby determine and count successive identical information frames. In dependence upon this count of identical frames the number of elemental samples averaged (accumulated) for each output signal sample is decreased, a multifold dot-type sequential sampling being employed (for example in Fig. 2b four frame periods are used to transmit the image, each time slot 5 being split into 4 time slots (dots) each comprising 16 Î 16 Î 2 = 10<SP>9</SP> average elemental samples that are transmitted in the numbered sequence indicated in the Figure). Thus as indicated in Fig. 2 as the number of successive identical frames increases the number of frame periods used to transmit the image increases in even powers of two, i.e. 2<SP>2</SP> frame periods Fig. 2b to 2<SP>10</SP> frame periods Fig. 2f whereas the number of elemental signal samples averaged to derive each output signal sample decreases in odd powers of two from 2<SP>11</SP>, Fig. 2a to 2<SP>1</SP> in Fig. 2f. Fig. 2 also indicates the number of signal samples P transmitted per image and the time taken in seconds. The total number of read-out samples transmitted (e.g. in a videotelephone system) or recorded (e.g. on a magnetic tape) comprises 80 per frame, i.e. 80¸0À04=2000 per sec. and these in the embodiment described are made at a rate of 2100 Hz obtained by dividing the field frequency 50 Hz by 42. Thus the transmitted bandwidth reduced signal occupies a frequency band of 2100Œ1050 Hz (duty cycle of 50%) and carries information on 32 light intensity levels (binary coded during bandwidth reduction process to 5 bits, the resulting output being fed via a D/A converter to the transmission line) in the selected time slots 5-10 indicated in Figs. 2a- 2f.