Patent GB-1193497-A (GB)
Title: Improvements in and relating to the Digital Transmission of Television
Inventor: SALTER MARTIN THOMAS ARDLEY
IPC: H04N5/14
Filed: 1967-08-03
Abstract: 1,193,497. Television. BRITISH BROADCASTING CORP. 1 Aug., 1968 [3 Aug., 1967], No. 35777/67. Heading H4F. The invention relates to a method of reducing abrupt changes in brightness in a television picture caused by the discrete levels produced during quantization of a digital video signal. The number of discrete levels is limited and hence the difference between adjacent levels may not be sufficient to portray subtle changes of light and shade, for example as occurs in the protrayal of clouds and faces. The method according to the invention provides a means for detecting those pulses in the digital video waveform that are going to cause the effect outlined above and reducing the abrupt change to a number of steps. This is illustrated in Fig. 1 wherein (a) represents the digital video waveform divided into four discrete levels and (h) represents the modified signal after processing. Only two of the steps, 23 and 24 have been altered, the remaining steps at A and B being assumed to represent true abrupt changes of picture content. To be operated on in this way, the pulse (C in this case) must fulfil two conditions: first the amplitude change must be not greater than a predetermined magnitude, and second the amplitude change must occur at an interval of time, greater than a predetermined interval, away from the last amplitude change. In the embodiments described, the above mentioned predetermined magnitude is one quantum and the predetermined interval is 1À6 microseconds or T in Fig. 1(e). Fig. 2 illustrates an apparatus for achieving the object of the invention with reference to Fig. 1. The waveform (a) is applied to an arithmetic unit 10 which produces three outputs 01, 02 and 03 shown as (b), (c) and (d) in Fig. 1 and also to a digital to analogue converter (not shown) from whence it passes to a delay unit 22 which brings the waveform (a) into time synchronism with the correction signal (g) at adder 21. Output 01 consists of a pulse occurring whenever there is any change in the numerical value of waveform (a), output 02 is as 01 for changes in (a) not greater than one quantum, output 03 has a zero value when the change in (a) is negative and a positive value when the change in (a) is positive. Output 01 is fed to a divide-by-sixteen counter 12 arranged to count clock pulses, synchronized with the groups of digits and having a recurrence period of 0À1 Ásec., which are applied at 13 whenever a gate G1 is open. Counter 12 counts sixteen clock pulses (i.e. 1À6 Ásec.) from the open gate G1 and then sends positive pulse via lead 14 to close G 1  and stop the counting. However if, before the count of sixteen, a pulse from 01 arrives, the counter is reset and counting recommences, so producing a waveform (e) at the output of the counter. When there is a positive output (e) from counter 12 a gate G 2  is opened. This allows output 02 to pass through this gate to a divide-by-four counter 15 which, together with gate G3, performs a similar operation as units G 1  and 12 except that the counter is reset after a maximum of four clock pulses. Waveform (e) is delayed in unit 16 to ensure that a pulse of (c) can pass gate G 2  before this gate is closed by (e). The outputs of all of the stages of counter 15 are fed to a weighting network 19 which converts them to a four-level " ramp " signal (g) whose peak to peak amplitude equals one quantum. Signal 03 is then combined with signal (g) to determine its polarity in dependance on the waveform (d) in a polarity control 20, the signal (g) then being added at 21 to the signal (a) to providuce the corrected waveform (h). Alternatively the correction signal (g) may be applied before the analogue to digital conversion with certain modifications to the equipment. In a further embodiment (Fig. 4, not shown) the modified signal is made to approximate to an inverted V with the centre of the V at the centre of the pulse C of waveform (a). This is achieved by converting waveform (a) into the form shown in Fig. 3(k) using four quantum steps beginning before C and ending after C. In all previous embodiments, correction signals are generated by examining digital words corresponding to successive picture elements within a television line, hence effecting horizontal correction only. Fig. 5 (not shown) illustrates a means by which both horizontal and vertical correction may be made.