Patent GB-1052438-A (GB)
IPC: H04N7/01
Abstract: 1,052,438. Television standards converters. BRITISH BROADCASTING CORPORATION. Nov. 18, 1964 [Nov. 18, 1963; Dec. 6, 1963], Nos. 45506/63 and 48370/63. Heading H4F. Television signals at 50 fields per second are applied to a delay circuit 23, Fig. 4a, having a total delay equal to the duration of one field and an output is derived from a tapping point 24 which is traversed from the position of maximum delay to the position of zero delay during the time of five fields and is then returned rapidly to the position of maximum delay. The output fields 1<SP>1</SP>, 2<SP>1</SP>, 31 etc., Fig. 4b, are thus reduced in duration corresponding to a rate of 60 per second and one additional field 511 is inserted in each five by repetition of the fifth field 5 (which is within the delay circuit) when the tapping point is moved back rapidly to the position of maximum delay. Conversion from 60 to 50 fields per second is effected by an arrangement in which the tapping is traversed from the zero to the maximum delay positions, so as to increase the duration of the output fields, and is returned rapidly to the zero position so as to eliminate one field (i.e. the field within the line) in six. Where it is desired to change the line rate as well, Fig. 12 (not shown), this may be done by preceding the delay circuit by a line standards converter as described in Specifications 928,730 and 966,318. Figs. 4a and 4b illustrate only the principle of the invention, the practical realization involves yarying the delay in discrete steps. Thus the delay circuitmay have fixed tapping points spaced apart by intervals of one input line. If the delay is reduced by one tap after every five lines, the sixth line is eliminated, whilst if the delay is increased, the fifth line is repeated. Such an arrangement may thus be utilized to change the television field duration (and hence frequency) in the ratio five to six either way with a corresponding change in the number of lines in each field. Extensions of this arrangement are described, Figs. 5-9 (not shown), for converting both ways from 525 line, 60 fields per second signals to 625 line, 50 fields per second signals. The need for a delay circuit with a large number of tapping points is avoided by the use of a binary series of delay circuits (i.e. circuits having delays of t, 2t, 4t, 8t &c.) and a switching system whereby the circuits may be assembled in all possible combinations. Delays greater than the nominal value of the source of the individual delays may be introduced by passing a signal through some delay elements more than once. The systems do not include line standard converters, since the line duration in the two standards is approximately the same, but effects minor changes in the line duration by means of an auxiliary one line duration delay circuit (which may also be in binary form) which is controlled according to a predetermined programme or automatically by a system in which line synchronizing signals accompanying the television signals are compared with standard synchronizing signals. Switching of the binary circuits in the main delay system is effected either by a separate control system or by keying signals included in the television signals. The arrangement described above with reference to Fig. 4a may be modified for converting line frequency. The circuit is given a delay corresponding to the duration of an input line whereby each rapid transition between the extreme delay value inserts or eliminates one line. Arrangements are described, Figs. 10 and 11 (not shown), for converting between signals of the same field frequency but having 600 and 500 lines per picture respectively. The delay circuits which are tapped and may be in binary form are arranged so that the delays between adjacent taps correspond to a fraction of picture element time. Arrangements are described with reference to Figs. 1-3 (not shown), for field frequency conversion which use tapped delay circuits having only a few taps (5 or 6) and hence an appreciable delay between taps. With these arrangements 50 lines are omitted from each output field, the timing of operations being such that 25 are omitted at the beginning and 25 at the end. Line standards conversion is introduced by a converter as described in Specifications 928,730 and 966,318 the converter being operated so as to compress or expand the lines to preserve picture aspect ratio. It is shown that in a receiver arranged to overscan the picture mask little picture information is lost. Ultra-sonic delay lines and arrangements based on magnetic recording and reproduction are referred to for use in the delay circuits. The television signals may be translated through the apparatus using frequency or pulse modulation. Undesirable defects which are introduced into the converted signals as a result of the elimination of lines and frames may be removed by interpolation techniques based on the arrangements described in Specifications 928,730 and 966,318. Although the detailed disclosures are of arrangements which insert or eliminate only a single line or field during the period of the beat frequency between the input and output line and field frequencies, the invention may also operate at small integral multiples, i.e. two, three and four times, of these periods.