GB966318A - Improvements in and relating to the conversion of television signals from one standard to another - Google Patents

Abstract

966,318. Television. BRITISH BROADCASTING CORPORATION. April 25, 1962 [April 27, 1961], No. 15300/61. Heading H4F. Apparatus for generating from an input signal an output signal of different line frequency comprises a plurality of storage means 31, equal in number to the number of picture elements into which a scanning line is to be resolved, first switch means 25 operating to scan the storage means in succession at the line frequency of the input signal to apply to each storage means a voltage or current representative of the instantaneous amplitude of the input signal, second switch means 31 operating to scan the storage means in succession at the line frequency of the output signal to apply to an output terminal 35 a voltage or current representative of the voltage or currents in each storage means at the instants of scanning and, between the source of input signal and the first switch means 25, means 20-23 arranged to apply to the switch means signal information representative of at least two adjacent lines and for varying the proportions of the signal derived from the two lines at a frequency equal to the difference between input and output signal line frequencies. The invention stems from that described in Specification 928,730, which makes use of low-pass filters as the storage means, and is based on the realization that the output available from a filter is dependent partly on the input from the latest line scanned and partly on the input from at least the previous line scanned, the proportions of the contributions depending on the instant at which the output is scanned to derive the signal at the changed line rate and hence varying throughout a scanning cycle at the difference between the input and output line rates. The invention substitutes simpler and cheaper storage means, e.g. capacitors 31, for the low-pass filters and provides means 20-23 for applying to the capacitors appropriate proportions of the signal for at least two lines so as to simulate the functions of the low-pass filters. As illustrated, the invention is based on the simulation of filters with sine-squared pulse response characteristics which produce outputs dependent on two (the latest and previous) lines. For this purpose there is provided a one-line duration delay unit 21 so that signals due to the latest and previous lines are made available simultaneously and these signals are then passed through modulators 20 and 22 and combined in an adder 23 for application to switch 23. The modulators are varied in gain, so as to vary the contributions of the two signals to the combined output, by a signal S from a function generator 38 which is controlled by a sawtooth generator 36 triggered by the input and output synchronizing sources 27 and 34 so as to operate at the difference frequency. For sine-squared filters the signal S is based on a cosine wave which changes in phase by 180 at the end of each halfcycle. The wave is applied directly to modulator 20 and via a phase inverter 40 to modulator 23. The precise form of wave S and its phase relationship to the difference frequency scanning cycle is discussed with reference to Figs. 4 and 5 (not shown). Where it is desired to simulate filters producing a triangular pulse response characteristic, function generator 38 produces a linearly varying output. For simulation of a sin #/# response more than one one-line-duration delay unit and associated modulators is provided. The apparatus shown in Fig. 3 is intended for operation where the line scanning rate is reduced. When it is desired to increase the scanning frequency, it is necessary to provide additional switch arms on switch 25. For a factor of increase between 1 and 2, two arms are provided located 180 degrees (i.e. half a line) apart and are supplied with signals at appropriate phases from separate circuit arrangements 20, 22, 23, 40, 36 and 38. For a factor of increase between 2 and 3, three arms 120 degrees apart are provided. Before the signals are applied to the storage means they may be converted to digital form; reference is made in this respect to Proc. I.R.E., Vol. 48, No. 9, 1960.