GB1243974A - Improvements in split-band compansion systems for sound recording or transmission - Google Patents

Abstract

1,243,974. Companders. EMI Ltd. 12 Nov., 1968 [12 Aug., 1967], No. 37138/67. Heading H4R. In a compander system the signal band to be compressed or expanded is split into two partial bands each of which is compressed or expanded independently of the other, and the slopes of the cross-over region of the separating filters employed to derive the partial bands in the compressor and in the expander are related by a factor p which is the reciprocal of the power (1/p) of the amplitude relationship between the input and output of the compressor and equal to the power (p) of the amplitude relationship between the input and output of the expander, i.e. for the compressor: Voltage out = (voltage in)<SP>1/#</SP> and for the expander: Voltage out = (voltage in)<SP>#</SP> In addition the partial ranges of the signals after compression or expansion have to be combined with a 90 degrees phase shift between the partial ranges in order to maintain a level frequency response. This may be achieved using a Norton filter with an odd order Butterworth response to separate the partial bands from the signal band. With an expansion law where # = 2, providing simple compressors and expanders; although a filter as referred to above may be used in the expander the compresser filter will be a Norton filter with an even order Butterworth response which will require an additional phase shifting network to provide the required phase relation in the partial bands. If p = 3, however, both filters will be of odd order eliminating the need for additional phase shift networks. Circuits for providing the square law or cube law compressor and expander using multipliers or dividers are described with respect to Figs. 3 to 6 (not shown). In addition backward or forward acting multiplier circuits using logarithmic amplifiers employing semiconductors, e.g. zener diodes, to generate the logarithmic characteristic are described with reference to Figs. 7 and 8 (not shown) to provide compansion factors of any desired value.