Patent CA-1219810-A (CA)
Title: Audio compressors and expanders
Inventor: DOLBY RAY M
IPC: H03G7/00
Filed: 1986-02-13
Granted: 1987-03-31
Abstract: Abstract According to a first aspect of the invention, compressor and expander circuits are arranged in series, the circuits each having bi-linear character-istics in which there is a low level portion of substan-tially constant gain up to a threshold, an intermediate level portion, above the threshold, of changing gain providing a maximum compression ratio or expansion ratio, and a high level portion of substantially constant gain different from the gain of the low level portion.  The intermediate level changing gain portions of the characteristics are staggered among the series circuits such that the resulting overall characteristic has a maximum compression or expansion ratio substan-tially no greater than that of any single circuit while providing an increased difference between the gains at low and high input levels (that is, providing increased compression or expansion) than for any of the circuits individually.  In accordance with a second aspect of the invention, signals in the high and/or low frequency extremes of a signal transmission system, in which those extremes are subject to errors in the transmission channel, are attenuated so as not to control the action of a compressor feeding the transmission channel and so as to be attenuated in the compressor output to the channel.  Consequently, a complementary expander fed by the channel is not subject to control by signals at frequencies subject to transmission channel errors. A filter network or networks are used having abrupt roll-offs and attenuation characteristics selected to attenuate signals in the frequency extreme or extremes and well within the usable frequency range of signals normally carried in the system.  To restore an overall flat frequency response a complementary network (or networks) may be employed with the expander.  According to a third aspect of the invention, a frequency depen-dent circuit is located in the main signal path of a dual path compressor or expander in which the main signal path is linear with respect to dynamic range and a further path provides a signal which boosts or bucks the main path by way of a combining circuit, but which is so limited that in the upper part of the input dynamic range the further path signal is smaller than the main path signal.  The frequency dependent circuit provides, in the case of a compressor, an attenuation in the frequency band affected by overload in a trans-mission channel fed by the compressor, and in the case of an expander fed by the channel, a complementary boost.  The arrangement provides level dependent equalization without significantly affecting the noise reduction effect of a compressor/expander system because in dual path compressors and expanders the output of the circuits at very low signal levels is provided mostly by the further path.  Thus, at high levels, where channel overload is a problem, the frequency dependent circuit is effective, while at low levels, where it is not desired or needed, the equali-zation effect is reduced.  The disclosure also describes combinations of two or more aspects of the invention. In one particular combined embodiment, C-type noise reduction is provided.  Additionally, arrangements for compressor/expander switching and B-type/C-type switch-ing are disclosed.