In a noise reduction system including a compressor and an expander having a complementary frequency response characteristic to the frequency response characteristic of the compressor, the compressor includes a high frequency emphasis variable frequency filter and a low frequency emphasis variable gain filter which are connected in a series circuit between input and output terminals and which respectively vary their parameters in response to the respective high and low frequency components of the compressor output. The expander includes a high frequency de-emphasis variable frequency filter and a low frequency de-emphasis variable gain filter which are connected in a series circuit between input and output terminals and which respectively vary their parameters in response to the respective high and low frequency components of the expander input.
A decoding arrangement for decoding audio signals previously encoded in accordance with the dbx format, includes a wideband expander instead of a full dbx expander. The wideband expander employs a relatively simple, and therefore inexpensive, peak detector in place of the true RMS detector of a full dbx expander, and does not include apparatus for spectral expansion of the audio signal. Although the decoding is noncomplementary with respect to the encoding, the stereo audio reproduction as perceived by a listener, is surprisingly good.
In a first aspect of the invention, compressor and expander circuits are arranged in series, the dynamic actions of the circuits being substantially separated with respect to level. The 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 substantially no greater than that of any single circuit while providing more compression or expansion than for any of the circuits individually. A second aspect, referred to as "spectral skewing", effects a reduction of the influence of signals in the high and/or low frequency extremes of a signal transmission system on the action of a compressor feeding the transmission channel. Consequently, a complementary expander fed by the channel is less likely to be controlled by signals at frequencies subject to transmission channel errors. A third aspect provides an anti-saturation effect. This aspect of the invention takes advantage of the properties of dual path compressors and expanders, in which the output of the circuits at very low signal levels is provided mostly by the further path and at high levels, mostly by the main path. 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 noise reduction switching are disclosed.
A noise reducing apparatus comprises a first variable gain control circuit for giving a level compression characteristic to an input signal by a first composite control voltage, to produce an output signal to a transmission path, a first control voltage producing circuit for dividing an output signal of the first variable gain control circuit into a plurality of frequency bands, and producing a plurality of control voltages respectively given with mutually different integration time constants for each of the divided frequency bands, a first adding circuit for respectively additionally composing the plurality of control voltages obtained from the first control voltage producing circuit, to obtain the first composite control voltage, a second variable gain control circuit for giving a level expansion characteristic complementary to the level compression characteristic to a level compressed signal obtained through the transmission path by a second composite control voltage, to produce an output signal, a second control voltage producing circuit for dividing an input signal of the second variable gain control circuit into a plurality of frequency bands, and producing a plurality of control voltages respectively given with mutually different integration time constants for each of the divided frequency bands, and a second adding circuit for respectively additionally composing the plurality of control voltages obtained from the second control voltage producing circuit, to obtain the second composite control voltage.
A signal compression system includes a plurality of channels. A plurality of these channels include a bandpass filter (for filtering out all but a portion of an input signal), an intensity detector (for deriving a spectrally weighted estimate of the intensity of a broader spectral portion of the input signal than the bandpass filtered spectral portion), and a divider (for compressing the bandpass filtered spectral portion using a variable gain having a preselected functional relationship to the spectrally weighted intensity estimate). The signal compression system preserves cross-channel information.
Apparatus for reducing distortion in a peak limited audio siganl. The amount of low-frequency spectral energy in the input audio signal is determined for a peak limited signal and used to control low-frequencies in the output audio signal. Identical low-pass filters with cut-off frequencies of 2.2 kHz are used; one passing a peak limited signal, the other a non-peak limited signal. When no noticeable low-frequency spectral energy is present in the peak limited (clipped) signal, the peak level is controlled below 2.2 kHz, without distortion, by a VCA and above 2.2 kHz by a clipper. When low-frequency spectral energy is present, the VCA reduces gain on each low-frequency wave to prevent uncontrolled safety clipping.
