Post filter for audio signals
In some embodiments, a pitch filter for filtering a preliminary audio signal generated from an audio bitstream is disclosed. The pitch filter has an operating mode selected from one of either: (i) an active mode where the preliminary audio signal is filtered using filtering information to obtain a filtered audio signal, and (ii) an inactive mode where the pitch filter is disabled. The preliminary audio signal is generated in an audio encoder or audio decoder having a coding mode selected from at least two distinct coding modes, and the pitch filter is capable of being selectively operated in either the active mode or the inactive mode while operating in the coding mode based on control information.
1. A decoder system for decoding a bit stream signal as an audio time signal, including:
a decoding section for decoding a bit stream signal as a preliminary audio time signal; and
an interharmonic noise attenuation post filter for filtering the preliminary audio time signal to obtain an audio time signal, wherein the interharmonic noise attenuation is controlled by a post filter gain, comprising a control section adapted to selectively disable the post filter, by setting the post filter gain to zero, responsive only to post-filtering information encoded in the bit stream signal, the post-filtering information being indicative of an encoder-side decision whether to disable post filtering, wherein the preliminary audio time signal is output as the audio time signal.
2. The decoder system of claim 1 , wherein the post-filtering is adapted to attenuate only such spectral components which are located below a predetermined cut-off frequency.
3. The decoder system of claim 1 , wherein the decoding section further comprises:
a code-excited linear prediction, CELP, decoding module; and
a transform-coded excitation, TCX, decoding module for decoding a bit stream signal as an audio time signal,
the control section being adapted operate the decoder system in at least the following modes:
a) the TCX module is enabled and the post filter is disabled;
b) the CELP module and the post filter are enabled; and
c) the CELP module is enabled and the post filter is disabled, wherein the preliminary audio time signal and the audio time signal coincide.
4. The decoder system of claim 1 , said decoding section including a speech decoding module, wherein a pitch frequency estimated by a long-term prediction section in the encoder is encoded in the bit stream signal, and wherein the post filter is adapted to attenuate spectral components located between harmonics of the pitch frequency.
5. The decoder system of claim 1 , wherein the bit stream signal contains a representation of a pitch frequency and the post filter is adapted to attenuate spectral components located between harmonics of the pitch frequency.
6. The decoder system of claim 4 ,
wherein the post filter is adapted to attenuate only such spectral components which are located below a predetermined cut-off frequency, and
wherein the decoding section further comprises an Advanced Audio Coding, AAC, decoding module for decoding a bit stream signal as an audio time signal,
the control section being adapted to operate the decoder system also in the following mode:
d) the AAC module is enabled and the post filter is disabled.
7. The decoder system of claim 1 , wherein the bit stream signal is segmented into time frames and the control section is adapted to disable an entire time frame or a sequence of entire time frames.
8. The decoder system of claim 7 , wherein the control section is further adapted to receive, for each time frame in a Moving Pictures Experts Group, MPEG, bit stream, a data field associated with this time frame and is operable, responsive to the value of the data field, to disable the post filter.
9. The decoder system of claim 1 , wherein the control section is adapted to decrease and/or increase the gain of the post filter gradually.
10. A method of decoding a bit stream signal as an audio time signal, including the steps of:
decoding a bit stream signal as a preliminary audio time signal; and
post-filtering the preliminary audio time signal by attenuating interharmonic noise, thereby obtaining an audio time signal, wherein the interharmonic noise attenuation is controlled by a post filter gain,
wherein the post-filtering step is selectively omitted, by setting the post filter gain to zero, responsive only to post-filtering information encoded in the bit stream signal, the post-filtering information being indicative of an encoder-side decision whether to disable post filtering.
11. A non-transitory computer readable storage medium containing a program of instructions which, when executed by one or more processors, cause the one or more processors to perform the method of claim 10 .