Calculating and adjusting the perceived loudness and/or the perceived spectral balance of an audio signal
The invention relates to the measurement and control of the perceived sound loudness and/or the perceived spectral balance of an audio signal. An audio signal is modified in response to calculations performed at least in part in the perceptual (psychoacoustic) loudness domain. The invention is useful, for example, in one or more of: loudness-compensating volume control, automatic gain control, dynamic range control (including, for example, limiters, compressors, expanders, etc.), dynamic equalization, and compensating for background noise interference in an audio playback environment. The invention includes not only methods but also corresponding computer programs and apparatus.
1. A method for adjusting a level of an audio signal in an audio processing apparatus, the method comprising:
dividing an audio signal into a plurality of frequency bands;
obtaining modification parameters for at least one of the plurality of frequency bands, the modification parameters comprising filter coefficients and amplitude scale factors, each amplitude scale factor respectively operating in a frequency band of a plurality of frequency bands;
deriving gain factors for at least one of the plurality of frequency bands, the gain factors determined based on the amplitude scale factors;
smoothing the gain factors, wherein the smoothing of the gain factors is optional;
determining a level of noise from noise compensation factors;
applying the gain factors to at least some of the frequency bands to generate gain adjusted frequency bands;
adjusting the level of noise based on the gain adjusted frequency bands;
filtering at least one of the frequency bands with a filter generated with the filter coefficients;
synthesizing the plurality of frequency bands to generate an output audio signal;
wherein the gain factors are both time and frequency varying.
2. The method of claim 1 , further comprising band smoothing of the amplitude scale factors.
3. The method of claim 1 , wherein the plurality of frequency bands are derived from an analysis filter bank.
4. The method of claim 1 , wherein the adjusting of the level of noise is performed by a synthesis filterbank.
5. An audio processing apparatus for adjusting a level of an audio signal, the audio processing apparatus comprising:
an analysis filterbank for dividing an audio signal into a plurality of frequency bands;
a parameter generator for obtaining modification parameters for at least one the plurality of frequency bands, the modification parameters comprising filter coefficients and amplitude scale factors, each amplitude scale factor respectively operating in a frequency band of a plurality of frequency bands;
a first processor for deriving gain factors for at least one of the plurality of frequency bands, the gain factors determined based on from the amplitude scale factors;
a smoother for smoothing the gain factors, wherein the smoothing of the gain factors is optional;
a second processor for determining a level of noise from noise compensation factors;
a first adjuster for applying the gain factors to at least one of the frequency bands to generate gain adjusted frequency bands;
a second adjuster for adjusting the level of noise based on the gain adjusted frequency bands;
a filter for filtering at least some of the frequency bands, the filter generated with the filter coefficients; and
a synthesis filterbank for synthesizing the plurality of frequency bands to generate an output audio signal;
wherein the gain factors are both time and frequency varying.
6. The audio decoder of claim 5 , further comprising a band smoother for smoothing the amplitude scale factors.
7. A non-transitory computer readable medium, storing software instructions for controlling a perceptual loudness of a digital audio signal, which when executed by one or more processors cause performance of the steps of method claim 1 .