Dynamic range and peak control in audio using nonlinear filters

Patent metadata

Field	Value
Publication number	US-2016351202-A1
Application number	US-201514723337-A
Country	US
Kind code	A1
Filing date	May 27, 2015
Priority date	May 27, 2015
Publication date	Dec 1, 2016
Grant date	—

Abstract

An audio encoding device is described herein. The audio encoding device includes a compressor that is based on a nonlinear filter. In particular, the nonlinear filter may be selected from the class of edge-preserving smoothing filters, which avoids common artifacts of conventional compressors. Edge-preserving smoothing filters have been used in image processing algorithms for their de-noising properties while preserving edges in the image. These properties are useful for audio compression because macro-dynamic loudness changes can be tracked precisely while micro-dynamic loudness changes can be ignored for the compression. Due to these advantages, more aggressive compression can be achieved with less distortion.

First claim

What is claimed is:

1 . A method for encoding an audio signal, comprising:
processing samples of the audio signal using a loudness model to determine the instantaneous loudness of the audio signal for a set of the samples; processing the instantaneous loudness using a primary nonlinear filter to generate a smoothed loudness; mapping the smoothed loudness to a dynamic range compression gain value; and combining the dynamic range compression gain value with the audio signal to generate a compressed audio signal.

2 . The method of claim 1 , wherein the primary nonlinear filter is an edge-preserving smoothing filter.

3 . The method of claim 1 , wherein the primary nonlinear filter is one of (1) a median filter; (2) a bilateral filter; (3) a guided filter; (4) a weighted least squares filter; and (5) an anisotropic diffusion filter.

4 . The method of claim 1 , wherein when the smoothed loudness is above a threshold value, the dynamic range compression gain value is set at a first level and when the smoothed loudness is below the threshold value, the dynamic range compression gain value is set at a second level, wherein the first level is below the second level.

5 . The method of claim 1 , further comprising:
reducing peaks in the compressed audio signal by (1) limiting the instantaneous loudness to a maximum loudness threshold value and (2) distorting the limited instantaneous loudness using a mapping function.

6 . The method of claim 1 , further comprising:
down-sampling the instantaneous loudness prior to processing by the primary nonlinear filter; and up-sampling the dynamic range compression gain value prior to combination with the audio signal.

7 . The method of claim 1 , wherein the primary nonlinear filter is a median filter, the method further comprising:
evaluating a local extremum of a time-derivative of the smoothed loudness to classify the audio signal into separate content segments, wherein the extremum indicate where a maximum slope steepness is reached, wherein a boundary between two segments is indicated when a magnitude of a steepness of the time-derivative of the smoothed loudness exceeds a steepness threshold.

8 . The method of claim 1 , further comprising:
processing the instantaneous loudness using a secondary nonlinear filter and an adaptation controller to determine a duration of peaks and valleys in the instantaneous loudness; slowing decay to the instantaneous loudness when the determined duration is below a decay threshold; and reducing the size of the primary nonlinear filter when the determined duration of the instantaneous loudness is below a filter threshold.

9 . The method of claim 8 , wherein the secondary nonlinear filter is a median filter.

10 . An audio encoding device for encoding an audio signal, comprising:
a loudness model to process samples of the audio signal to determine the instantaneous loudness of the audio signal for a set of the samples; a primary nonlinear filter to process the instantaneous loudness to generate a smoothed loudness; and a primary mapping unit to map the smoothed loudness to a dynamic range compression gain value.

11 . The audio encoding device of claim 10 , further comprising:
a multiplier to combine the dynamic range compression gain value with the audio signal to generate a compressed audio signal.

12 . The audio encoding device of claim 10 , wherein the dynamic range compression gain value is added as metadata to the audio signal before being transmitted to an audio playback device.

13 . The audio encoding device of claim 10 , wherein the primary nonlinear filter is an edge-preserving smoothing filter.

14 . The audio encoding device of claim 10 , wherein the primary nonlinear filter is one of (1) a median filter; (2) a bilateral filter; (3) a guided filter; (4) a weighted least squares filter; and (5) an anisotropic diffusion filter.

15 . The audio encoding device of claim 10 , wherein when the smoothed loudness is above a threshold value, the dynamic range compression gain value is set at a first level and when the smoothed loudness is below the threshold value, the dynamic range compression gain value is set at a second level, wherein the first level is below the second level.

16 . The audio encoding device of claim 12 , further comprising:
a maximum value unit to reduce peaks in the compressed audio signal by limiting the instantaneous loudness to a maximum loudness threshold value; and a secondary mapping unit to distort the limited instantaneous loudness using a mapping function to further reduce peaks in the compressed audio signal.

17 . The audio encoding device of claim 10 , further comprising:
a down-sampling unit to down-sample the instantaneous loudness prior to processing by the primary nonlinear filter; and an up-sampling unit to oversample the dynamic range compression gain value.

18 . The audio encoding device of claim 10 , further comprising:
a secondary nonlinear filter and an adaptation controller to (1) process the instantaneous loudness to determine a duration of peaks and valleys in the instantaneous loudness and (2) reduce the size of the primary nonlinear filter when the determined duration of the instantaneous loudness is below a filter threshold; and a decay generator to slow decay to the instantaneous loudness when the determined duration is below a decay threshold.

19 . The audio encoding device of claim 16 , wherein the secondary nonlinear filter is a median filter.

20 . A non-transitory computer readable medium that stores instructions that when executed by a processor of an audio encoding device cause the audio encoding device to:
process samples of the audio signal using a loudness model to determine the instantaneous loudness of the audio signal for a set of the samples; process the instantaneous loudness using a nonlinear filter to generate a smoothed loudness; and map the smoothed loudness to a dynamic range compression gain value.

21 . The non-transitory computer readable medium of claim 20 , wherein the computer readable medium stores further instructions that when executed by the processor of the audio encoding device cause the audio encoding device to:
combine the dynamic range compression gain value with the audio signal to generate a compressed audio signal.

22 . The non-transitory computer readable medium of claim 20 , wherein the computer readable medium stores further instructions that when executed by the processor of the audio encoding device cause the audio encoding device to:
add the dynamic range compression gain value as metadata to the audio signal.

23 . The non-transitory computer readable medium of claim 20 , wherein the nonlinear filter is an edge-preserving smoothing filter.

24 . The non-transitory computer readable medium of claim 20 , wherein the nonlinear filter is one of (1) a median filter; (2) a bilateral filter; (3) a guided filter; (4) a weighted least squares filter; and (5) an anisotropic diffusion filter.

Assignees

• Apple Inc

Inventors

• Baumgarte Frank M

Classifications

• G10L19/26 Primary

  Pre-filtering or post-filtering · CPC title

• H03G7/002

  in untuned or low-frequency amplifiers, e.g. audio amplifiers (H03G7/007, H03G7/001, H03G7/008, H03G7/02, H03G7/06 take precedence) · CPC title

• H03G7/007

  of digital or coded signals · CPC title