Quality improvement techniques in an audio encoder

• US 7,917,369 B2
• Filed: 04/18/2007
• Issued: 03/29/2011
• Est. Priority Date: 12/14/2001
• Status: Expired due to Fees

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Microsoft Corporation
• Inventors: Lee, Ming-Chieh, Thumpudi, Naveen, Chen, Wei-Ge
• Primary Examiner(s): Wozniak; James S
• Assistant Examiner(s): Serrou; Abdelali
• Application Number:
  US11/737,072
• Publication Number:
  US 20070185706A1
• Time in Patent Office: 1,441 Days
• Field of Search: 704/500, 704/503, 704/200.1, 704/230, 704/E19.005, 704/E19.015
• US Class Current: 704/500
• CPC Class Codes: G10L 19/002 Dynamic bit allocation for perceptual audio coders G10L19/032
• CPC Class Codes: G10L 19/008 Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
• CPC Class Codes: G10L 19/02 using spectral analysis, e.g. transform vocoders or subband vocoders

Abstract

An audio encoder implements multi-channel coding decision, band truncation, multi-channel rematrixing, and header reduction techniques to improve quality and coding efficiency. In the multi-channel coding decision technique, the audio encoder dynamically selects between joint and independent coding of a multi-channel audio signal via an open-loop decision based upon (a) energy separation between the coding channels, and (b) the disparity between excitation patterns of the separate input channels. In the band truncation technique, the audio encoder performs open-loop band truncation at a cut-off frequency based on a target perceptual quality measure. In multi-channel rematrixing technique, the audio encoder suppresses certain coefficients of a difference channel by scaling according to a scale factor, which is based on current average levels of perceptual quality, current rate control buffer fullness, coding mode, and the amount of channel separation in the source. In the header reduction technique, the audio encoder selectively modifies the quantization step size of zeroed quantization bands so as to encode in fewer frame header bits.

Claims

1. In a transform-based audio encoder, a method of improved band truncation, the method comprising:

performing a frequency transform on each of multiple input channels of a multi-channel input audio signal to produce multiple input channels of transform domain coefficients;

performing a multi-channel transformation on the multiple input channels of transform domain coefficients for the multi-channel input audio signal to produce a plurality of joint coding channels;

on a channel-by-channel basis, selectively suppressing at least one of the joint coding channels as a function of at least quality of reproduction, rate control buffer fullness, and channel separation, wherein the selectively suppressing comprises scaling the at least one joint coding channel by a scaling factor having a value varying based on a current average level of quality, current rate control buffer fullness and amount of channel separation; and

for at least one of the joint coding channels;

selecting as an open-loop process a first portion of the transform domain coefficients for band truncation as a function of a target quality measurement;

suppressing the selected first portion of the transform domain coefficients from encoding in a compressed audio data stream;

measuring an achieved quality measurement of the input audio signal encoded with the selected first portion of the transform domain coefficients suppressed;

selecting as a closed-loop process a second portion of the transform domain coefficients for second band truncation as a function of the achieved quality measurement; and

suppressing the selected second portion of the transform domain coefficients from encoding in a second compressed audio data stream.

2. The method of claim 1 wherein the target quality measurement is a target noise-to-excitation ratio for the input audio signal.

3. In a transform-based audio encoder, a method of encoding a multi-channel audio input signal, the method comprising:

performing a frequency transform on each of multiple input channels of the multi-channel audio input signal to produce multiple input channels of frequency coefficient data;

performing a multi-channel transformation on the multiple input channels of frequency coefficient data for the multi-channel audio input signal to produce a plurality of joint coding channels;

on a channel-by-channel basis, selectively suppressing at least one of the joint coding channels as a function of at least quality of reproduction, rate control buffer fullness, and channel separation, wherein the selectively suppressing comprises scaling the at least one joint coding channel by a scaling factor having a value varying based on a current average level of quality, current rate control buffer fullness and amount of channel separation; and

encoding the frequency coefficient data for the multi-channel audio input signal with said selective suppression of said at least one joint coding channel.

4. The method of claim 3 further comprising measuring the current average level of quality as a noise-to-excitation ratio for a portion of the multi-channel audio input signal.

5. The method of claim 3 wherein the selectively suppressing the at least one joint coding channel is also a function of a rate setting of the transform-based audio encoder.