Harmonic Post-Filter for Audio Inter-Harmonic Noise Reduction
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Solution Overview
Problem
Transform-based audio codecs introduce inter-harmonic noise when processing harmonic audio signals, particularly at low bitrates, which degrades the subjective quality of the audio.
Innovation Solution
A harmonic post-filter based on a long-term prediction filter working in the time-domain is applied to the decoded audio signal. This post-filter uses a transfer function with a numerator comprising a gain value and a denominator comprising the integer and fractional parts of the pitch lag, along with a multi-tap filter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If transform-based audio codec operates at low bitrate, then compression efficiency is improved, but inter-harmonic noise increases
Solution Approach 1:
A post-filter is introduced as an intermediary component between the transform decoder and the audio output. This post-filter acts as a mediator that processes the decoded audio signal to remove inter-harmonic noise while preserving the harmonic content, thus resolving the contradiction between compression efficiency and noise reduction
Solution Approach 2:
The harmful inter-harmonic noise is extracted and removed from the decoded audio signal through the post-filter. The filter selectively extracts and eliminates the noise components that fall between harmonic frequencies, allowing the desired audio signal to pass through with reduced noise
2Speed
If transform size is reduced for low delay operation, then processing speed is improved, but frequency resolution deteriorates
Solution Approach 1:
The time-domain post-filter replaces the need for larger transform sizes to achieve good frequency resolution. Instead of relying on mechanical/algorithmic transform operations with large sizes, the solution substitutes a signal processing approach using filter banks that can achieve frequency selectivity independently of transform size, thus maintaining fast processing speed while improving frequency resolution
3Object-generated harmful factors
If pre-filter is applied to improve harmonic quality, then subjective quality is improved, but system stability deteriorates
Solution Approach 1:
Instead of applying a pre-filter before the transform encoder (which causes stability issues), the solution inverts the approach by applying a post-filter after the transform decoder. This reversal of the filtering timing and position eliminates the stability problems associated with pre-filters while still achieving the goal of improving harmonic quality in the decoded audio
Data Source
AI summary
An apparatus for processing an audio signal having associated therewith a pitch lag information and a gain information, includes a domain converter for converting a first domain representation of the audio signal into a second domain representation of the audio signal; and a harmonic post-filter for filtering the second domain representation of the audio signal, wherein the post-filter is based on a transfer function including a numerator and a denominator, wherein the numerator includes a gain value indicated by the gain information, and wherein the denominator includes an integer part of a pitch lag indicated by the pitch lag information and a multi-tap filter depending on a fractional part of the pitch lag.