Apparatus and method for processing an audio signal using a harmonic post-filter

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This invention relates to an apparatus and method for processing audio signals using a harmonic post-filter that reduces unwanted noise (inter-harmonic noise) which often occurs when audio codecs compress and decompress harmonic or tonal audio, especially at low bitrates. The core idea is to use a post-filter—applied after audio decoding—which takes into account both the integer and fractional parts of pitch lag and gain information to more precisely filter the audio and suppress noise, thereby improving the perceived quality of the audio output without the need for a pre-filter before encoding.

Use CasesContent extracted from patent full text and abstract with AI.

• Improving voice call quality in telecommunications, especially in mobile or VoIP calls at low data rates.
• Enhancing compressed music or streaming audio where tonal clarity is important despite strong compression.
• Audio post-processing in devices such as smartphones, hearing aids, audio players, or conferencing systems to reduce artifacts from low-bitrate coders.
• Integration into broadcast encoding/decoding systems for higher quality music or speech transmission.
• Audio playback in embedded or consumer devices where data storage or transmission bandwidth is limited.

BenefitsContent extracted from patent full text and abstract with AI.

• Significantly reduces inter-harmonic noise artifacts, leading to better perceived sound quality.
• Optimized for low bitrate or low-delay codecs where traditional filters are less effective or introduce distortion.
• Adaptively uses transmitted pitch and gain information already present in the bitstream, minimizing added complexity and overhead.
• Avoids issues of instability and added noise that can result from using harmonic pre-filters before the codec.
• Design flexibility—the filter parameters can be adjusted based on bitrate and signal characteristics, allowing broad applicability across various devices and scenarios.

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Patent Abstract

An apparatus for processing an audio signal having associated therewith a pitch lag information and a gain information, comprises a domain converter (100) for converting a first domain epresentation of the audio signal into a second domain representation of the audio signal; and a harmonic post-filter (104) for filtering the second domain representation of the audio signal, wherein the post-filter is based on a transfer function comprising a numerator and a denominator, wherein the numerator comprises a gain value indicated by the gain information, and wherein the denominator comprises 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.