EP-4742241-A2 - CROSS PRODUCT ENHANCED HARMONIC TRANSPOSITION

Abstract

The present invention relates to audio coding systems which make use of a harmonic transposition method for high frequency reconstruction (HFR). A system and a method for generating a high frequency component of a signal from a low frequency component of the signal is described. The system comprises an analysis filter bank providing a plurality of analysis subband signals of the low frequency component of the signal. It also comprises a non-linear processing unit to generate a synthesis subband signal with a synthesis frequency by modifying the phase of a first and a second of the plurality of analysis subband signals and by combining the phase-modified analysis subband signals. Finally, it comprises a synthesis filter bank for generating the high frequency component of the signal from the synthesis subband signal.

Inventors

• VILLEMOES, LARS
• HEDELIN, PER

Assignees

• Dolby International AB

Dates

Publication Date

20260513

Application Date

20100115

Claims (9)

1. A system for encoding a signal, comprising: a splitting unit for splitting the signal into a low frequency component and into a high frequency component; a core encoder for encoding the low frequency component; a frequency determination unit for determining a fundamental frequency Ω of the signal; and a parameter encoder for encoding the fundamental frequency Ω, wherein the fundamental frequency Ω is for use in regenerating the high frequency component of the signal.
2. The system according to claim 1, further comprising: an envelope determination unit for determining the spectral envelope of the high frequency component; and an envelope encoder for encoding the spectral envelope.
3. A method for encoding a signal, comprising: splitting the signal into a low frequency component and into a high frequency component; encoding the low frequency component; determining a fundamental frequency Ω of the signal; and encoding the fundamental frequency Ω, wherein the fundamental frequency Ω is for use in regenerating the high frequency component of the signal.
4. The method according to claim 3, further comprising: determining the spectral envelope of the high frequency component; and encoding the spectral envelope.
5. An encoded signal, comprising: an encoded low frequency component of an audio signal; and an indication of the fundamental frequency Ω of the audio signal, wherein the fundamental frequency Ω is for use in regenerating a high frequency component of the audio signal.
6. The encoded signal of claim 5, further comprising: an encoded spectral envelope of the high frequency component of the audio signal.
7. The encoded signal of claim 5 or claim 6, wherein the encoded signal is determined by performing the method of claim 3 or claim 4.
8. A software program adapted for execution on a processor and for performing the method steps of claim 3 or claim 4 when carried out on a computing device.
9. A storage medium comprising a software program adapted for execution on a processor and for performing the method steps of claim 3 or claim 4 when carried out on a computing device.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a European divisional application of European patent application 25180486.0 (reference: D08072EP11), for which EPO Form 1001 was filed 3 June 2025. TECHNICAL FIELD The present invention relates to audio coding systems which make use of a harmonic transposition method for high frequency reconstruction (HFR). BACKGROUND OF THE INVENTION HFR technologies, such as the Spectral Band Replication (SBR) technology, allow to significantly improve the coding efficiency of traditional perceptual audio codecs. In combination with MPEG-4 Advanced Audio Coding (AAC) it forms a very efficient audio codec, which is already in use within the XM Satellite Radio system and Digital Radio Mondiale. The combination of AAC and SBR is called aacPlus. It is part of the MPEG-4 standard where it is referred to as the High Efficiency AAC Profile. In general, HFR technology can be combined with any perceptual audio codec in a back and forward compatible way, thus offering the possibility to upgrade already established broadcasting systems like the MPEG Layer-2 used in the Eureka DAB system. HFR transposition methods can also be combined with speech codecs to allow wide band speech at ultra low bit rates. The basic idea behind HRF is the observation that usually a strong correlation between the characteristics of the high frequency range of a signal and the characteristics of the low frequency range of the same signal is present. Thus, a good approximation for the representation of the original input high frequency range of a signal can be achieved by a signal transposition from the low frequency range to the high frequency range. This concept of transposition was established in WO 98/57436, as a method to recreate a high frequency band from a lower frequency band of an audio signal. A substantial saving in bit-rate can be obtained by using this concept in audio coding and/or speech coding. In the following, reference will be made to audio coding, but it should be noted that the described methods and systems are equally applicable to speech coding and in unified speech and audio coding (USAC). In a HFR based audio coding system, a low bandwidth signal is presented to a core waveform coder and the higher frequencies are regenerated at the decoder side using transposition of the low bandwidth signal and additional side information, which is typically encoded at very low bit-rates and which describes the target spectral shape. For low bit-rates, where the bandwidth of the core coded signal is narrow, it becomes increasingly important to recreate a high band, i.e. the high frequency range of the audio signal, with perceptually pleasant characteristics. Two variants of harmonic frequency reconstruction methods are mentioned in the following, one is referred to as harmonic transposition and the other one is referred to as single sideband modulation. The principle of harmonic transposition defined in WO 98/57436 is that a sinusoid with frequency ω is mapped to a sinusoid with frequency Tω where T > 1 is an integer defining the order of the transposition. An attractive feature of the harmonic transposition is that it stretches a source frequency range into a target frequency range by a factor equal to the order of transposition, i.e. by a factor equal to T. The harmonic transposition performs well for complex musical material. Furthermore, harmonic transposition exhibits low cross over frequencies, i.e. a large high frequency range above the cross over frequency can be generated from a relatively small low frequency range below the cross over frequency. In contrast to harmonic transposition, a single sideband modulation (SSB) based HFR maps a sinusoid with frequency ω to a sinusoid with frequency ω + Δω where Δω is a fixed frequency shift. It has been observed that, given a core signal with low bandwidth, a dissonant ringing artifact may result from the SSB transposition. It should also be noted that for a low cross-over frequency, i.e. a small source frequency range, harmonic transposition will require a smaller number of patches in order to fill a desired target frequency range than SSB based transposition. By way of example, if the high frequency range of (ω, 4ω] should be filled, then using an order of transposition T = 4 harmonic transposition can fill this frequency range from a low frequency range of 14ω,ω. On the other hand, a SSB based transposition using the same low frequency range must use a frequency shift of Δω=34ω and it is necessary to repeat the process four times in order to fill the high frequency range (ω, 4ω]. On the other hand, as already pointed out in WO 02/052545 A1, harmonic transposition has drawbacks for signals with a prominent periodic structure. Such signals are superimpositions of harmonically related sinusoids with frequencies Ω,2Ω,3Ω,..., where Ω is the fundamental frequency. Upon harmonic transposition of order T, the output sinusoids have frequencies