Apparatus and method for encoding and decoding an audio signal using an aligned look-ahead portion

an audio signal and look-ahead technology, applied in the field of audio coding, can solve the problems of requiring a whole future frame, requiring additional delay of 20 ms, disadvantage of encoders, etc., and achieve the effect of reducing bitrate and facilitating and low efficiency switching over from one mod

Active Publication Date: 2015-04-21
FRAUNHOFER GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG EV
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  • Claims
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AI Technical Summary

Benefits of technology

[0020]In accordance with the present invention, a switched audio codec scheme is applied having a transform coding branch and a prediction coding branch. Importantly, the two kinds of windows, i.e., the prediction coding analysis window on the one hand and the transform coding analysis window on the other hand are aligned with respect to their look-ahead portion so that the transform coding look-ahead portion and the prediction coding look-ahead portion are identical or are different from each other by less than 20% of the prediction coding look-ahead portion or less than 20% of the transform coding look-ahead portion. It is to be noted that the prediction analysis window” is used not only in the prediction coding branch, but it is actually used in both branches. The LPC analysis is also used for shaping the noise in the transform domain. Therefore, in other words, the look-ahead portions are identical or are quite close to each other. This ensures that an optimum compromise is achieved and that no audio quality or delay features are set into a sub-optimum way. Hence, for the prediction coding in the analysis window it has been found out that the LPC analysis is the better the higher the look-ahead is, but, on the other hand, the delay increases with a higher look-ahead portion. On the other hand, the same is true for the TCX window. The higher the look-ahead portion of the TCX window is, the better the TCX bitrate can be reduced, since longer TCX windows result in lower bitrates in general. Therefore, in accordance with the present invention, the look-ahead portions are identical or quite close to each other and, particularly, less than 20% different from each other. Therefore, the look-ahead portion, which is not desired due to delay reasons is, on the other hand, optimally used by both, encoding / decoding branches.
[0035]On the other hand, when there is a switch from ACELP to TCX, a specific transition window is applied which immediately starts at the beginning of the current frame, i.e., the frame immediately after the switchover, with a non-overlapping portion so that any data do not have to be reconstructed in order to find overlap “partners”. Instead, the non-overlap portion of the synthesis window provides correct data without any overlapping and without any overlap-add procedures necessitated in the decoder. Only for the overlap portions, i.e., the third portion of the window for the current frame and the first portion of the window for the next frame, an overlap-add procedure is useful and performed in order to have, as in a straightforward MDCT, a continuous fade-in / fade-out from one block to the other in order to finally obtain a good audio quality without having to increase the bitrate due to the critically sampled nature of the MDCT as also known in the art under the term “time-domain aliasing cancellation (TDAC).

Problems solved by technology

This means that, by using this LPC analysis window, an additional delay of 20 ms, i.e., a whole future frame is necessitated.
Therefore, the G.718 encoder is not a switched encoder and, therefore, this encoder is disadvantageous in that it only provides a single speech coding mode within the core layer.
Hence, quality problems will occur when this coder is applied to other signals than speech signals, i.e., to general audio signals, for which the model behind CELP encoding is not appropriate.
Hence, although the USAC encoder provides a good audio quality due to its switched nature, the delay is considerable due to the LPC analysis window look-ahead portion 518 in FIG. 5d.
Hence, for the prediction coding in the analysis window it has been found out that the LPC analysis is the better the higher the look-ahead is, but, on the other hand, the delay increases with a higher look-ahead portion.

Method used

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Embodiment Construction

[0055]FIG. 1 a illustrates an apparatus for encoding an audio signal having a stream of audio samples. The audio samples or audio data enter the encoder at 100. The audio data is introduced into a windower 102 for applying a prediction coding analysis window to the stream of audio samples to obtain windowed data for a prediction analysis. The windower 102 is additionally configured for applying a transform coding analysis window to the stream of audio samples to obtain windowed data for a transform analysis. Depending on the implementation, the LPC window is not applied directly on the original signal but on a “pre-emphasized” signal (like in AMR-WB, AMR-WB+, G718 and USAC). On the other hand the TCX window is applied on the original signal directly (like in USAC). However, both windows can also be applied to the same signals or the TCX window can also be applied to a processed audio signal derived from the original signal such as by pre-emphasizing or any other weighting used for e...

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Abstract

An apparatus for encoding an audio signal having a stream of audio samples has: a windower for applying a prediction coding analysis window to the stream of audio samples to obtain windowed data for a prediction analysis and for applying a transform coding analysis window to the stream of audio samples to obtain windowed data for a transform analysis, wherein the transform coding analysis window is associated with audio samples within a current frame of audio samples and with audio samples of a predefined portion of a future frame of audio samples being a transform-coding look-ahead portion, wherein the prediction coding analysis window is associated with at least the portion of the audio samples of the current frame and with audio samples of a predefined portion of the future frame being a prediction coding look-ahead portion, wherein the transform coding look-ahead portion and the prediction coding look-ahead portion are identically to each other or are different from each other by less than 20%; and an encoding processor for generating prediction coded data or for generating transform coded data.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation of copending International Application No. PCT / EP2012 / 052450, filed Feb. 14, 2012, which is incorporated herein by reference in its entirety, and additionally claims priority from U.S. Provisional Application No. 61 / 442,632, filed Feb. 14, 2011, which is also incorporated herein by reference in its entirety.BACKGROUND OF THE INVENTION[0002]The present invention is related to audio coding and, particularly, to audio coding relying on switched audio encoders and correspondingly controlled audio decoders, particularly suitable for low-delay applications.[0003]Several audio coding concepts relying on switched codecs are known. One well-known audio coding concept is the so-called Extended Adaptive Multi-Rate-Wideband (AMR-WB+) codec, as described in 3GPP TS 26.290 B10.0.0 (2011-03). The AMR-WB+ audio codec contains all the AMR-WB speech codec modes 1 to 9 and AMR-WB VAD and DTX. AMR-WB+ extends the AMR-WB cod...

Claims

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Application Information

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Patent Type & AuthorityPatents(United States)
IPC IPC(8): G10L19/12G10L19/00G10L19/012G10K11/16G10L19/005G10L19/03G10L19/22G10L19/04G10L19/02G10L25/06G10L19/025G10L19/107
CPCG10L19/00G10L19/012G10K11/16G10L19/04G10L19/005G10L19/12G10L19/0212G10L19/03G10L25/06G10L19/025G10L19/107G10L19/22
InventorRAVELLI, EMMANUELGEIGER, RALFSCHNELL, MARKUSFUCHS, GUILLAUMERUOPPILA, VESABAECKSTROEM, TOMGRILL, BERNHARDHELMRICH, CHRISTIAN
OwnerFRAUNHOFER GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG EV