Enhancing perceptual performance of SBR and related HFR coding methods by adaptive noise-floor addition and noise substitution limiting

Abstract

Methods and an apparatus for enhancement of source coding systems utilizing high frequency reconstruction (HFR) are introduced. The problem of insufficient noise contents is addressed in a reconstructed highband, by using Adaptive noise-floor Addition. New methods are also introduced for enhanced performance by means of limiting unwanted noise, interpolation and smoothing of envelope adjustment amplification factors. The methods and apparatus used are applicable to both speech coding and natural audio coding systems.

Description

be the scale factors of the original signal at a given time, and
P₂=[p₂₁, . . . , p_2N]  eq. 8
the corresponding scale factors of the transposed signal, where every element of the two vectors represents sub-band energy normalised in time and frequency. The required amplification factors for the spectral envelope adjustment filterbank is obtained as

$\begin{array}{cc}G=\left[g_1,\dots ,g_N\right]=\left[\sqrt{\frac{p_{11}}{p_{21}}},\dots ,\sqrt{\frac{p_{1N}}{p_{2N}}}\right].& \mathrm{eq}.9\end{array}$

By observing G it is trivial to determine the frequency bands with unwanted noise substitution, since these exhibit much higher amplification factors than the others. The unwanted noise substitution is thus easily avoided by applying a limiter to the amplification factors, i.e. allowing them to vary freely up to a certain limit, g_max. The amplification factors using the noise-limiter is obtained by
G_lim=[min(g₁,g_max), . . . , min(g_N,g_max)]  eq. 10

However, this expression only displays the basic principle of the noise-limiters. Since the spectral envelope of the transposed and the original signal might differ significantly in both level and slope, it is not feasible to use constant values for g_max. Instead, the average gain, defined as

$\begin{array}{cc}{G}_{\mathrm{avg}}=\sqrt{\frac{\sum _i{P}_{1i}}{\sum _i{P}_{2i}}},& \mathrm{eq}.11\end{array}$
is calculated and the amplification factors are allowed to exceed that by a certain amount. In order to take wide-band level variations into account, it is also possible to divide the two vectors P₁ and P₂ into different sub-vectors, and process them accordingly. In this manner, a very efficient noise limiter is obtained, without interfering with, or confining, the functionality of the level-adjustment of the sub-band signals containing useful information.

Claims

1. A method for enhancing a source encoding method, the source encoding method generating an encoded signal by encoding an original signal, the original signal having a low band portion and a high band portion, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, comprising the following steps:

estimating a noise-floor level of the high band portion of the original signal, the noise floor level being a measure for a difference between a first spectral envelope determined by local minimum points of a spectral representation of the original signal and a second spectral envelope determined by local maximum points of a spectral representation of the original signal; and

multiplexing the encoded signal including the low band portion of the original signal and the noise-floor level of the high band portion of the original signal to obtain an encoder output signal.

2. A method according to claim 1, in which the step of estimating includes the following step:

mapping the noise-floor level to several frequency bands to obtain a noise-floor level for each of the several frequency bands.

3. A method according to claim 2, in which the difference measure is additionally smoothed in time.

4. A method according to claim 2, further comprising the following steps:

providing an additional fine structured spectral representation of the original signal using a resolution which is lower than a resolution used in the step of providing the fine structured spectral representation;

performing the steps of applying a dip following action, applying a peak following action and forming a difference to obtain an additional difference measure; and

choosing between the additional difference measure and the noise-floor level values to obtain a largest noise-floor level estimate.

5. A method according to claim 1, in which the noise-floor level is represented using linear predictive coding, or any other polynomial representation.

6. A method according to claim 1, in which the step of estimating includes the following steps:

providing a fine structured spectral representation of the original signal using a resolution which is sufficient so that formants or single sinusoidals in the spectral representation are visible, the fine structured spectral representation having local minimum points and local maximum points;

applying a dip-following action on the fine structured spectral representation for interpolating along the local minimum points to obtain the first spectral envelope;

applying a peak following action on the fine structured spectral representation of the original signal for interpolating along the maximum points to obtain the second spectral envelope;

forming a difference between the first spectral envelope and the second spectral envelope to obtain a difference measure; and

smoothing the difference measure to obtain noise-floor level values.

7. A method according to claim 1, in which a spectral envelope of the high band portion of the original signal is estimated and additionally multiplexed into the encoder output signal to be used by a decoding method using a high-frequency reconstruction technique.

8. An apparatus for enhancing a source encoder, the source encoder generating an encoded signal by encoding an original signal, the original signal having a low band portion and a high band portion, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, comprising:

an estimator for estimating a noise-floor level of the original signal, the noise floor level being a measure for a difference between a first spectral envelope determined by local minimum points of a spectral representation of the original signal and a second spectral envelope determined by local maximum points of a spectral representation of the original signal; and

a multiplexer for multiplexing the encoded signal including the low band portion of the original signal and the noise-floor level of the high band portion of the original signal to obtain an encoder output signal.

9. An apparatus for enhancing a source decoder, the source decoder generating a decoded signal by decoding an encoded signal obtained by source encoding of an original signal, the original signal having a low band portion and a high band portion, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, wherein the decoded signal is used for high-frequency reconstruction to obtain a high-frequency reconstructed signal including a reconstructed high band portion of the original signal, comprising:

a demultiplexer for demultiplexing an input signal including the encoded signal and a noise-floor level of the high band portion of the original signal, the noise floor level being a measure for a difference between a first spectral envelope determined by local minimum points of a spectral representation of the original signal and a second spectral envelope determined by local maximum points of a spectral representation of the original signal;

means for obtaining a spectral envelope representation of the high band portion of the original signal;

a shaper for shaping a spectrum of a random noise signal in accordance to the spectral envelope representation of the high band portion of the original signal to obtain a spectrally shaped random noise signal;

an adjuster for adjusting the spectrally shaped random noise signal in accordance to the noise-floor level to obtain an adjusted spectrally shaped random noise signal; and

an adder for adding the adjusted spectrally shaped random noise signal to the high-frequency reconstructed signal to obtain an enhanced high-frequency reconstructed signal.

10. An apparatus according to claim 9, further comprising:

a combiner for combining the enhanced high-frequency reconstructed signal and the decoded signal to generate an output signal having the low band portion of the original signal and a reconstructed high band portion of the original signal.

11. A method for enhancing a source decoding method, the source decoding method generating a decoded signal by decoding an encoded signal obtained by source encoding of an original signal, the original signal having a low band portion and a high band portion, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, wherein the decoded signal is used for high-frequency reconstruction to obtain a high-frequency reconstructed signal including a reconstructed high band portion of the original signal, comprising the following steps:

demultiplexing an input signal including the encoded signal and a noise-floor level of the high band portion of the original signal, the noise floor level being a measure for a difference between a first spectral envelope determined by local minimum points of a spectral representation of the original signal and a second spectral envelope determined by local maximum points of a spectral representation of the original signal;

obtaining a spectral envelope representation of the high band portion of the original signal;

shaping a spectrum of a random noise signal in accordance to the spectral envelope representation of the high band portion of the original signal to obtain a spectrally shaped random noise signal;

adjusting the spectrally shaped random noise signal in accordance to the noise-floor level to obtain an adjusted spectrally shaped random noise signal; and

adding the adjusted spectrally shaped random noise signal to the high-frequency reconstructed signal to obtain an enhanced high-frequency reconstructed signal.

12. The method in according to claim 11, in which the spectral envelope representation includes an energy measure for an energy of the high-frequency reconstructed signal and the noise-floor, the method further comprising the following step:

adjusting the high-frequency reconstructed signal so that a combined energy of the high-frequency reconstructed signal and the adjusted spectrally shaped random noise signal corresponds to the energy measure of the spectral envelope representation.

13. The method according to claim 11, in which the step of adjusting the spectrally shaped random noise signal includes a step of smoothing a level of the spectrally shaped random noise signal in time and/or frequency.

14. The method according to claim 11, in which a spectral envelope of the high-frequency reconstructed signal is adjusted using interpolation.

15. The method according to claim 11, in which a spectral envelope of the high-frequency reconstructed signal is adjusted using smoothing of envelope adjustment amplification factors.

16. An apparatus for enhancing a source decoder, the source decoder generating a decoded signal by decoding an encoded signal obtained by source encoding of an original signal, the original signal having a low band portion and a high band portion, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, wherein the decoded signal is used for high-frequency reconstruction to obtain a high-frequency reconstructed signal including a reconstructed high band portion of the original signal, comprising:

an adjuster for adjusting a spectral envelope of the high-frequency reconstructed signal, the adjuster including a limiter for limiting of envelope adjustment amplification factors.

17. An apparatus for enhancing a source decoder, the source decoder generating a decoded signal by decoding an encoded signal obtained by source encoding of an original signal, the original signal having a low band portion and a high band portion, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, wherein the decoded signal is used for high-frequency reconstruction to obtain a high-frequency reconstructed signal including a reconstructed high band portion of the original signal, comprising:

a high frequency reconstruction module for generating a signal, the high-frequency reconstruction module having a summer for summing several high-frequency reconstructed signals, originating from different low band frequency ranges of the decoded signal to obtain the signal, and

an analyzer for analyzing the low band portion of the decoded signal and for providing control data to the summer.

18. Encoder comprising:

an audio coder for encoding an audio signal to obtain an encoded signal, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, comprising:

a noise estimation device for estimating a level of noise to be added in a high-frequency regeneration process at a decoder; and

an envelope extraction unit for extracting a spectral envelope of the original signal to be used for adjusting a reconstructed high-band portion of the original signal.

19. Encoder in accordance with claim 18, in which the noise level is determined such that noise to be added to the reconstructed high-band results in a noise contents in the reconstructed high-band, which is similar to the noise content in the high-band of the original signal.

20. Encoder in accordance with claim 18, in which the noise estimator is operated to perform an analysis by synthesis approach for determining the noise level.

21. Encoder in accordance with claim 18, in which the noise estimator includes a decoder and is operative to assess a correct value of the amount of adaptive noise required.

22. Encoding method comprising:

encoding an audio signal to obtain an encoded signal, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, comprising:

estimating a level of noise to be added in a high-frequency regeneration process at a decoder; and

extracting a spectral envelope of the original signal to be used for adjusting a reconstructed high-band portion of the original signal.