Enhancing perceptual performance of SBR and related HFR coding methods by adaptive noise-floor addition and noise substitution limiting
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.
1. 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 high 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 a high-frequency reconstruction to obtain a high-frequency reconstructed signal including a reconstructed high band portion of the original signal, comprising:
a high-frequency reconstructor for generating a reconstructed high band from the decoded signal; and
a noise adder for adaptively adding noise to the reconstructed high band, wherein the noise adder is operative to add such a noise level that a high-frequency reconstructed signal having a noise content similar to the noise content of the original signal is obtained.
2. Apparatus in accordance with claim 1 , in which the noise adder is operative to shape noise in accordance to a spectral envelope representation of the high band and add the shaped noise at such a level to the high-frequency reconstructed signal that the high-frequency reconstructed signal has a noise content similar to the noise content of the original signal.
3. Apparatus in accordance with claim 1 , in which the noise adder is operative to obtain a measure of the amount of adaptive noise and to add an amount of noise to the reconstructed high band, the amount being determined by the measure of the amount of adaptive noise.
4. The apparatus in accordance with claim 3 , in which the measure of noise is a noise floor level, and in which the noise adder is operative to add noise in accordance with the noise floor level.
5. Apparatus in accordance with claim 1 , further comprising a high band adjuster, which is operative to adjust the regenerated high frequency signal to obtain a correct total signal level after adding the noise to the signal.
6. Apparatus in accordance with claim 5 , in which the high band adjuster is operative to use an adjustment factor as defined below:
adjustFactor
(
k
,
l
)
=
1
1
+
nf
(
k
,
l
)
wherein adjustFactor is an adjustment factor, k is a frequency band index, l is a time index, and nf is a noise-floor level.
7. 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 high 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 a high-frequency reconstruction to obtain a high-frequency reconstructed signal including a reconstructed high band portion of the original signal, comprising:
generating a reconstructed high band from the decoded signal; and
adaptively adding noise to the reconstructed high band, wherein such a noise level is added that a high-frequency reconstructed signal having a noise content similar to the noise content of the original signal is obtained.