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. 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 interpolator for interpolating spectral envelope data, the spectral envelope data including a scale factor for a group of filterbank channels, the group having a plurality of filterbank channels, to obtain an interpolated scale factor for each filterbank channel in the group of filterbank channels;
a transposer for generating the high-frequency reconstructed signal using the decoded signal;
an analyzer for analyzing the high-frequency reconstructed signal to obtain a scale factor for each filterbank channel in the group of filterbank channels;
an amplification factor calculator for calculating an amplification factor for each filterbank channel in the group using the scale factors and the interpolated scale factors; and
an adjuster for adjusting a spectral envelope of the high-frequency reconstructed signal using the amplification factors for the channels in the group.
2. Apparatus in accordance with claim 1 , in which the interpolator is operative to assign the scale factor for a group of channels to each channel in the group.
3. Apparatus in accordance with claim 1 , in which the adjuster is operative to limit or smooth calculated amplification factors.
4. A method of 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:
interpolating spectral envelope data, the spectral envelope data including a scale factor for a group of filterbank channels, the group having a plurality of filterbank channels, to obtain an interpolated scale factor for each filterbank channel in the group of filterbank channels;
generating the high-frequency reconstructed signal using the decoded signal;
analyzing the high-frequency reconstructed signal to obtain a scale factor for each filterbank channel in the group of filterbank channels;
calculating an amplification factor for each filterbank channel in the group using the scale factors and the interpolated scale factors; and
adjusting a spectral envelope of the high-frequency reconstructed signal using the amplification factors for the channels in the group.