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. A method for enhancement of source coding systems using high-frequency reconstruction, where said source coding system comprises an encoder representing all operations performed prior to storage or transmission, and a decoder representing all operations performed after storage or transmission, characterized by:
at said encoder, estimating the noise-floor level of an original signal;
at said decoder, shaping random noise in accordance to a spectral envelope representation, and adjusting said noise in accordance to said noise-floor level estimated in said encoder;
at said decoder, adding said noise to the high-frequency reconstructed signal.
2. A method according to claim 1 , characterized in that said noise-floor level is mapped to several frequency bands.
3. A method according to claim 1 , characterized in that said noise-floor level is represented using LPC, or any other polynomial representation.
4. A method according to claim 1 , characterized in that said noise-floor level is estimated using dip- and peak- followers applied to a spectral representation of said original signal.
5. A method according to claim 1 , characterized in that said noise-floor level is smoothed in time and/or frequency.
6. A method according to claim 1 , characterized in that the level of said noise generated in the decoder is smoothed in time and/or frequency.
7. A method according to claim 1 , characterized in that the spectral envelope of said high-frequency reconstructed signal is adjusted using limiting of the envelope adjustment amplification factors.
8. A method according to claim 1 , characterized in that the spectral envelope of said high-frequency reconstructed signal is adjusted using interpolation.
9. A method according to claim 1 , characterized in that the spectral envelope of said high-frequency reconstructed signal is adjusted using smoothing of the envelope adjustment amplification factors.
10. A method according to claim 1 , characterized in that the high-frequency reconstruction generates a signal which is the sum of several high-frequency reconstructed signals, originating from different lowband frequency ranges, and that an envelope adjustment device analyses said lowband and provides control data to said summation.
11. An apparatus for enhancement of source coding systems using high-frequency reconstruction, the apparatus comprising:
an encoder for encoding a signal to be decoded by a decoder; and
an estimator for estimating the noise-floor level of an original signal.
12. An apparatus for enhancement of source coding systems using high-frequency reconstruction, where said apparatus comprises a decoder, for decoding a coded signal encoded by an encoder, characterized by:
means for shaping random noise in accordance to a spectral envelope representation, and adjusting said noise in accordance to said noise-floor level estimated in said encoder;
means for adding said noise to the high-frequency reconstructed signal.