Universal Speech and Audio Codec

Linear Prediction Domain processing

Philippe Gournay, Bruno Bessette, Roch Lefebvre Université de Sherbrooke Département de Génie Electrique et Informatique Sherbrooke, Québec, Canada

Outline

• • • The 3GPP AMR-WB+ Standard – Source of inspiration for LPD processing in USAC Changes brought to LPD processing – Forward Aliasing Cancellation – Frequency-Domain Noise Shaping – Other changes Conclusion – More efficient LPD processing – Better unification of LPD and non-LPD FD coders

Context

• The 3GPP AMR-WB+ Standard – Hybrid codec – Time (ACELP) and Frequency (TCX) Domain – Very efficient on speech and speech-over music contents

The AMR-WB+ Encoder Audio ACELP 1 frame Mode Selection TCX 1, 2 or 4 frames

Mode Index, ISF

Bitstream

AMR-WB+ Frame Structure (a) (c) ACELP (b) Short TCX ACELP ACELP Long TCX ACELP ACELP Medium TCX

One super-frame = 1024 samples

• Three out of the 26 possible ACELP/TCX coding configurations

Transitions from ACELP to TCX • Zero-input response (ZIR) of LPC weighting filter provides pseudo-windowing Decoded TCX window ACELP Frame 1/8 overlap

Transitions from TCX to ACELP

• Redundant windowed TCX samples are discarded Decoded TCX window Frame

Limitations of the AMR-WB+ model • • • Non-critically sampled transforms – FFT vs. MDCT Inefficiencies at transitions between modes – Sub-optimal windowing (from ACELP to TCX) – Discarded samples (from TCX to ACELP) – Transform windows not aligned with ACELP grid – LPC analysis window also shifted to the right Even worse when switching with AAC – Time-Domain Aliasing Cancellation (TDAC) – Transitions between LPD and non-LPD processing

Changes brought to the LPD processing • • • • Replaced FFTs by MDCTs Introduced Frequency Domain Noise Shaping Introduced Forward Aliasing Cancellation Other changes

Frequency Domain Noise Shaping • • • To unify processing of AAC and TCX frames, the MDCT transform in TCX is applied in the original signal domain Noise shaping for TCX frames is performed in the MDCT domain based on LPC filters mapped to the MDCT domain FDNS allows a smooth (sample-by-sample) time domain noise envelope by applying a 1st-order filtering to the MDCT coefficients (similar in principle to TNS)

Effect of FDNS on the spectral shape and the time-domain envelope of the noise Noise gains g 1 [m] calculated at time position A A C Interpolated gains seen in the time domain, for each of the M bands Noise gains g 2 [m] calculated at time position B B time axis (n)

Frequency-Domain Noise Shaping • FDNS allows a smooth (sample-by-sample) time domain noise envelope by applying a 1st-order filtering to the MDCT coefficients (similar in principle to TNS)

Forward Aliasing Cancellation • Introduced to compensate windowing and time domain aliasing in MDCT-coded switching to and from ACELP frames frames when Windowing effect and Time Domain Aliasing ACELP synthesis TCX frame output + Next ACELP frame

Forward Aliasing Cancellation • • • FAC is applied in the original signal domain FAC is quantized in the LPC weighted domain so that quantization noises of FAC and decoded MDCT are of the same nature For transition from ACELP to TCX, the ACELP synthesis can be taken into account; this reduces the bitrate needed to encode FAC

Computation of FAC targets for transitions from and to ACELP (encoder) + + LPC1 LPC2 Signal in the original domain Line 1 ACELP synthesis TCX frame output Windowed and folded ACELP synth Windowed ACELP ZIR ACELP contribution + ACELP error TCX frame error (including ACELP contribution) + Next ACELP frame Line 2 Line 3 Line 4 FAC target FAC target

Quantization of FAC targets LPC1 Filter memory (ACELP error) Zero memory LPC1 1/W 1 (z) ZIR W 1 (z) DCT-IV Q DCT-IV -1 1/W 1 (z) Transmit to decoder FAC target Transition from ACELP to TCX LPC2 Filter memory (TCX frame error) Zero memory FAC synthesis LPC2 W 2 (z) DCT-IV Q DCT-IV -1 1/W 2 (z) FAC synthesis FAC target Transmit to decoder Transition from TC to ACELP

Other changes brought to the LPD processing • • • Critical sampling – MDCT vs. FFT – FAC+FDNS Scalar quantizer + adaptive arithmetic coder for TCX (AMR-WB+ uses AVQ) Variable bit rate – LPC quantizer – Bit reservoir adaptation

Conclusion • • • USAC makes use of LPD and non-LPD processing – LPD mode inspired by AMR-WB+ – Non-LPD mode derived from AAC Substantial changes were brought to the LPD processing, and new tools were introduced to make it more efficient – Frequency Domain Noise Shaping (FDNS) – Forward Aliasing Cancellation (FAC) USAC is a real unification of two coding models