Vivek Rajendran

EVRC-Wideband: The New 3GPP2 Wideband Vocoder Standard

In this paper, the latest wideband vocoder standard adopted by the cdma2000 standardization body, 3GPP2, is described. Christened enhanced variable rate codec-wideband (EVRC-WB), the proposed codec encodes wideband speech (16 KHz sampling frequency) at a maximum bit-rate of 8.55 kbit/s. EVRC-WB is based on a split band coding paradigm in which two different coding models are used for the signal components in the low frequency (LF) (0-4 KHz) and the high frequency (HF) (3.5-7 KHz) bands. The coding model used for the former is based on the EVRC-B narrowband (0-4 KHz) codec, modified to encode the LF band signal at a maximum bitrate of 7.75 kbit/s. The HF band coding model is a LPC based coding scheme where the excitation is derived from the coded LF band excitation using non-linear processing. Mean opinion scores from 3GPP2 characterization tests are provided to demonstrate that the EVRC-WB codec (8.55 kbit/s, max.) performs statistically significantly better than the adaptive multirate wideband (12.65 kbit/s, max.).

Overview of the EVS codec architecture

The recently standardized 3GPP codec for Enhanced Voice Services (EVS) offers new features and improvements for low-delay real-time communication systems. Based on a novel, switched low-delay speech/audio codec, the EVS codec contains various tools for better compression efficiency and higher quality for clean/noisy speech, mixed content and music, including support for wideband, super-wideband and full-band content. The EVS codec operates in a broad range of bitrates, is highly robust against packet loss and provides an AMR-WB interoperable mode for compatibility with existing systems. This paper gives an overview of the underlying architecture as well as the novel technologies in the EVS codec and presents listening test results showing the performance of the new codec in terms of compression and speech/audio quality.

Improved error resilience for volte and VoIP with 3GPP EVS channel aware coding

A highly error resilient mode of the newly standardized 3GPP EVS speech codec is described. Compared to the AMR-WB codec and other conversational codecs, the EVS channel aware mode offers significantly improved error resilience in voice communication over packet-switched networks such as Voice-over-IP (VoIP) and Voice-over-LTE (VoLTE). The error resilience is achieved using a form of in-band forward error correction. Source-controlled coding techniques are used to identify candidate speech frames for bitrate reduction, leaving spare bits for transmission of partial copies of prior frames such that a constant bit rate is maintained. The self-contained partial copies are used to improve the error robustness in case the original primary frame is lost or discarded due to late arrival. Subjective evaluation results from ITU-T P.800 Mean Opinion Score (MOS) tests are provided, showing improved quality under channel impairments as well as negligible impact to clean channel performance.

Super-wideband bandwidth extension for speech in the 3GPP EVS codec

This paper describes the time-domain bandwidth extension (TBE) framework employed to code wideband and super-wideband speech in the newly standardized 3GPP EVS codec. The TBE algorithm uses a nonlinear harmonic modeling technique that incorporates principles of time-domain envelope-modulated noise mixing. At 13.2 kbps, the super-wideband coding of speech uses as low as 1.55 kbps for encoding the spectral content from 6.4-14.4 kHz. Subjective evaluation results from ITU-T P.800 Mean Opinion Score (MOS) tests are provided, showing significantly improved quality compared to the other standardized SWB codecs under both clean speech and speech with background noise.

Low bit rate high-quality MDCT audio coding of the 3GPP EVS standard

This paper presents a low bit-rate MDCT coder, which is adopted as a part of the recently standardized codec for Enhanced Voice Services. To maximize codec performance for NB to SWB input signals for low bit-rates (7.2 to 16.4 kbps), new adaptive bit-allocation and spectrum quantization schemes, which emphasize perceptually important spectrum while efficiently coding full spectrum, was introduced into the low bit-rate MDCT coder. Further, small symbol switched Huffman coding is exploited for reducing the bits consumption for quantizing band energies of the spectrum. Finally, the performance of the coder is illustrated with some listening test results.

Recent Speech Coding Technologies and Standards

This chapter presents an overview of recent developments in conversational speech coding technologies, important new algorithmic advances, and recent standardization activities in ITU-T, 3GPP, 3GPP2, MPEG and IETF that offer a significantly improved user experience during voice calls on existing and future communication systems. User experience is determined by speech quality, hence network operators are very concerned about quality of speech coders. Operators are also concerned about capacity, hence coding efficiency is another important measure. Advanced speech coding technologies provide the capability to both improve coding efficiency and user experience. One option to improve quality is to extend the audio bandwidth from traditional narrowband to wideband (16 kHz sampling) and super-wideband (32 kHz sampling). Another method is in increasing the robustness of the coder against transmission errors. Error concealment algorithms are used which substitute the missing parts of the audio signal as far as possible. In packet-switched applications (VoIP systems), special mechanisms are included in jitter buffer management (JBM) algorithms to maximize sound quality. It is of high importance to ensure standardization and deployment of speech coders that meet quality expectations. As an example of this, we refer to the Enhanced Voice Services (EVS) project in 3GPP that is developing the next generation speech coder in 3GPP. The basic motivation for 3GPP to start the EVS project was to extend the path of codec evolution by providing super-wideband experience at around 13 kb/s and better quality for music and mixed content in conversational applications. Optimized behavior in VoIP applications is achieved through the introduction of high error robustness, jitter buffer management, inclusion of source-controlled variable bit rate operation, support of various audio bandwidths, and stereo.

Linear prediction based comfort noise generation in the EVS codec

A Discontinuous transmission (DTX) system, which is widely adopted in speech codecs, is an important function for speech communication systems that can reduce the transmission bandwidth by at least a half. Within a DTX system, the comfort noise generation (CNG) plays a key role in the overall quality. Critical performance parameters with respect to the CNG including the transition quality from active to comfort noise (CN) frame, the quality of CN spectrum estimation, wider bandwidth rendering and the DTX efficiency have all been found to be very important. This paper describes a series of new technologies developed for the EVS codec aiming to address the performance of the CNG: A new hangover based CN analysis technique provides improved CNG transition quality. A new entropy based CN spectrum estimation technique and a new hybrid CNG scheme improve the CN spectrum estimation. Finally, a novel bandwidth extension technique for efficient rendering of high-frequency CN and a novel technique improving the DTX efficiency by controlling the DTX hangover length are described.