Sharath Manjunath

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.).

Multimode variable bit rate speech coding: an efficient paradigm for high-quality low-rate representation of speech signal

The speech signal consists of a time-varying ensemble of different types of segments with distinct characteristics, which require different degrees of coding resolution in order to retain an overall high voice quality. A fixed-rate coder can capture such time-varying characteristics only if it operates at a high enough bit rate. At a low bit rate, a fixed-rate coder will not be able to capture all of these various segments well and will fail to render high voice quality. A multimode variable bit rate (VBR) coder uses an arsenal of modes, operating at different bit rates. These modes are designed to represent these different speech segments optimally with the right amount of coding resolution. Thus, a multimode VBR codec adapts the coding mechanism to the input speech and delivers high quality at low (average) rates. This paper presents the essential framework and the unique advantages of a multimode VBR codec and suggests algorithms for the different modes.

Upfront intra-refresh decision for low-complexity wireless video telephony

An adaptive intra-refresh (IR) technique is proposed for low-complexity video encoding on resource-constrained wireless platforms. The IR decision is made upfront without requiring any pre-encoding, which significantly reduces the complexity and power consumption in real-time communication. To allow upfront mode decision, a novel closed-form solution is provided requiring no algorithmic iteration or exhaustive searches. The IR scheme adapts to multiple factors including the content texture information, the frame-to-frame pixel-value variation, and the estimated channel loss probability. The macroblock-based refresh can be performed using various patterns (i.e., cyclic, random etc.). Experimental results demonstrate superior performance of the adaptive scheme over fixed schemes at all channel loss rates