VMR-WB — Source-controlled Variable Bit Rate Wideband Compression The Variable Rate Multi-Mode Wideband (VMR-WB) codec is the mandatory speech codec for cdma2000® wideband telephony and multimedia streaming services. Designed by Nokia and VoiceAge, VMR-WB is based on the AMR-WB/G.722.2 codec. VMR-WB operates in the range from 0.8 to 13.3 kbps, including one mode of operation (mode 3) that is interoperable with AMR-WB/G.722.2 (at 12.65 kbps and below).

Topics on this page Quality Technical Background Rate Sets Operating Modes Narrowband Operation Technical Highlights Applications VMR-WB Standard for cdma2000® Telephony RTP Packet Format Definition

Quality

This table compares the quality of VMR-WB with source control at several average bit rates for a speech signal that is 60% active speech and 40% background noise/silence.

VMR-WB mode	VMR-WB at average bit rate	Quality	G.722 at fixed bit rate
1	5.5 kbps	=	56 kbps
2	4.5 kbps	>	48 kbps
3*	6.7 kbps	=	56 kbps

* Interoperable with AMR-WB/G.722.2.

Back to top

Technical Background
VMR-WB is a source-controlled variable bit rate (VBR) codec. The VBR speech coding concept is crucial for optimal operation of CDMA systems (such as cdmaOne and cdma2000). In source-controlled VBR coding, the codec operates at several bit rates, and a rate selection mechanism is used to determine the bit rate suitable for encoding each speech frame based on the characteristics of the speech signal (e.g., voiced, unvoiced, transient, background noise). The goal is to attain the best speech quality at a given average data rate (ADR). The codec can operate in different modes by tuning its rate selection algorithm to obtain different ADRs. Codec performance is improved by increasing the ADR. The system imposes the mode depending on the network capacity and the desired quality of service. This gives the codec a mechanism of trade-off between speech quality and system capacity.

Rate Sets
CDMA systems use rate sets comprised of 4 bit rates referred to as full-rate (FR), half-rate (HR), quarter-rate (QR), and eighth-rate (ER). CdmaOne and cdma2000 systems support two rate sets, referred to as Rate-Set I and Rate-Set II. The following table shows the source-coding bit rates and corresponding channel bit rates inclusive of error protection bits for variable-rate codecs with a rate selection mechanism operating in Rate-Set I and Rate-Set II.

Operating rates of a variable-rate codec with rate selection mechanism operating in Rate-Set I and Rate-Set II

	FR	HR	QR	ER
Rate-Set I
Source-coding bit rate (kbps)	8.55	4.0	2.0	0.8
Channel bit rate including error detection bits (kbps)	9.6	4.8	2.4	1.2
Rate-Set II
Source-coding bit rate (kbps)	13.3	6.2	2.7	1.0
Channel bit rate including error detection bits (kbps)	14.4	7.2	3.6	1.8

Back to top

Operating Modes
VMR-WB has four operating modes in Rate-Set II and one in Rate-Set I.

Modes 0 to 3 operate in Rate-Set II. Modes 0, 1, and 2 are specific to CDMA systems (i.e., TIA/EIA/IS-95, cdma2000) with mode 0 providing the highest quality and mode 2 the lowest ABR. Mode 3 is interoperable with AMR WB/G.722.2 at 12.65 kbps and below. Its ADR is slightly higher than the ABR of mode 0.

Mode 4 is compliant with cdma2000 Rate-Set I with an ADR slightly lower than mode 2 and with quality equivalent to or better than mode 2.

VMR-WB operating modes

VMR-WB mode	Description	Approximate speech average data rate (kbps)	Bit rate for 40% speech activity (kbps)
0	“Premium”	12.8	5.7
1	“Standard”	10.5	4.8
2	“Economy”	8.1	3.8
3	Interoperable with AMR-WB/G.722.2	13.3	6.1
4	Rate-Set I mode	~ 8.1	~ 3.8

The system selects one of the operational modes depending on the traffic conditions. The transition from one mode to another is seamless and memoryless (i.e., there is no transition period to achieve the ADR of the new mode), and there is no need to transmit the mode information to the decoder. The system can also force maximum and minimum rates.

Back to top

Narrowband Operation
The narrowband codecs developed in CDMA for Rate-Set I operation consist of QCELP8 (IS-95), EVRC (IS-127), and SMV. In Rate-Set II operation, QCELP13 (IS-733) is used.

While the design of VMR-WB focused on wideband input and output signals, VMR-WB accepts narrowband input signals sampled at 8 kHz, and it can synthesize narrowband speech. The codec lookahead varies depending on the sampling frequency of the input and the output, being 13.75 ms for wideband input and output and 15.0625 ms for narrowband input and output.

Technical Highlights

Applications