Vocoders

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http://wiki.multimedia.cx/index.php?title=Category:Vocoders

Vocoders are voice codecs— audio codecs that are optimized for coding human speech at very low bitrates.

There are 9 articles in this category, as of 2006.05.24.

ACELP.net: [verse] Voice codec created by VoiceAge. http://www.voiceage.com/acelpnet.php RealNetworks uses this codec under the name "RealAudio sipro".

Windows Media Audio Voice, the voice codec from Microsoft's line of Windows Media audio codecs, is actually ACELP.net as well.

GSM 06.10

GSM 06.10 is a GSM vocoder standard that also occurs in some multimedia files.

Nelly Moser

Speech codec found in older Flash Video files.

Qualcomm Voice Codec

Qualcomm Voice Codec is most commonly found in Apple QuickTime. The version in QuickTime is not the same as the standardized one.

Qualcomm PureVoice is standardized as TIA IS-733, specs could be downloaded from the 3GPP2 specs site.

RealAudio 14.4

RealNetworks' 14.4 kbits/sec codec. Actual data rate is 8 kbits/sec. Equivalent to EIA/TIA IS-54 VSELP.

Speex

Speex is an open audio compression format designed for speech that claims to be patent-free. It is available under the Xiph variant of the BSD license.

Website: http://speex.org[]

VSELP

Equivalent to RealAudio 14.4.

ACELP

http://www.voiceage.com/prodacelpnet.php

ACELP.net is the preferred low bit rate speech codec in RealAudio and is widely deployed in both Windows Media Player and Audible ready equipment.

Technology

Encoded bandwidth

Dual-rate & fixed-rate: ~200-3400 Hz
Wideband: ~50-7000 Hz

Coding type

ACELP(r) (Algebraic Code Excited Linear Prediction)

Bit rate

Dual-rate: 8.5/6.5 kbps
Fixed-rate: 5.0 kbps
Wideband: 16 kbps

Delay (ms):

Frame size:

Dual-rate: 18
Fixed-rate: 30
Wideband: 10

Lookahead: Half a frame

Quality: Toll at 8.5 kbps; near-toll at 6.5 kbps

VSELP

http://en.wikipedia.org/wiki/VSELP

Vector Sum Excited Linear Prediction, or VSELP, is a speech coding method used in the IS-54 standard. This codec was used in early TDMA cell phones in the United States. It was also used in the first version of RealAudio for audio over the Internet. The IS-54 VSELP standard was published by the Telecommunications Industry Association in 1989.

IS-54 VSELP specifies an encoding of each 20 ms of speech into 159-bit frames, thus achieving a raw data rate of 7.95 kbit/s. In an actual TDMA cell phone, the vocoder output is packaged with error correction and signaling information, resulting in an over-the-air data rate of 16.2 kbit/s. For internet audio, each 159-bit frame is stored in 20 bytes, leaving 1 bit unused. The resulting file thus has a data rate of exactly 8 kbit/s.

A major drawback of VSELP is its limited ability to encode non-speech sounds, so that it performs poorly when encoding speech in the presence of background noise. For this reason, use of VSELP has been gradually phased out in favor of newer codecs.