Vector Sum Excited Linear Prediction
Encyclopedia
Vector sum excited linear prediction (VSELP) is a speech coding
method used in several cellular standards. The VSELP algorithm is an analysis-by-synthesis coding technique and belongs to the class of speech coding algorithms known as CELP (Code Excited Linear Prediction).
Variations of this codec
have been used in several 2G
cellular telephony standards, including IS-54, IS-136 (D-AMPS), GSM (Half Rate
speech) and iDEN
. 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.
D-AMPS (IS-54 and IS-136) 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.
GSM half-rate VSELP (GSM 06.20) uses 20 ms frames with 112 bits per frame, giving a raw data rate of 5.6 kbit/s. The iDEN VSELP coder has three modes:
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.
VSELP is also the encoding method used in some early Motorola Type II trunking systems. It was phased out in favor of the newer IMBE
method.
Speech coding
Speech coding is the application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting...
method used in several cellular standards. The VSELP algorithm is an analysis-by-synthesis coding technique and belongs to the class of speech coding algorithms known as CELP (Code Excited Linear Prediction).
Variations of this codec
Codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
have been used in several 2G
2G
2G is short for second-generation wireless telephone technology. Second generation 2G cellular telecom networks were commercially launched on the GSM standard in Finland by Radiolinja in 1991...
cellular telephony standards, including IS-54, IS-136 (D-AMPS), GSM (Half Rate
Half Rate
Half Rate is a speech coding system for GSM, developed in the early 1990s.Since the codec, operating at 5.6 kbit/s, requires half the bandwidth of the Full Rate codec, network capacity for voice traffic is doubled, at the expense of audio quality. It is recommended to use this codec when the...
speech) and iDEN
Integrated Digital Enhanced Network
Integrated Digital Enhanced Network is a mobile telecommunications technology, developed by Motorola, which provides its users the benefits of a trunked radio and a cellular telephone...
. It was also used in the first version of RealAudio
RealAudio
RealAudio is a proprietary audio format developed by RealNetworks and first released in April 1995. It uses a variety of audio codecs, ranging from low-bitrate formats that can be used over dialup modems, to high-fidelity formats for music. It can also be used as a streaming audio format, that is...
for audio over the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
. The IS-54 VSELP standard was published by the Telecommunications Industry Association
Telecommunications Industry Association
The Telecommunications Industry Association is accredited by the American National Standards Institute to develop voluntary, consensus-based industry standards for a wide variety of ICT products, and currently represents nearly 400 companies...
in 1989.
D-AMPS (IS-54 and IS-136) VSELP specifies an encoding of each 20 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
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
Vocoder
A vocoder is an analysis/synthesis system, mostly used for speech. In the encoder, the input is passed through a multiband filter, each band is passed through an envelope follower, and the control signals from the envelope followers are communicated to the decoder...
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.
GSM half-rate VSELP (GSM 06.20) uses 20 ms frames with 112 bits per frame, giving a raw data rate of 5.6 kbit/s. The iDEN VSELP coder has three modes:
- 30 ms frames at 126 bits per frame with a raw data rate of 4.2 kbit/s,
- 22.5 ms frames at 99 bits per frame with a raw data rate of 4.4 kbit/s and
- 22.5 ms frames at 180 bits per frame with a raw data rate of 8.0 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.
VSELP is also the encoding method used in some early Motorola Type II trunking systems. It was phased out in favor of the newer IMBE
IMBE
Improved multi-band excitation is a proprietary vocoder developed by Digital Voice Systems, Inc. .It is the predecessor of their Advanced Multi-Band Excitation .- External links :*...
method.