Editing Selectable Mode Vocoder

{{Short description|Speech codec used by CDMA2000 networks}}
{{multipleissues|
{{confusing|date=May 2009}}
{{technical|date=June 2024}}}}

'''Selectable Mode Vocoder''' ('''SMV''') is [[variable bitrate]] [[speech coding]] standard used in [[CDMA2000]] networks.<ref name="3gpp2 smv">{{Cite web|url=http://www.3gpp2.org/public_html/specs/C.S0030-0_v3.0_040325.pdf|title=3GPP2 C.S0030-0 Selectable Mode Vocoder (SMV) Service Option for Wideband Spread Spectrum Communication Systems|access-date=2009-05-26|publisher=3rd Generation Partnership Project 2|year=2004|format=PDF|url-status=dead|archive-url=https://web.archive.org/web/20110723044752/http://www.3gpp2.org/public_html/specs/C.S0030-0_v3.0_040325.pdf|archive-date=2011-07-23}}</ref> SMV provides multiple modes of operation that are selected based on input speech characteristics.
==Technical specification==
===Codecs===
The SMV for [[Wideband]] [[code-division multiple access|CDMA]] is based on 4 codecs: full rate at 8.5&nbsp;kbit/s, half rate at 4&nbsp;kbit/s, quarter rate at 2&nbsp;kbit/s, and eighth rate at 800&nbsp;bit/s.<ref name="3gpp2 smv"/> The full rate and half rate are based on the [[Code Excited Linear Prediction|CELP]] [[algorithm]]<ref name="3gpp2 smv"/> that is based on a combined closed-loop-open-loop-analysis (COLA). In SMV the signal frames are first classified as:

* Silence/Background noise
* Non-stationary unvoiced
* Stationary unvoiced
* Onset
* Non-stationary voiced
* Stationary voiced
===Algorithm===
The algorithm includes [[voice activity detection]] (VAD) followed by an elaborate [[Frame (Artificial intelligence)|frame]] classification scheme. Silence/background noise and stationary unvoiced frames are represented by [[frequency spectrum|spectrum]]-[[modulated]] noise and coded at 1/4 or 1/8 rate. The SMV uses 4 subframes for full rate and two/three subframes for half rate. The stochastic (fixed) codebook structure is also elaborate and uses sub-codebooks each tuned for a particular type of speech. The sub-codebooks have different degrees of pulse sparseness (more sparse for noise like excitation). SMV scores a high of 3.6 [[Mean Opinion Score|MOS]]<ref name="nokiamos">{{Cite web|url=http://europe.nokia.com/library/files/docs/Makinen2.pdf|title=Performance Comparison of Source Controlled GSM AMR and SMV Vocoders|access-date=2009-05-26|publisher=Nokia Research Center, Multimedia Technologies Laboratory|author1=J. Makinen|author2=P. Ojala|author3=H. Toukomaa|format=PDF}}{{dead link|date=May 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> at full rate with clean speech.

The coder works on a frame of 160 speech samples (20&nbsp;ms) and requires a look ahead of 80 samples (10&nbsp;ms) if noise-suppression option B is used. An additional 24 samples of look ahead is required if noise-suppression option A is used. So the algorithmic delay for the coder is 30&nbsp;ms with noise-suppression option B and 33&nbsp;ms with noise-suppression option A.

The next evolution of CDMA speech codecs is [[VMR-WB]] which provides much higher speech quality with [[wideband]] while fitting to the same networks.

SMV can be also used in 3GPP2 container file format – [[3GP|3G2]].

==References==
{{reflist}}

==External links==
* {{IETF RFC|3558|link=no}} - RTP Payload Format for Enhanced Variable Rate Codecs (EVRC) and Selectable Mode Vocoders (SMV)

{{Compression formats}}

[[Category:Speech codecs]]
[[Category:3rd Generation Partnership Project 2 standards]]
[[Category:Mobile telecommunications standards]]
[[Category:Code division multiple access]]