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`!Selectable Mode Vocoder`! (`!SMV`!) is `F33f`_`[variable bitrate`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Variable_bitrate]`_`f `F33f`_`[speech coding`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Speech_coding]`_`f standard used in `F33f`_`[CDMA2000`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=CDMA2000]`_`f networks.`:cite-ref-3gpp2-smv-1-0[`F5bf`_`[1`#cite-note-3gpp2-smv-1]`_`f] SMV provides multiple modes of operation that are selected based on input speech characteristics.
>>Contents
• `F0af`_`[Technical specification`#technical-specification]`_`f
• `F0af`_`[Codecs`#codecs]`_`f
• `F0af`_`[Algorithm`#algorithm]`_`f
• `F0af`_`[References`#references]`_`f
• `F0af`_`[External links`#external-links]`_`f
-─
>>Technical specification
>>>Codecs
The SMV for `F33f`_`[Wideband`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Wideband]`_`f `F33f`_`[CDMA`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Code-division_multiple_access]`_`f is based on 4 codecs: full rate at 8.5 kbit/s, half rate at 4 kbit/s, quarter rate at 2 kbit/s, and eighth rate at 800 bit/s.`:cite-ref-3gpp2-smv-1-1[`F5bf`_`[1`#cite-note-3gpp2-smv-1]`_`f] The full rate and half rate are based on the `F33f`_`[CELP`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Code_Excited_Linear_Prediction]`_`f `F33f`_`[algorithm`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Algorithm]`_`f`:cite-ref-3gpp2-smv-1-2[`F5bf`_`[1`#cite-note-3gpp2-smv-1]`_`f] 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 `F33f`_`[voice activity detection`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Voice_activity_detection]`_`f (VAD) followed by an elaborate `F33f`_`[frame`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Frame_(Artificial_intelligence)]`_`f classification scheme. Silence/background noise and stationary unvoiced frames are represented by `F33f`_`[spectrum`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Frequency_spectrum]`_`f-`F33f`_`[modulated`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Modulated]`_`f 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 `F33f`_`[MOS`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Mean_Opinion_Score]`_`f`:cite-ref-nokiamos-2-0[`F5bf`_`[2`#cite-note-nokiamos-2]`_`f] at full rate with clean speech.
The coder works on a frame of 160 speech samples (20 ms) and requires a look ahead of 80 samples (10 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 ms with noise-suppression option B and 33 ms with noise-suppression option A.
The next evolution of CDMA speech codecs is `F33f`_`[VMR-WB`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=VMR-WB]`_`f which provides much higher speech quality with `F33f`_`[wideband`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Wideband]`_`f while fitting to the same networks.
SMV can be also used in 3GPP2 container file format – `F33f`_`[3G2`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=3GP]`_`f.
>>References
`:cite-note-3gpp2-smv-1`!1.`! `F0af`_`[↑`#cite-ref-3gpp2-smv-1-0]`_`f "3GPP2 C.S0030-0 Selectable Mode Vocoder (SMV) Service Option for Wideband Spread Spectrum Communication Systems" (PDF). 3rd Generation Partnership Project 2. 2004. Archived from the original (PDF) on 2011-07-23. Retrieved 2009-05-26.
`:cite-note-nokiamos-2`!2.`! `F0af`_`[↑`#cite-ref-nokiamos-2-0]`_`f `:citerefj-makinenp-ojalah-toukomaa`aJ. Makinen; P. Ojala; H. Toukomaa. "Performance Comparison of Source Controlled GSM AMR and SMV Vocoders" (PDF). Nokia Research Center, Multimedia Technologies Laboratory. Retrieved 2009-05-26.
>>External links
• RFC 3558 - RTP Payload Format for Enhanced Variable Rate Codecs (EVRC) and Selectable Mode Vocoders (SMV)
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