Selectable Mode Vocoder
In today's world, Selectable Mode Vocoder is an issue that has gained relevance in various areas of society. Whether in the personal, work, social or political sphere, Selectable Mode Vocoder has captured the attention of millions of people around the world. Its impact has generated debate, controversy and a growing interest in understanding its implications and consequences. From different perspectives, Selectable Mode Vocoder has managed to influence our lives in unexpected ways, challenging established paradigms and posing new challenges for the future. In this article, we will fully explore the impact of Selectable Mode Vocoder and how it has transformed our reality in recent years.
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Selectable Mode Vocoder (SMV) is variable bitrate speech coding standard used in CDMA2000 networks. SMV provides multiple modes of operation that are selected based on input speech characteristics.
The SMV for Wideband CDMA 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. The full rate and half rate are based on the CELP algorithm 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
The algorithm includes voice activity detection (VAD) followed by an elaborate frame classification scheme. Silence/background noise and stationary unvoiced frames are represented by 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 MOS 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 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 – 3G2.
References
- ^ a b c "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.
- ^ J. 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.[permanent dead link]
External links
- RFC 3558 - RTP Payload Format for Enhanced Variable Rate Codecs (EVRC) and Selectable Mode Vocoders (SMV)