From Wikipedia, the free encyclopedia
| This article
may need to be rewritten
to comply with Wikipedia's
quality standards
, as too confusing, outline due to outline being non existent altogether.
You can help
. The
talk page
may contain suggestions.
(
May 2009
)
|
Selectable Mode Vocoder
(
SMV
) is
variable bitrate
speech coding
standard used in
CDMA2000
networks.
[1]
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.
[1]
The full rate and half rate are based on the
CELP
algorithm
[1]
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
[2]
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
[
edit
]
External links
[
edit
]
- RFC
3558
- RTP Payload Format for Enhanced Variable Rate Codecs (EVRC) and Selectable Mode Vocoders (SMV)