Medias Mediatrix 1204, Справочное руководство

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Voice & Fax Codecs Reference Manual (SIP Version)
Mediatrix 1204 81
Analog Modem
The Mediatrix 1204 can send modem transmissions in clear channel (G.726). It supports 9.6 kbps to 33.6 kbps
analog modems (V.34 support over clear channel). If configured adequately, modems with higher rate
capabilities (for instance, V.90) will automatically fall back in the transmission range supported, which is
usually near 33.6 kbps.
Quality of modem transmissions is dependent upon the system configuration, quality of the analog lines, as
well as the number of analog-to-digital and digital-to-analog conversions. Modem performance may therefore
be reduced below the optimum values stated above.
G.723.1
Specified in ITU-T Recommendation G.723.1, dual-rate speech coder for multimedia communications
transmitting at 5.3 kbit/s and 6.3 kbit/s. This Recommendation specifies a coded representation that can be
used to compress the speech signal component of multi-media services at a very low bit rate. The audio is
encoded in 30 ms frames.
A G.723.1 frame can be one of three sizes: 24 octets (6.3 kb/s frame), 20 octets (5.3 kb/s frame), or 4 octets.
These 4-octet frames are called SID frames (Silence Insertion Descriptor) and are used to specify comfort
noise parameters.
G.729
Specified in ITU-T Recommendation G.729, coding of speech at 8 kbit/s using conjugate structure-algebraic
code excited linear prediction (CS-ACELP). For all data rates, the sampling frequency (and RTP timestamp
clock rate) is 8000 Hz.
A voice activity detector (VAD) and comfort noise generator (CNG) algorithm in Annex B of G.729 is
recommended for digital simultaneous voice and data applications; they can be used in conjunction with G.729
or G.729 Annex A. A G.729 or G.729 Annex A frame contains 10 octets, while the G.729 Annex B comfort
noise frame occupies 2 octets.
Comfort noise Supports comfort noise as defined in draft-ietf-avt-rtp-cn-05.txt . See
“G.726 Codecs Parameters” on page 87 for more details.
Table 49: G.726 Features (Continued)
Feature Description
Caution:
The Mediatrix 1204 cannot use G.723 and G.726 at the same time due to memory size limitation.
You must restart the Mediatrix 1204 when enabling or disabling these two codecs. If both G.723 and G.726
are enabled when you restart the Mediatrix 1204, G.723 is the codec used.
Table 50: G.723.1 Features
Feature Description
Packetization time Range of 30 ms to 120 ms with increment of 30 ms. See “G.723 Codec
Parameters” on page 87 for more details.
Voice Activity Detection (VAD) The Mediatrix 1204 supports the annex A. Annex A is the built-in support
of VAD in G.723.1.
Caution:
The Mediatrix 1204 cannot use G.723 and G.726 at the same time due to memory size limitation.
You must restart the Mediatrix 1204 when enabling or disabling these two codecs. If both G.723 and G.726
are enabled when you restart the Mediatrix 1204, G.723 is the codec used.