Medias Mediatrix 1204, Справочное руководство

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Chapter 16 - Voice Transmissions Codec Descriptions
200 Mediatrix 1204
G.726
Specified in ITU-T Recommendation G.726: 40, 32, 24, 16 kbit/s adaptive differential pulse code modulation
(ADPCM). It describes the algorithm recommended for conversion of a single 64 kbit/s A-law or U-law PCM
channel encoded at 8000 samples/sec to and from a 40, 32, 24, or 16 kbit/s channel. The conversion is applied
to the PCM stream using an Adaptive Differential Pulse Code Modulation (ADPCM) transcoding technique.
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.
Table 125: G.726 Features
Feature Description
Packetization time Range of 10 ms to 100 ms with increment of 10 ms. The preferred is
20 ms. See
“Packetization Time” on page 203 for more details.
Voice Activity Detection (VAD) Uses the G.711 VAD settings. Can be enabled or disabled. When
enabled, two levels of detection are available: transparent or
conservative. See
“G.711 and G.726 VAD” on page 210 for more details.
Comfort noise Uses the G.711 comfort noise settings. Supports white and custom
comfort noise as defined in
RFC 3389 . See “Comfort Noise” on
page 211 for more details.
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 126: G.723.1 Features
Feature Description
Packetization time Range of 30 ms to 120 ms with increment of 30 ms. See “Packetization
Time” on page 203 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.