Media5 Mediatrix 4104, Справочное руководство

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Chapter 15 - Voice Transmissions Codec Descriptions
230 Mediatrix 4104
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 4104 can send modem transmissions in clear channel (G.726). If configured adequately,
modems with higher rate capabilities (for instance, V.90) will automatically fall back in the transmission range
supported
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.
Table 144: 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 232 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 242 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 244 for more details.
Table 145: 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 232 for more details.
Voice Activity Detection (VAD) The Mediatrix 4104 supports the annex A. Annex A is the built-in support
of VAD in G.723.1.