GatesAir PTR-255 Program Audio Codec Module
Intraplex Products Issue 2.1, February 2006
audio mode. Also, at higher data rates there will be less deterioration of the audio
quality after multiple generations of encoding and decoding.
The tradeoff is that using more of the aggregate circuit (T1 or 2 MB) for the audio
leaves less room to carry other channels on the same circuit. When the PTR-255 is used
in an IntraLink ISDN shelf, the maximum bandwidth available is 128 kbps (2 ISDN B
channels).
2.4.2 G.722 Coding
The ITU-T G.722 audio encoding standard defines a method for encoding 7 kHz audio into 64
kbps. G.722 uses a form of sub-band Adaptive Differential Pulse Code Modulation (ADPCM)
audio compression, which provides end-to-end processing delay of less than 30 ms. To ensure
byte alignment when the card is used in an ISDN multiplexer, the G.722 decoder uses statistical
framing.
When the G.722 monaural mode is selected, only the channel 1 (left channel) audio input is
active. The monaural audio output appears on both audio channel output connectors.
G.722 allows for the transmission of an embedded auxiliary data channel at rates up to 9.6 kbps
(see Section 2.5, Auxiliary Data Channel). The auxiliary data channel is transmitted using some
of the bits from audio channel 1; thus, when the auxiliary data channel is active in G.722 mode,
the audio coding bitrate for channel 1 is reduced as illustrated in Table 2-3.
In G.722 dual monaural mode, there is still only one auxiliary data channel, associated with audio
channel 1. Audio channel 2 is always coded at the full 64 kbps.
Table 2-3
G.722 Modes
Maximum Aux Data Rate
(RS-232, asynchronous)
While not suitable for high fidelity program audio, G.722 coding is ideal for high quality voice
talkback circuits and for transmission of voice-based programming such as talk shows or sports
coverage.
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