GatesAir Intraplex PTR-255 - Page 16

GatesAir PTR-255 Program Audio Codec Module
Intraplex Products Issue 2.1, February 2006
12
When using the PTR-255 on an ISDN line to talk to another manufacturer’s codec, the choice of
MPEG Layer 2 or MPEG Layer 3 may be limited by the algorithm available on the far end codec;
where this is not the case, the user may select either algorithm for program audio.
Both MPEG Layer 2 and MPEG Layer 3 allow for the transmission of an embedded auxiliary
data channel (see Section 2.5, Auxiliary Data Channel).
The PTR-255 modules implement these MPEG algorithms in a way that allows the user to create
the optimum transmit signal for each application.
On the transmit side, the user must set all of the parameters described below. On the receive side,
it is only necessary to set the desired data rate. The card will automatically detect the coding
algorithm (MPEG Layer 2 or MPEG Layer 3), sample rate and audio mode. Depending on the
card settings, this auto-detection feature takes from 1 to 15 seconds; when auto-detection is
complete, the SYNC LED lights.
The user-settable parameters are:
1. The sample rate, which determines nominal audio bandwidth of the signal. Sampling
rate options are 32 and 48 ksps in MPEG Layer 2 and 16, 24, 32 and 48 ksps MPEG
Layer 3, providing nominal audio bandwidth of 7.5, 11, 15 and 20 kHz, respectively.
Note however, that while the nominal audio bandwidth is fully achieved when the data
rate is high enough, at very low bit rates the actual audio bandwidth may not reach the
nominal.
2. The audio mode. Mode options are monaural, dual monaural, stereo and joint stereo,
defined as follows:
 Monaural transmits a single audio channel. Either the left or the right input channel
or the sum of the two channels, can be selected for transmission. A received
monaural audio signal will be placed on both the left and right outputs.
 Dual monaural transmits two completely independent audio channels. For a given
level of compression, dual monaural requires twice the transmission bandwidth of
monaural.
 Stereo transmits discretely encoded, phase-aligned left and right channel audio.
Since the channels are encoded separately, this method requires the same bandwidth
as dual monaural; however, the number of bits allocated to each subband is the same
for both the left and right channels of a stereo signal.
 Joint stereo is a coding process that takes advantage of the fact that much of the
information at any given moment in a stereo music signal is the same on both the left
and right channels. By encoding this common information only once and then
adding the additional information that is unique to each channel, it enables higher
fidelity audio to be sent at lower data rates.
3. The transmission data rate, which is the amount of bandwidth on the aggregate circuit
devoted to carrying the encoded audio signal. Data rate options range from 64 kbps to
384 kbps (320 kbps maximum for MPEG Layer 3). The higher the data rate setting, the
less data compression takes place and the better the sound quality will be for any given