Table of Contents
Advertisement
Voice Modules (VCMs & DPVs)
VCM Voice Compression
Three types of voice compression are available: ADPCM, VLBRV, and CELP. Voice compression
is intended for voice signals and not for modem or FAX tones.
•
ADPCM (Adaptive Differential Pulse Code Modulation) compression on basecard
provides ADPCM compression to 32, 24 or 16 kbps. ADPCM compression resides on the
basecard. ADPCM echo cancellation requires an optional echo canceller plug-in card that
provides echo cancellation for use on voice circuits with long delays; a built-in residual
echo suppressor provides additional attenuation of echo signals.
•
VLBRV (Very Low Bit Rate Voice) provides voice compression to bit rates of 9.6, 4.8,
and 2.4 kbps. VLBRV compression and echo cancellation is provided by a plug-in card.
•
CELP (Codebook Excited Linear Predictive) provides voice compression to bit rates of
9.6, 6.4, and 4.8 kbps. CELP compression and echo cancellation is provided by a plug-in
card.
•
The VLBRV and CELP types also have optional internal FAX modems.
Note
Echo cancellers should be disabled if passing modem or FAX signals. Refer to Chapter 2 for more
information on echo cancellation.
VCM Level Adjustment
It is important that you adjust the VF level of the voice modules correctly. A level that is too high
produces distortion, and a level that is too low produces poor volume and excessive noise.
The voice channel module has its transmit gain adjustment and base levels set by hardware switches
on the card
(Table
VCM Echo Cancellation
The subjective effect of echo depends on the magnitude of the signal reflection at a 2W/4W hybrid,
and the round-trip delay to the talker. An echo canceller can be used to prevent echoes. Most echo
cancellers also contain a residual echo suppressor to attenuate any echo signals that were not
completely canceled.
Note
The ADPCM VCM has an optional echo canceller plug-in which is recommended for round-trip circuit
delays of over 35 ms. The VLBRV and CELP VCMs have built-in echo cancellers.
The VCM's SLAC (Subscriber Line Audio Circuit) device has an adaptive hybrid balance feature.
This adapts within limits to the impedance of the 2-wire port, minimizing the magnitude of the
reflected signal. The adaptive hybrid may reduce the need for the echo canceller in applications with
round-trip delays of less than 35 ms, but its performance in increasing the echo path loss is not as
good as an echo canceller.
The adaptive hybrid may be enabled or disabled by means of a switch on the VCM basecard. It
should always be disabled if any echo canceller is present in the circuit, otherwise poor performance
may result. It is automatically disabled for VLBRV applications. The adaptive hybrid or echo
canceller may not work properly if modem tones are sent over a voice channel.
6-2
6-1). The receive gain adjustment is set at the TMS Controller.
Office Connunications Manager OCM-1000/OCM-2000
Installation & Operation
VCM Overview
036R340-000
Issue 16
Advertisement
Table of Contents
