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SECTION 4
PRINCIPLES OF OPERATION
OVERVIEW
This section describes the general circuit functions of the CELP Voice
Channel Module. Signal flow and processing through the CELP Module is
described to show the relationship of the hardware and interconnection
with external equipment.
Analog voice information is directed into the transmit section of the
Base Card
CELP base card via a standard 600-ohm transformer coupled telephone
Operation
interface. This signal is sent to an analog level conditioning circuit.
The standard input transmit levels are –16.0 and 0 dBm. These levels are
selectable through a DIP switch and can be adjusted through hardware in
0.5-dB steps in the range of –6.0 dB to +1.5 dB.
This conditioned analog signal then goes through an
analog-to-digital
conversion process and is sent to the CELP piggyback card for bit rate re-
duction.
The reduced bit rate signal is received from the piggyback at 9.6, 6.4 or
4.8 kHz and then passed via an elastic buffer to the multiplexer transmit
data bus. The elastic buffer absorbs any jitter acquired in the data stream
before its passage into the multiplexer.
Receive voice data is sent from the multiplexer at the reduced bit rate and
passed through a receive elastic buffer (similar to the transmit buffer) on
its way to the CELP piggyback card.
The receive data comes from the piggyback as regular PCM data at 64
kHz. It is sent to the base card and converted to an analog signal. This
analog signal passes through a level conditioning circuit (the output pad),
where its level is adjusted before being output to the user via a standard
transformer coupled 600-ohm line. The output pad for the receive
analog
signal consists of three sections.
The standard receive output levels are 0 and +7.0 dBm. These levels are
selectable through a DIP switch and is adjusted through software in 0.5-dB
steps in the range –6.0 dB to +1.5 dB.
The -001 version of the CELP Voice Channel Module uses overhead con-
E&M Signaling
trol bandwidth to pass E&M signaling through the network. The amount
(-001 Version)
of control bandwidth required depends upon various parameters.
This method works fine for simple ON-HOOK/OFF-HOOK applications
(with either DTMF or no dialing). The amount of control bandwidth re-
quired is not critical in these applications.
For pulse dialing applications, extra overhead bandwidth is required if
the dial pulse distortion is required to meet certain standards (e.g.,
4-1
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