AT&T MERLIN LEGEND System Reference Manual page 472

Functional Description
Signal Processing
Signal Conversion
Quantizing
Input/Output Bus. The I/O bus contains a 16-bit address bus and an 8-bit
data bus. The address bus selects the module that receives instructions from
the 68000 microprocessor in the processor module. The microprocessor
provides instructions to the port processors and DSEs through the 8-bit data
bus.
Time-Division Multiplex Bus. The TDM bus connects the DSEs to allow
voice or data to flow in and out of the system. The TDM bus is parallel, 8 bits
wide, and runs at 2.048 MHz (256 time slots x 8 kHz = 2.048 MHz). Each
TDM cycle has 256 time slots for voice, data, tones, and clocks. The frame
repetition rate is 8 kHz, providing a 64-kbps channel on each time slot (8-bit
bus x 8 kHz = 64 kbps).
The built-in data modem connects to the TDM bus; this permits access from
a local or remote PC or workstation equipped with a 1200-bps modem. The
TDM bus connects with the built-in diagnostics that enable the processor to
read and write to dedicated TDM test slots.
Voice signals on the TDM bus are encoded in Mu-Law 255 pulse code
modulation (PCM) format for domestic use. The system provides a circuit-
switched connection for transmission of digital data signals up to 64 kbps.
Because the system is internally a digital system in a world of both analog and
digital devices, it must accurately translate analog signals. Doing this involves
signal conversion, quantizing, commanding, and switching.
The first step in analog-to-digital conversion is to sample the analog signal by
taking samples of the height of the analog waveform at 8000 times per second.
This sampling is called pulse amplitude modulation (PAM). Because the analog
signal is sampled at more than twice its bandwidth (for analog voice the
bandwidth is 3300 Hz), both the amplitude and frequency content of the original
analog signal are preserved. The amplitude of each sample is digitally encoded
and transmitted. At the receiving end, the digital signals are decoded and then
reconstructed to their original analog form by the analog-to-digital converters
(codecs) in the system.
A codec digitizes each sample of the analog signal into a distinct 8-bit PCM
binary code (per CCITTG.711).
The digitally encoded signals are routed from one interface port to another
interface port by assigning source and destination to specific time slots on the
TDM bus. In this way, signals can be transmitted to one or several destinations
and reinstructed at the original amplitude. The result is no signal loss during
switching and transmission from one point to another.
The process of converting PAM samples into discrete PCM values is called
quantizing. In the case of voice-grade signals, 8 bits are commonly used to
encode one PAM sample. Quantizing with 8 bits allows 256 unique values to
represent the range of amplitudes in the analog signal being sampled.
Control Unit Operation 4-3