Digital Signal Processing Reference
In-Depth Information
Channel1
Channel 3
Channel 2
Channel 4
T
A
T
A
T
A
T
A
T
A
T
A
8-Bit
8-Bit
8-Bit
8-Bit
8-Bit
8-Bit
Channel
1
Channel
2
Channel
3
Channel
4
Time-
multiplex
4·8-Bit
4·8-Bit
4·8-Bit
4·8-Bit
4·8-Bit
4·8-Bit
Illustration 268:
Time-division multiplex process
In this case the digitalization of analog signals comprises four phases which are represented by means of
four analog signals: scanning, quantization, coding, and time-division multiplexing.
The four (parallel) channels are all scanned at the same scanning rate (e.g. f
A
= 8 kHz or T
A
= 125
s).
This, however, is done phase-shifted by 90 degrees each or time-shifted by T
A
/4. This results in a total
scanning rate of 4
<
μ
8 kHz = 32 kHz for the four channels.
Quantization and coding are executed in one stage. The result is available in the form of four digital - i.e.
time and value discrete - signals which in this case have a width of 8 bits. The basic pulse rate of each
individual digital signal is 32 kHz.
Time-division multiplexing is a parallel-serial conversion in which the transmission rate of 32 kbit/s is
quadrupled and increases to 128 kbit/s.
The receiver conducts this procedure in reverse order: first, demultiplexing, i.e. serial-parallel conversion
and then D/A conversion.
In telecommunications at least 30 different channels are combined using time-division multiplexing (PCM
30). Similar to frequency-division multiplexing, several small groups can be combined to form larger units
(e.g. PCM 120 etc).
