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Baseband Pulse and Digital Signaling Chap. 3
From source 1
2 kHz, analog
TDM
TDM
PAM signal
4 bit
PCM signal
16 ksamples/s A/D 64 kb/s
From source 2
f 1
f 2
4 kHz, analog
f 1 = 4 kHz
From source 3
2 kHz, analog
From source 4
7.2 kb/s, digital
Pulse
stuffing
8 kb/s
8 kb/s
From source 5
7.2 kb/s, digital
Pulse
stuffing
TDM PCM
Output signal
128 kb/s
f 2 = 8 kHz
From source 11
7.2 kb/s, digital
Pulse
stuffing
8 kb/s
Figure 3–39 TDM with analog and digital inputs as described in Example 3–17.
Now suppose that the analog sources will be converted into 4-bit PCM words and, for simplicity,
that frame sync will be provided via a separate channel and synchronous TDM lines are used. To
satisfy the Nyquist rate for the analog sources, sources 1, 2, and 3 need to be sampled at 4, 8, and
4 kHz, respectively. As shown in Fig. 3–39, this can be accomplished by rotating the first
commutator at f 1 = 4 kHz and sampling source 2 twice on each revolution. This produces a
16-ksampless TDM PAM signal on the commutator output. Each of the analog sample values is
converted into a 4-bit PCM word, so that the rate of the TDM PCM signal on the ADC output is
64 kbitss. The digital data on the ADC output may be merged with the data from the digital
sources by using a second commutator rotating at f 2 = 8 kHz and wired so that the 64-kbitss
PCM signal is present on 8 of 16 terminals. This arrangement provides an effective sampling rate
of 64 kbitss. On the other eight terminals, the digital sources are connected to provide a data
transfer rate of 8 kbitss for each source. Since the digital sources are supplying a 7.2-kbits data
stream, pulse stuffing is used to raise the source rate to 8 kbitss.
The preceding example illustrates the main advantage of TDM: It can easily accommodate
both analog and digital sources. Unfortunately, when analog signals are converted
to digital signals without redundancy reduction, they consume a great deal of digital system
capacity.