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Baseband Pulse and Digital Signaling Chap. 3
delays of the reproduced speech at the system output, and have poorer speech quality. (For
more details about speech coders and the trade-off involved, see the literature [e.g., Gershio,
1994; Spanias, 1994; Budagavi and Gibson, 1998].)
3–9 TIME-DIVISION MULTIPLEXING †
DEFINITION. Time-division multiplexing (TDM) is the time interleaving of samples
from several sources so that the information from these sources can be transmitted serially
over a single communication channel.
Figure 3–35 illustrates the TDM concept as applied to three analog sources that are multiplexed
over a PCM system. For convenience, natural sampling is shown together with
the corresponding gated TDM PAM waveform. In practice, an electronic switch is used
for the commutation (sampler). In this example, the pulse width of the TDM PAM signal
is T s 3 = 1(3f s ), and the pulse width of the TDM PCM signal is T s (3n), where n is the
number of bits used in the PCM word. Here f s = 1T s denotes the frequency of rotation
for the commutator, and f s satisfies the Nyquist rate for the analog source with the
largest bandwidth. In some applications in which the bandwidth of the sources is
markedly different, the larger bandwidth sources may be connected to several switch
positions on the sampler so that they will be sampled more often than the smaller bandwidth
sources.
At the receiver, the decommutator (sampler) has to be synchronized with the incoming
waveform so that the PAM samples corresponding to source 1, for example, will appear on the
channel 1 output. This is called frame synchronization. Low-pass filters are used to reconstruct
the analog signals from the PAM samples. ISI resulting from poor channel filtering
would cause PCM samples from one channel to appear on another channel, even though perfect
bit and frame synchronization were maintained. Feedthrough of one channel’s signal into
another channel is called crosstalk.
Frame Synchronization
Frame synchronization is needed at the TDM receiver so that the received multiplexed data can
be sorted and directed to the appropriate output channel. The frame sync can be provided to the
receiver demultiplexer (demux) circuit either by sending a frame sync signal from the transmitter
over a separate channel or by deriving the frame sync from the TDM signal itself.
Because the implementation of the first approach is obvious, we will concentrate on that of the
latter approach, which is usually more economical, since a separate sync channel is not needed.
As illustrated in Fig. 3–36, frame sync may be multiplexed along with the information words
in an N-channel TDM system by transmitting a unique K-bit sync word at the beginning of
† In this method, a common channel or system is shared by many users. Other methods for sharing a common
communication system are discussed under the topic of multiple access techniques in Sec. 8–5.