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Baseband Pulse and Digital Signaling Chap. 3
filtering. In a similar way, PPM may be converted to PAM by using the clock pulse to reset the
integrator to zero and start the integration. The PPM pulse is then used to stop the integration.
The final value of the ramp is the PAM sample that is used to regenerate the analog signal.
Pulse time modulation signaling is not widely used to communicate across channels,
because a relatively wide bandwidth channel is needed, especially for PPM. However, PTM
signals may be found internally in digital communications terminal equipment. The spectra of
PTM signals are quite difficult to evaluate because of the nonlinear nature of the modulation
[Rowe, 1965]. The main advantage of PTM signals is that they have a great immunity to additive
noise compared to PAM signaling, and they are easier to generate and detect than PCM,
which requires ADCs.
3–12 SUMMARY
In this study of baseband digital signaling, we concentrated on four major topics: (1) how the
information in analog waveforms can be represented by digital signaling, (2) how to compute
the spectra for line codes, (3) how filtering of the digital signal, due to the communication
channel, affects our ability to recover the digital information at the receiver [i.e., the intersymbol
interference (ISI) problem], and (4) how we can merge the information from several
sources into one digital signal by using time-division multiplexing (TDM). The United States
and worldwide standards for TDM telecommunications systems were given.
PCM is an analog-to-digital conversion scheme that involves three basic operations: (1)
sampling a bandlimited analog signal, (2) quantizing the analog samples into M discrete values
and (3) encoding each sample value into an n-bit word where M = 2 n . There are two
sources of noise in the signal that is recovered at the receiver output: (1) quantizing noise due
to the approximation of the sample values using the M allowed values and (2) noise due to receiver
bit detection errors caused by channel noise or by ISI that arises because of improper
channel frequency response. If the original analog signal is not strictly bandlimited, there will
be a third noise component on the receiver output due to aliasing.
In studying the effect of improper channel filtering in producing ISI, the raised cosinerolloff
Nyquist filter was examined. Here it was found that the minimum bandwidth required
to pass a digital signal without ISI was equal to one-half of the baud rate. A channel bandwidth
equal to the baud rate (r = 1) was found to be more realistic.
The channel bandwidth may be reduced if multilevel signal techniques are used (for a
given data rate R).
This chapter has focused on baseband signaling. In the next chapter, we will be concerned
with modulating baseband signals onto a carrier so that the spectrum will be concentrated
about some desired frequency called the carrier frequency.
3–13 STUDY-AID EXAMPLES
SA3–1 PAM Signal Spectrum and Bandwidth An analog waveform, w(t) is converted
into a flat-topped PAM signal by using a sampling rate of 8 kHz and a pulse width of 100 µs.
Assume that W(f) = 2 (fB), where B = 3 kHz.