is the technique where the value of each sample (i.e., the modulating
signal) systematically changes the characteristics of a carrier
signal (e.g., amplitude (height) or frequency (timing)). The resulting
modulated wave "carries" the data. Conversely, removing the carrier
signal results in the return of the original measurement.
The TDM stream produced by the basic multiplexer
scheme is accomplished via Pulse Code Modulation or PCM. Three other modulation forms are also used: Pulse Duration Modulation
(PDM), Pulse Position Modulation (PPM), and Pulse Amplitude Modulation
(PAM). The resulting waveforms from these modulation techniques for a
simple analog data signal are shown below.
The PAM data stream signal is transmitted from the
multiplexer in a uniformly spaced sequence of constant-width pulses. The
intensity of each pulse is modulated by amplitude. This is similar to
AM radio broadcast, except the carrier is a pulse rather than a sine wave.
Since amplitudes are degraded by noise, the multiplexed data stream is
usually converted to a constant-amplitude pulse modulation scheme. PDM
carries the information in the pulse width, which varies directly to the
amplitude of the signal. PPM results if the PDM waveform is differentiated,
then rectified. The distance between the two pulses represents the sampled
amplitude of the sine wave, with the first pulse as the zero time reference.
Average system power for PPM is much lower than that required for PDM,
but at the expense of greater bandwidth.
Both PDM and PPM use constant-amplitude pulses, but are still analog
representations of an analog signal. In a PCM system, each pulse is encoded
into its binary equivalent before transmission. During PCM encoding, the
serial output stream is conditioned for the communication link. In many
cases, PCM data is not only transmitted, but also stored. When considering
recording or transmitting requirements, you must establish the patterns
used to represent logical one and zero values.
Over the years, a number of PCM codes have been designed to represent
logic one and zero levels while achieving the greatest performance for
a given application. These are shown below.