Page 79 - Concise Encyclopedia of Robotics

Digital to analog
The scheme for digital-to-analog (D/A) conversion depends on whether
the signal is binary or multilevel. The D/A conversion process is carried
out by a D/A converter (DAC).
In a binary DAC, a microprocessor reverses the A/D conversion
process done in recording or transmission. Multilevel digital signals can
be converted back to analog form by “smoothing out” the pulses. This
can be intuitively seen by examining Fig. 1. Imagine the train of pulses
being smoothed into the continuous curve. Data Conversion
Digital signals lend themselves to repeated reproduction without loss
of integrity. Digital signals are also relatively immune to the effects of
noise in wireless and long-distance cable circuits. For this reason, even if
the initial input and final output signals are analog in nature, such as
moving images or human voices, there are advantages to using digital
format in the intervening medium.
Digital signals can be clarified by means of digital signal processing
(DSP) to enhance the signal-to-noise (S/N) ratio, thereby minimizing the
number of communication errors and necessary bandwidth while maxi-
mizing the data transfer rate. This is true whether the ultimate input and
output signals are analog or digital.
Serial versus parallel
Binary data can be sent and received one bit at a time along a single line
or channel. This is serial data transmission. Higher data speeds can be
obtained by using multiple lines or a wideband channel, sending inde-
pendent sequences of bits (high and low, or 1 and 0) along each line or
subchannel. This is parallel data transmission.
In parallel-to-serial (P/S) conversion, bits are received from multiple
lines or channels, and transmitted one at a time along a single line or
channel. A buffer stores the bits from the parallel lines or channels
while they are awaiting transmission along the serial line or channel.
In serial-to-parallel (S/P) conversion, bits are received from a serial line
or channel, and sent in batches along several lines or channels. The
output of an S/P converter cannot go any faster than the input, but the
circuit is useful when it is necessary to interface between a serial-data
device and a parallel-data device.
Figure 2 illustrates a communications circuit in which a P/S converter
is used at the source and an S/P converter is used at the destination. In
this example, the data characters are 8-bit bytes; the illustration shows
the transfer of one character.

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