Page 130 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 6: Data Communication

6.4.2.1 Sampling Notes
The amplitude of a signal is measured at regular intervals. The interval is designated as ts, and
is called the sample interval. The sample interval must be chosen to be short enough that the
signal does not change greatly between measurements. The sampling rate, which is the inverse of
the sample interval should be greater than twice the highest frequency component of the signal
which is being sampled. This sample rate is known as the Nyquist frequency. If you sample at a
lower rate, you run the risk of missing some information, known as aliasing.

6.4.2.2 Encoding
Once the samples are obtained, the must be encoded into binary. For a given number of bits,
each sample may take on only a finite number of values. This limits the resolution of the
sample. If more bits are used for each sample, then a higher degree of resolution is obtained.
For example, if the sampling is 8-bit, each sample may only take on 256 different values. 16-bit
sampling would give 65,536 unique values for the signal in each sample interval. Higher bit
sampling requires more storage for data and requires more bandwidth to transmit.
6.4.3 Digital Data with Digital Signals

We have already discussed how computers use a binary number system to perform operations. In its
simplest form, digital data is a collection of zeroes and ones, where the value at any one time is called
a bit. In order for two digital users (like computers) to communicate there must be an agreement on
the format used. There are several different ways in which a binary number by be formatted. This
is called pulse code modulation or PCM. The most straightforward PCM format is designated as
NRZ-L, for non return to zero level. In this format, the level directly represents the binary value:
low level = 0, high level = 1.
There are many other varieties, which are explained below:

(a) NRZ-M ( non return to zero mark). 1: no change in level from last pulse. 0: level changes
from last pulse.

(b) NRZ-S (non return to zero space). This is the same as NRZ-M but with the logic levels
reversed. 1: level changes from last pulse. 0: no change in level from last pulse.
(c) Bi-Phase-L (bi-phase level). The level always changes in the middle of the pulse. 1: level
changes from high to low. 0: level changes from low to high.

(d) Bi-Phase-M. (bi-phase mark). The level always changes at the beginning of each pulse.
1: level changes in the middle of the pulse. 0: no level change in the middle of the
pulse.
(e) Bi-Phase-S (bi-phase space). This is the same as Bi-Phase-L but with the logic levels reversed.
1: no level change in middle of pulse. 0: level changes in the middle of the pulse.

(f) DBi-Phase-M (differential bi-phase mark). The level always changes in the middle of the
pulse. 1: no level change at beginning of the pulse. 0: level change at beginning of the
pulse.

(g) DBi-Phase-S (differential bi-phase space). This is the same as DBi-phase-M but with the
logic levels reversed. 1: level change at beginning of the pulse. 0: no level change at the
beginning of the pulse.

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