Fundamentals - The MPEG - page 21

designer chooses, can have the same performance as analog at much lower cost. Compression is one of the
techniques used to lower the cost, but it has the potential to lower the quality as well.
Any analog signal source can be characterized by a given useful bandwidth and signal-to-noise ratio. Video signals
have very wide bandwidth extending over several megaHertz but require only 50 dB or so SNR whereas audio
signals require only 20 kHz but need much better SNR.
Although there are a number of ways in which audio and video waveforms can be represented digitally, there is
one system, known as pulse code modulation (PCM) which is in virtually universal use. Figure 2.2 shows how PCM
works. Instead of being continuous, the time axis is represented in a discrete or stepwise manner. The waveform is
not carried by continuous representation, but by measurement at regular intervals. This process is called sampling
and the frequency with which samples are taken is called the sampling rate or sampling frequency F s . The
sampling rate is generally fixed and is not necessarily a function of any frequency in the signal, although in
component video it will be line-locked for convenience. If every effort is made to rid the sampling clock of jitter, or
time instability, every sample will be made at an exactly even time step. Clearly if there are any subsequent
timebase errors, the instants at which samples arrive will be changed and the effect can be detected. If samples
arrive at some destination with an irregular timebase, the effect can be eliminated by storing the samples
temporarily in a memory and reading them out using a stable, locally generated clock. This process is called
timebase correction which all properly engineered digital systems employ. It should be stressed that sampling is an
analog process. Each sample still varies infinitely as the original waveform did.
Figure 2.2: In pulse code modulation (PCM) the analog waveform is measured periodically at the sampling rate.
The voltage (represented here by the height) of each sample is then described by a whole number. The whole
numbers are stored or transmitted rather than the waveform itself.
Figure 2.2 also shows that each sample is also discrete, or represented in a stepwise manner. The length of the
sample, which will be proportional to the voltage of the waveform, is represented by a whole number. This process
is known as quantizing and results in an approximation, but the size of the error can be controlled until it is
negligible. If, for example, we were to measure the height of humans to the nearest metre, virtually all adults would
register two metres high and obvious difficulties would result. These are generally overcome by measuring height
to the nearest centimetre. Clearly there is no advantage in going further and expressing our height in a whole
number of millimetres or even micrometres. An appropriate resolution can be found just as readily for audio or
video, and greater accuracy is not beneficial. The link between quality and sample resolution is explored later in
this chapter. The advantage of using whole numbers is that they are not prone to drift. If a whole number can be
carried from one place to another without numerical error, it has not changed at all. By describing waveforms
numerically, the original information has been expressed in a way which is better able to resist unwanted changes.
Essentially, digital systems carry the original waveform numerically. The number of the sample is an analog of time,
and the magnitude of the sample is an analog of the signal voltage. As both axes of the waveform are discrete, the
waveform can be accurately restored from numbers as if it were being drawn on graph paper. If we require greater
accuracy, we simply choose paper with smaller squares. Clearly more numbers are required and each one could
change over a larger range.
Discrete numbers are used to represent the value of samples so that they can readily be transmitted or processed
by binary logic. There are two ways in which binary signals can be used to carry sample data. When each digit of
the binary number is carried on a separate wire this is called parallel transmission. The state of the wires changes
at the sampling rate. This approach is used in the parallel video interfaces, as video needs a relatively short
wordlength; eight or ten bits. Using multiple wires is cumbersome where a long wordlength is in use, and a single
wire can be used where successive digits from each sample are sent serially. This is the definition of pulse code
modulation. Clearly the clock frequency must now be higher than the sampling rate.