Fundamentals - The MPEG - page 34

Figure 2.23: Using a phase-locked loop, the H-sync pulses of input video can be used to derive the 27 MHz master
clock of MPEG and the 13.5 MHz and 6.75 MHz sampling clocks of 4:2:2 video.
The numerically locked loop is a digital relative of the phase-locked loop. Figure 2.24 shows that the input is an
intermittently transmitted value from a counter. The input count is compared with the value of a local count and the
difference is used to control the frequency of a local oscillator. Once lock is achieved, the local oscillator and the
remote oscillator will run at exactly the same frequency even though there is no continuous link between them.
Figure 2.24: In the numerically locked loop, the state of a master counter is intermittently sent. A local oscillator is
synchronized to the master oscillator by comparing the states of the local and master counters.
2.11 Quantizing
Quantizing is the process of expressing some infinitely variable quantity by discrete or stepped values. Quantizing
turns up in a remarkable number of everyday guises. Figure 2.25 shows that an inclined ramp enables infinitely
variable height to be achieved, whereas a stepladder allows only discrete heights to be had. A stepladder
quantizes height. When accountants round off sums of money to the nearest pound or dollar they are quantizing.
Figure 2.25: An analog parameter is continuous whereas a quantized parameter is restricted to certain values.
Here the sloping side of a ramp can be used to obtain any height whereas a ladder only allows discrete heights.
In audio the values to be quantized are infinitely variable voltages from an analog source. Strict quantizing is a
process which is restricted to the voltage domain only. For the purpose of studying the quantizing of a single
sample, time is assumed to stand still. This is achieved in practice either by the use of a track/hold circuit or the
adoption of a quantizer technology which operates before the sampling stage.
Figure 2.26 (a) shows that the process of quantizing divides the voltage range up into quantizing intervals Q . In
applications such as telephony these may be of differing size, but for digital audio and video the quantizing
intervals are made as identical as possible. If this is done, the binary numbers which result are truly proportional to
the original analog voltage, and the digital equivalents of filtering and gain changing can be performed by adding
and multiplying sample values. If the quantizing intervals are unequal this cannot be done. When all quantizing
intervals are the same, the term uniform quantizing is used. The term linear quantizing will be found, but this is, like
military intelligence, a contradiction in terms.