Digital transmission technology l: source encoding - Signals, Processes, and Systems - page 344

Decoding also starts with the standard lexicon described. It again contains the entries from
0 to 255. In our example "L" is again the longest known "chain of signs". It is therefore
given and stored in the variable P (as in "prefix"). The next entry is also a known sign "Z".
This is first stored as the variable C.

Now the content of P and C is bracketed and the result "LZ" included in the lexicon under
the index number (256). The whole thing proceeds in this way and the lexicon and the
original input sign sequence are created by means of the decoding algorithm.

Try encoding and then decoding a different input sign chain according to the LZW
principle. Only then will you notice that the input sign chain must have a special form for
compression to be carried out quickly and efficiently ("the rain in Spain falls mainly on
the plain").

Source encoding of audio signals
A/D and D/A conversion was already dealt with in Chapter 7 (section "Quantisation") and
above all in Chapter 9. Illustration 184 shows the principle of an A/D converter/encoder,
which outputs "measurement" serially as a sequence of 5 bit strings of numbers. From a
technical point of view at present 8 bit to 24 bit A/D and D/A converters are usual.

This process is generally referred to as PCM (pulse code modulation). Every measure-
ment is allocated - as in ASCII encoding - a code of the same length independent of the
frequency of occurrence. According to Illustration 221 a transmission rate of 172 kbyte/s
(roughly 620 Mbytes per hour) results for an audio-stereo-signal.

The compression strategies described up to now ought to be applicable to audio-signals.
These are, after all, available as a sequence of bit patterns. Usually processes are used in
A/D conversion which themselves have a "compression effect". In addition, with this
process the share of analog circuitry is reduced and is replaced by DSP (digital signal
processing).

Delta encoding or delta modulation
So-called screencam videos are very popular in order for example to record the
installation or use of a programme in the form of a screen video. This is also used
frequently on the CD accompanying this manuscript. Most of the time the cursor with
which menu items are clicked on to simply moves around the screen.

It would not make sense here to save the entire content of the screen 25 times a second.

Note:

in mathematics and in the technical and scientific field the Greek
capital letter

Δ is used to describe a difference or change. Thus, Δ t

is the time difference between two points t 1 and t 2 .

The notation Δ -modulation is more usual than delta modulation.