Digital Signal Processing Reference
In-Depth Information
Encoding and decoding of digital signals and data
The general expression for the numerical representation of symbols (e.g. letters or
measurements) and for the systematic modification of digital signals or data is the term
"
encoding
".
Illustration 222 shows an important example of encoding. In this "ASCII encoding" all
the important signs (symbols) in written communication are allocated a number between
0 and 127. 128 (= 27) different signs can only be coded in this way. A 7- Bit code is
sufficient for this purpose. Since 1963 this type of encoding has been a worldwide
standard for computers.
Note:
More than 128 different signs still cannot be transmitted via the internet and all
other computer networks. German umlauts (e.g. ä) and the sign "ß" are not included.
If only one bit more had been taken originally (8 Bit = 1 Byte) the possible number
of signs would have been twice as large. A 2 byte (16 bit) code (65,536 signs) would
have produced a really universal code for the electronic networking of the whole
world. But it is the good old 7 bit ASCII that makes its way through the networks.
The coded (digital) signal can be partly or completely transformed back into ins original
form in the receiver. These two processes are often called encoding and decoding. The
word code is used for the actual encoding process.
In digital transmission technology the term encoding is used above all in connection with
the following processes:
•
A/D and D/A converters (really A/D encoders)
•
Data compression
•
Error protection encoding
•
Coding of digital data
Compression
The compression of digital signals is the general expression for processes (algorithms) or
programs which transform a simple data format into an optimally compact format. In the
final analysis, the number of bits and bytes are to be reduced as far as possible. Decom-
pression reverses this process.
In this sense the ASCII encoding - see Illustration 222 - is badly encoded. Each of the
many symbols has the same length of 7 bits, no matter whether it occurs frequently or
very, very rarely. Thus, in text the space separating two words is by far the most frequent.
Small letters are more frequent than capitals. The letter "e" is certainly more frequent than
"x".
