Digital Signal Processing Reference
In-Depth Information
Chapter 11
Digital transmission technology l: source encoding
Modern microelectronics offers applications in digital signal processing (DSP) which a
few years ago would not have been thought possible. Mobile telephony is only one
example, the global internet another. The general public is not so aware of the fascinating
applications of medical technology and the radio and television technology of the future:
Digital Audio Broadcasting
DAB and
Digital Video Broadcasting
DVB.
A single new development of this kind can completely transform the market, ruin compa-
nies or catapult them forwards. Take the example of ADSL.
ADSL (asymmetric digital subscriber line) is a new development of this kind which
makes a data highway out of the old copper twin wire of an underground cable. ASDL
transports via a traditional telephone line up to 125 times that of an ISDN channel, i.e. up
to 8 Mbit/s from the network to the subscriber and roughly 700 kBit/s from the subscriber
to the network. The original ISDN channel on this line remains in existence and can be
used additionally. This is sufficient for 4 digital television channels which can reach the
subscriber simultaneously in real time.
The good old telephone line had been declared defunct. Investments by Deutsche
Telekom on a scale of 250 bn euros would have been necessary to connect every house-
hold by glass fibre as a medium of transmission. ASDL will make these huge investments
unnecessary in the foreseeable future and the competitors of Deutsche Telekom are in
for a rough ride. It owns the “last mile” - the telephone line - to the subscriber.
Illustration 221 shows the huge appetite of multimedia applications for transmission
capacity in which pictures, animations, speech, and music apart from text are used. Video
applications have the biggest appetite. A so-called CCIR 601video signal requires 250
Mbit/s for the video data stream alone. The required bandwidth is more than even high
speed networks can cope with. The ATM technology which is mainly used at the moment
- the data from the transmitters are split up into small data packages of 53 Byte and input-
ted into a transmission channel in the sequence in which they arrive (asynchronic transfer
mode) - usually provides only 155 Mbit/s.
DSP (digital signal processing) provides the solution. (Mathematical) signal theory has
created procedures which make possible the effective compression of data. In addition, it
offers solutions as to how data can be protected efficiently from interference such as
noise. Both processes - compression and error protection encoding - can be applied to
real signals using the computer. They have for instance made it possible to transmit
needle-sharp video pictures from the furthest stars of our solar system over many millions
of kilometers. The transmitter of the space probe had a power of only 6 W.
