MULTIPLEXING
There is one programme stream multiplexer for each individual programme, and several (typically five) programme streams are combined in a transport stream multiplexer (Fig. 12.11 again) to form a multiplex which in transmission occupies a single channel slot of the same width as is used by a standard analogue transmission, e.g. 8 MHz for a terrestrial system, 27 MHz for a satellite transponder.
CONDITIONAL ACCESS
The MPEG-2 system layer makes provision for encryption of services, permitting programme providers to use electronic subscriber addressing and authorisation. Scrambling does not necessarily have to be used, and no specific encryption systems are specified, except that the transport stream packet header must remain in the clear so that users of the other services on the multiplex are not affected. The header contains a scrambling control field to indicate the encryption system in use; and an adaptation field for the conveyance of encryption keys and similar codes for the control of access to specific services by individual users.
MODULATION SYSTEMS
The three main media for DTV broadcasting and distribution are terrestrial transmitters, satellites and cable networks, and although all of them use phase-modulation each of them has different requirements of its modulation system for optimum results. A huge advantage of digital transmissions is their relative immunity to noise and interference. For a good picture, terrestrial a.m. broadcasts need a carrier-to-noise ratio of about 40 dB; for f.m. (satellite) analogue transmissions the ratio is 11–12 dB, while for digital reception it need be no more than 10 dB. The benefit can be reaped in terms of lower- power transmitters for the same coverage area; smaller receiving aerials; or a combination of both.
QPSK
Satellite DTV transmissions lend themselves best to the modulation system called QPSK, Quadrature Phase Shift Keying, which we met in Chapter 9 in connection with Nicam TV sound. As there, the bit- stream for transmission is broken down into bit-pairs or symbols, of which there are only four possible types: 00, 01, 10 and 11. Each type of symbol is represented by a different carrier phase as shown in Fig.
12.12. The rate of change of carrier phase depends on the data speed or symbol rate, typically 15–30 million per second, but in some cases much lower. The receiver needs to be programmed for the expected symbol rate in order to lock onto and interpret the datastream. The modulation system also incorporates Forward Error Correction (FEC) which inserts additional bits into the datastream. The FEC size is quoted as a fraction like ^ or _ the latter signifying that an additional bit is added to each three ‘real’ ones. The receiver must be programmed with this figure for each transmission it will receive so that it can make sense of the data being broadcast.
The QPSK transmission mode is optimum for satellite use, in which noise has the effect of ‘twisting’ the carrier vectors: here each vector has a wide phase tolerance in the receiver’s detector, which is in itself simple in principle, and relatively immune to noise in the received signal. The high carrier frequency involved with satellite transmis- sions affords a very fast symbol rate with a single carrier.