Originally Posted by Spot:
“Yes - though the muxes will typically be transmitted at lower power, so coverage will be more limited than other services from the same transmitter.”
Not necessarily. Digital is not like analogue, where you needed the same signal-to-noise ratio to get the same picture quality, regardless. For digital, you can trade off capacity for coverage: you can use a lower modulation density - fewer bits per carrier - and you can include a greater or lesser amount of redundant information, for error correction.
The two PSB standard-definition multiplexes (carrying the BBC channels on one, and ITV1, C4 and C5 on the other, plus some of ITV and C4's 'portfolio' channels) use 64QAM, FEC 2/3. The three commercial multiplexes use 64QAM, FEC 3/4.
QAM means 'quadrature-amplitude modulation'. The relative amplitude (size) of the carrier can be modified, and so can the phase (time-shift). The available options are QPSK (2 bits per carrier), 16QAM (4 bits per carrier) and 64QAM (6 bits per carrier). QPSK = Quadrature Phase Shift Keying; only the phase changes, not the size of each carrier. The more different steps there are, the better the signal-to-noise ratio has to be to accurately determine which one was meant.
FEC is Forward Error Correction. This means that the signal carries redundant information to allow the receiver to infer what the correct data is. The basic code is rate 1/2 - that is, two output bits are transmitted for each input bit - but the code can be 'punctured' to reduce the overhead. Rate 2/3 means three output bits are transmitted for each two input bits, rate 3/4 means four for every three, etc. You can say that rate 2/3 means the capacity is 2/3 of the potential, while rate 3/4 means the capacity is 3/4 of the potential, and so on. The more redundant information is transmitted, the easier it is to determine which value is correct, and the lower the signal-to-noise ratio required.
The mode used by the local TV services will depend on the winning multiplex operator. Generally all have gone for the QPSK 3/4 mode. This requires 7.2 dB of signal-to-noise ratio, according to the DVB-T standard, if the receiver has mostly line-of-sight to the transmitter, with some echoes. The equivalent for the PSB 64QAM 2/3 mode is 17.3 dB. That means that the local TV multiplex can be as much as 10 dB less powerful - that is, one-tenth the power - compared to the BBC and ITV/C4 multiplexes.