[mythtv-users] DVB technical issues about scan

[Simon Hobson]

Philip Brady <Phil.Brady at hotmail.co.uk> wrote:

> that error recovery you suggest is true of the Internet but not of adjacent TV transmitters.
> 
> A single transmitter will typically transmit a number of frequencies = multiplexes.  Each frequency/multiplex will carry a number of channels.  It will send out packets of data 'BBC1 picture', BBC2 picture', BBC1 sound' etc.

Actually, part of many digital transmission is provision for single frequency networks. Certainly DAB is designed for that, and I vaguely recall DVB-T is as well. Ahh, a quick look reveals that it is :
https://en.wikipedia.org/wiki/Single-frequency_network#DVB-T_SFN
DVB-T uses OFDM - rather than a single high-speed data carrier, there are lots of narrow-frequency sub carriers, each of which carries a very low bit rate signal. Because the bit rate on any carrier is low, a significantly delayed echo can be accommodated without causing bits to overlap to the point where the signal can't be decoded. The delayed echo can be due to a reflection, or in a SFN it can be a weaker signal from a more distant transmitter.

Thus it's possible to transmit a multiplex across a wide area, from multiple transmitters, all on the same frequency. The main requirements are that the signals are very tightly synchronised, and that by the time you are far enough from a transmitter for it's signal to be too delayed for the signal to cope, it's signal strength is very low. The specific limit on delay depends on the bitrate of each subcarrier - so there's some tradeoff to be made in the transmission system. In case it isn't obvious, each transmitter must transmit **EXACTLY** the same bit stream.



> Here in the UK all BBC channels tend to be on one multiplex, all ITV on another, High definition on a third, and odd-balls on a fourth.

Actually, while BBC mainly has one mux to itself, the main re-digital channels (ie ITV, C4, C5) were put onto another mux. Partly this is so that for those users only able to get a signal from a restricted repeater (only repeating three muxes) would be able to get the "main" channels.

I strongly recommend https://ukfree.tv/maps/freeview for a wealth of information.