Ivan Methuselah's Digi-Box Ration Book

As with any sort of radio or TV broadcasting, digital telly is just a load of modulated electrons being farted through space. The modulation of choice is a geometrically advanced form of AM called QAM (quadrature amplitude modulation) which is far too elaborate to go into. The only real difference between analogue and digital broadcasting, save a bit of ghost-killing error-correction, is that you can pack more TV channels down the same tube. Think different coloured pulses being flashed down an optic fibre if you're very hi-tech, or just a load of thin wires down a bit of old drain-pipe if you're not. Digital is all about getting more channels into your home down the same bit of sky. Rather than do this in a chaotic and untidy sort of way, the powers that be have provided some neat elastic bands...

Digital telly is whacked out of the mast in six analogue-channel-sized chunks called multiplexes (abbrev.: mux.). So Emley Moor, for instance, transmits analogue C5 on Ch37 UHF, Mux2 on Ch40, analogue C4 on Ch41, MuxA on 43, B1 on 44, MuxB on 46, I1 on 47, MuxD on 49, MxC on 50, B2 on 51 and Mux1 on 52. Each multiplex is a chunk of bandwidth containing an array of channels. When the space was first dished out, the old five analogue channels (plus S4C) got half a multiplex each, with the rest being sold off to ONdigital. Cue furious selling of bandwidth: C5 sold up its half to S4C who were later bought out by ITV, while the ONdigitals got bought up by the BBC and the transmitter company Crown Castle (aka National Grid). The current state of affairs is:

Multiplexes 2 and A are transmitted at QAM-64, with the rest going at QAM-16. QAM is, as already outlined, the means of hurling data across the sky, and the numerics can be reasonably likened to the FAT system of data storage. QAM-64 can squeeze in more bits per second than QAM-16 (more space-wires down the same tube of nothing), with the downside that all that data rammed into one space can lead to increased interference. Error correction is applied to DTV as it is to a CD, and with a decent signal QAM-64 should apparently pose no problem. But in these early days of digital a decent signal is not always assured, especially on a portable aerial. BBC research suggests that QAM-16 reaches 5% more of the population than the more fragile QAM-64. All this then is partly why the BBC channels come into the back bedroom with less of a fight than those of ITV and C4. Note that the increased bitrate resulting from a QAM-64 transmission is used to jemmy in more channels as opposed to upping the picture quality. It is anticipated that when the analogue signals are finally killed the QAM-16 multiplexes will go over to QAM-64 to make space for terrestrial HDTV.

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MULTIPLEX 1	BBC	QAM-16	12 GOP	18100 kb/s
BBC One						In this table, each channel is allocated a chunk of space which very very approximately reflects the ammount of bandwidth it occupies in the real world. Channels placed alongside each other in a single row share a section of bandwidth with their neighbours. In such cases each channel's allocation is manipulated on the fly according to the demands of their respective output.
BBC Two	BBC Three / CBBC		BBC News

MULTIPLEX 2	ITV / C4	QAM-64	12 GOP	24100 kb/s
ITV 1		ITV 2	ITV 3	ITV 4
Channel 4		More4	E4	Channel 4 +1

MULTIPLEX A	SDN (ITV)	QAM-64	var. GOP	24100 kb/s
Five		Five US	Fiver / Television X	Setanta / TUTV	Teachers' TV / TUTV
QVC	bid tv	price-drop tv	CITV / Gems / Nuts / TUTV

MULTIPLEX B	BBC	QAM-16	18 GOP	18100 kb/s
BBC Four / CBeebies		BBCi 301		BBCi 302 / Community		BBC Parliament

MULTIPLEX C	National Grid Wireless		QAM-16	18 GOP	18100 kb/s
Sky Three	Sky News		Sky Sports News
E4 +1	Dave / Smile

MULTIPLEX D	National Grid Wireless		QAM-16	var. GOP	18100 kb/s
Film4 / Gems / NLX	4Music		TMF		ITV 2 +1		Virgin 1 / UK History	Ideal World