London DSO

Retuning is only required on most modern equipment for RF Channel moves only. changes to LCNs and the services on each of the multiplexes being configured in the background (of most kit) and on switch-on and EPG population with the rest.

Slightly unfair. Most SD digital set top boxes (Sky Digital and Freeview) came with a default 4:3 Centre-Cut setting on their output - long after 16:9 displays were in the majority.

Not so, a couple of Sony Bravias I 'look after' did actually manage to find all the channels again automatically after both stages of DSO at Hannington.

However, I had to bring them out of standby on the morning after DSO, for them to 'see' the presently stored RF positions had gone, then return them to standby for 5 mins. Everytime you put a Bravia into standby, then it performs an RF scan and if things have changed it rearranges etc the stored channels, so when it's next brought out of standby, everything is back to normal.

Of course, if you don't know it does that, and that's how it works, it's not very useful for a DSO style situation !
My mother in law reported BBC 1 had gone from her bedroom telly on DSO 1 morning, I turned up, switched the TV on, and of course there it was !


It is good however for any new channels that appear, because within a couple of standby cycles, those channels get invisibly added,

It's not exactly a scientific example but I've found virtually every Freeview set top box or integrated TV I've set up in the last few years has offered the 4:3/16:9 option as part of the set-up process, so at least viewers get given the option (rather than assuming that 4:3 is the default).

I've noticed on a couple of TVs recently that the options are called normal and wide. I wonder how many people just want a normal picture so choose that without really thinking or understanding what they are selecting. On my 2011 Sony Bravia, you have to hunt through the menus just to get it to switch for 4:3 content, by default it shows 4:3 content stretched to 16:9. There is no reason for it to be set like that at all.

All the multiplexes broadcast information about all the other multiplexes being transmitted from the same transmitter, including their frequencies, which I've found most modern TV/receivers pick up on and retune themselves to when required. That of course assumes you tune to a channel it can still receive however, as of course it needs to tune to one of the multiplexes to pick up the information on the others.

RF data is NOT transmitted, it can't be because relay stations have no abilty to alter the transport stream of their parent. PIDs are cross referred between muxes in the SI data, that's the only method available for receivers to know of alternative services, but they must perform an RF scan to work out what's where

While you are right about the issue of relays, I can assure you that they do transmit RF data regardless, as part of the NIT. To show this below is an analysis of the raw data being transmitted right now on PSB1 from the Winter Hill transmitter, highlighting the frequency data for all the other muxes from the trasmitter. This is without providing it information about the other muxes, so the data it shows is purely from what it is receiving from PSB1.



As you can see, the only mux it doesn't carry RF data for is the HD mux. Therefore I assume TVs that can retune themselves store the RF data and compare it to its own tuning results, so that if the frequencies match, it can then follow this data to look for changes.

Very interesting, and thanks for taking the trouble to grab that screen shot. However I still wonder whether that's not just your software usefully filling in the RF detail. Can your receiver see DVB-T2 signals, because if not that could explain the absence of the RF data for that mux? I have access to colleagues that design digital TV tuners, I'll get a definitive answer on Monday for us all!

I take it there are 7 muxes listed, because one is the local Channel M one?

I highly doubt it, as although the application I am using (Transedit) is available to users of DVB Viewer, it is standalone and can be used without tuning channels in DVB Viewer (I had to manually enter the frequency for PSB1 for it to analyse it). Also the seventh mux is the Channel M one as you suggest, which I don't have tuned in as the reception is patchy for me.

That's of course not to say it isn't using some sort of intelligence to work out the frequencies, but it doesn't seem like it to me. The ability to transmit that information is certainly available as part of the DVB-T standard regardless (I'm a broadcast engineering student so I've recently studied it in quite some detail).

The frequency information can be carried in the NIT (Network Information Table) and I believe it can include main and slave transmitter frequencies to cope with relay situations (i.e. identical data streams being carried on different frequencies).

More info in ETSI EN 300 468 which talks about 'transposers' within the same 'cell'.

Well, someone I know who sets up the coders for UK DVB-T sites tells me this:-

"The NIT only contains the centre frequency for the main station, this is DVB mandatory. But no relays are listed as alternates, though a flag is set to say alternative frequencies are in use. But these frequencies are not declared."

I stand corrected, you were quite right Orry.