[Stooky Bill]

Was the Vaizey Plan put out by the DCMS before DSO or after it?

I think I recall having a discussion about it when a lot of us were members of TV Forum. I'm quite stunned about the amount of decoders that are in use out there to transmit all of those TV stations at the moment. Going from what I have read in your post; are those 450 decoders are just for Freeview only or are they for other platforms as well?

Encoders not decoders. 

Yes it's surprising and something I hadn't appreciated until fairly recently...

Because the DTT muxes are statistically multiplexed (each service changes it bitrate according to need) each service needs to be encoded seperately for each version of the MUX. So for example even though Channel 5 HD for example is identical in every region, it still needs encoding seperately for every region. Otherwise if suddenly Look North was requiring more bits but elsewhere in the country something else needed more then it can cope with that. 

So for example if there's 17 versions of a MUX and there's 9 services on each that's 153 seperate encoding processes. Then double that so there's some resilience!

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My understanding is that they'll have one DVB-S transponder (for the legacy SD services) and the rest will be switched to DVB-S2 (for the HD ones)

This was what I thought would happen but the tweet above claims otherwise.
See the first image

And later in the year, SD versions of BBC channels on satellite will close entirely.

That's not contrary to what I said. 

The closure 'later in the year' will be the remaining DVB-S transponder I mentioned