What is a clean feed and a dirty feed?

I think this thread's a good example of how the same term can have umpteen different meanings in television!

Since producers don't always pay as much attention as they ought to, distribution tapes are run through a bit of kit known as a legaliser before the transmission copy is made - which just makes sure that the video doesn't exceed the specified voltages, etc. (This is often done as part of a tech review, which will also check for flashing pictures which could be harmful for viewers with photosensitive epilepsy)
I heard a story about a Janet Street-Porter programme on BBC Two (possibly DEF II) which had its colours and brightness whacked right up beyond the legal limits; and fell off the air because of it. I'm sure someone here will be able to fill in the details about that...

A backup copy could just be a spare copy of a programme tape - as reliable as videotapes are these days, they still break sometimes. It's slightly different from a standby, though.
Any programme that's live or delivered late will have a standby programme scheduled against it, sometimes more. Live events (eg sport) will have various standbys of various durations, in case the programme finishes early or the OB goes down, or in case it over-runs and later programmes need to be dropped and replaced with something shorter. These could be anything that's considered appropriate for the timeslot (but invariably on the BBC seem to include Animal Park).
Sometimes a standby will be a 'best of' programme - Newsround generally has a Newsround Extra as a standby, and Watchdog has 25 Years of Watchdog. I think I've seen Airport as a standby for Question Time though, so it doesn't always make sense. There's even a standby tape for Two Minutes Silence.

Generally yes - for a start, you're going to have trouble getting more bandwidth than a van full of tapes. They're a fairly safe bet for long term storage, and they're not going to crash or be corrupted just by sitting in a warehouse (unless the roof falls in, or something ridiculous like that).
That said, TV playout is generally done from server, so programmes will be delivered on tape and ingested onto server, and Flextech are moving into a tapeless environment: they'll get a distribution tape, it'll be ingested, and they'll do all their compliance edits, promo-making, subtitling (etc), and playout from server copies, rather than passing round tapes.
It's not entirely tapeless, though - the MPEG files are moved on to data tapes, which are kept in a big automated data archive vault thing for long-term storage.

You can usually pick up the output of most of TV Centre's studios from within TVC.

Ah, that's an interesting point too - news VTs will be recorded with interviews and camera sound on one channel, and voiceover on another, then this will be mono-ised before transmission: which, as Mr Sands says, makes things easier for archive use.

ISO - Isolation feed. A recording of a camera direct, so that in an edit you have got a different picture to cut to if the main recording is not suitable.

Or Independent Seperate Output

Each studio has a main output and several ISO's

For instance News24 has a main output which is routed to the transmission chain.
A spare output incase the main feed fails.

And four ISO feeds

ISO4 is identical to main but has a BBC WORLD Tower burnt on. This is what world take overnight
ISO3 has a BBC News bug burnt on and the ticker keyed off. This is the feed News Online take.
ISO2 and ISO1 can be routed for sundry reasons.

The BBC World studio has a similar set up.
The ISO2 feed from BBC World is routed to Washington and is used to feed the Washington plasma. The director in london can therefore directly control what goes into the Washington Plasma.

The news Studios also have a clean output which is devoid of any astons or Live Bugs. This is what is recorded when an interview is replayed later.

I was surprised when I was at a BBC OB in Belfast where the audio the presenter and crowd of assembled people heard was the actual programme audio off BBC ONE itself mixed with the gallery sound, not only was the ident and continuity played through, but the sound was turned down once studio presenter had asked OB presenter a question and turned up again after.

That said, TV playout is generally done from server, so programmes will be delivered on tape and ingested onto server, and Flextech are moving into a tapeless environment: they'll get a distribution tape, it'll be ingested, and they'll do all their compliance edits, promo-making, subtitling (etc), and playout from server copies, rather than passing round tapes.
It's not entirely tapeless, though - the MPEG files are moved on to data tapes, which are kept in a big automated data archive vault thing for long-term storage.

I was surprised when I was at a BBC OB in Belfast where the audio the presenter and crowd of assembled people heard was the actual programme audio off BBC ONE itself mixed with the gallery sound, not only was the ident and continuity played through, but the sound was turned down once studio presenter had asked OB presenter a question and turned up again after.

That's Clean Feed/IFB/Mix-Minus. The sound isn't turned down when the OB presenter is speaking, the audio from the OB is subtracted from the programme sound by the mixing desk.

Despite it being touted and promised a lot, there are very few 'tapeless environments' - it's the televisual equivalent of a 'paperless office'.

The area I worked in 6 years ago was supposed to be tapeless, that place has now been closed and superseeded by something brand new that still isn't!

I've only seen one genuinely tapeless TV station and that's Bloomberg in London

BFBS TV is going tapeless

That is the 'VT' clock of the sting that opens the next part of the programme.

In that case it is a Matte - a sequence with holes which is super'd onto the picture from the studio camera not full screen.

The gallery will leave that there during the break and then run it 5 seconds before they're due on air. It doesn't have to be there during the break, it's just good practise so they don't forget, also the transmission bods can see that they are ready to go before they cut them to air