Phase 1 of Audetel had the target of NICAM as the delivery to the home - Using the 11Kbit/s auxilliary channel
- i think because the Broadcaster involved was ITV whose VBI for anciallry services was felt to be too small!
(the BBC had for some reasons refused to take part)
NICAM had an obvious probelm - it was not carried through a Regional centre (either BBC or Itv)
and and not obvious one - if you exercised the two top bits in the 10 bit signal some NICAM decoders made rude sounds in the Programme sound!
(A large number of rented TV Sest were recalled to be modified!)
and a subtle one - it was not tied to Video pulses!
Having got the Principles both technical and editorial sorted out ...
The BBC was reapproached by the ITC as Project managers to take part in phase 2 - which ended up with Pilot services
the BBC was a lot more interested (I think because different people were asked)
and so took part ..... and teletext was used -
Unsually for a EU funded project two methods of play out were allowed/used
ITV played out a File like subtitles which had the packets as pre encoded CELP audio
on the complemnetary line (22) to the subtitles on line 335
the BBC used a real time CELP encoder - Audio in CELP in Teletext out
The BBC aslo had far more complex techniques for combining Teletext
and also could use a line pair (20/333) for subtlkes thus could put AD muxed with subtitles!
(actaully if you think about it AD is rarely active when people are speaking
and a subtitles is a one shot say 4 packets for say 5 secs of programe dialogue
so subtitle packets are not there most of the time)
Subtitles got through the broadcaster infrastructure .....so this was good!
and BBC and ITV did a few hundred AD hours in the pilot!
And it was standardised in sect 6.7 of ETS 300 708
www.etsi.org
It used a stand alone decoder - made by Portset- this handed the Fade byte (and I think Pan)
but did not have reciever mix control but we wished it had!!!! Hence UK DTT having it!
It also to be a universal box had outputs which were remodulted UHF, Scart, Audio phonos and 3.5mm hedphones
all a bit too complex ....
With DTT coming rapdily over the Horizon it was decided that AD coudl be added to a normal TV / STB
and thus this was the route to go.... and UK DTT would not have teletext!
In the broadcasters infrastructure the BBC were going SDI with emebed audio for playout
so the AD on one track and the control tone on the other so Audio 3 &4 just whizzed through without anyone messing it up!
(Itv were still AES3 - which gave them some problems
and all AD police units etc needed both Audio and video Inputs and outputs!!)
Control tones meant that it worked as stereo signal over compressed routes!
After Audetel CELP coding was used for Talkback
- ITV used it for the breakfast contractor talkback over NICAM - but only 8 bit!!
and the BBC used it for News talkback over air after a incident where mobile phone network overloaed
but the mobile network got better so it was dicontinued.
BBC Wales had 128Kbit/sec APTX over Teletext for Welsh sound over radio links!
........... Some of fun I had in the 1990s!
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