The second BBC VHF radio transmitter.

[Synchrodyne]

This Wireless World 1972 September article recorded the start of the BBC PCM distribution system:

   

Cheers,

Steve 

[ppppenguin]

I wonder what sort of circuit the GPO provided for carrying the BBC PCM signals and how they were encoded for that channel. It's probably in the BBC R&D reports somewhere.

I remmber reading Hi Fi News back then. Angus Mackenzie used to report on the BBC failings and successes in this area. i don't know how he got information about regional quality as he lived just over a mile from me in north london.

[Terry]

Didn't I read somewhere - possibly on here - recently that the BBC were constrained by the GPO's monopoly - which was jealously guarded - which meant that the BBC were unable to use alternative distribution methods so long as the GPO could provide a link. The GPO's attitude was, basically, 'You get what your given, whether it's what you want or not!'

My college obtained an NTSC flying spot slide scanner from STC - allegedly fire damaged - following a fire at their Footscray factory although there was no evidence of any such damage. With the UK having decided to adopt PAL it was probably a good wheeze to get rid of it and, presumably, replace it with something more modern.

As the college had two 21" RCA NTSC colour sets they wanted to demonstrate them at the Dagenham Town Show using slides from the scanner back at the college. Fortunately there was line of sight between the the college and the show ground and someone sourced suitable microwave equipment for which the GPO issued a temporary licence.

When the GPO were asked to renew the licence the following year, the attitude was that the college had now had sufficient time to install a permanent (GPO!) link if they still needed it! I think it was grudgingly renewed but by the following year the colour service had started and colour was no longer such a novelty. In any case, they also had a 21"  Ekco NTSC set that I'd converted to simple PAL but, as I'd already left the college by then, I rather lost the plot.

[Hartley118]

I wonder what sort of circuit the GPO provided for carrying the BBC PCM signals and how they were encoded for that channel. It's probably in the BBC R&D reports somewhere.

I understand that there was a pretty close relationship between the BBC and PO Telephones back then.  The PO had installed a relatively broadband coax cable network for 405 line TV to the main VHF transmitters and ISTR this was used for the 13 bit digital audio network.

Martin

[ppppenguin]

It's certain that BBC and GPO worked closely to provide the necessary links. Renting a vision circuit to send audio must have been expensive unless there was a special deal, or multiple PCM signals were multiplexed into a video channel. Although multiple PCM audio signals will readily fit on a vision circuit there has to be some form of channel coding. Even if this is as simple as pulse shaping.

By 1978 it's clear that PCM distribution was well established: http://downloads.bbc.co.uk/rd/pubs/reports/1978-26.pdf

PCM was being considered in 1968, including sound in syncs: http://downloads.bbc.co.uk/rd/pubs/archi...aph_75.pdf

This report from 1970 shows it was possible to carry multiple stereo PCM audio signals over a single video link: http://downloads.bbc.co.uk/rd/pubs/reports/1970-34.pdf

This page has a lot of useful info: http://www.bbceng.info/Technical%20Revie...ts-fm.html

[Synchrodyne]

That’s an interesting bunch of articles.

Some sense as to how the BBC and the GPO worked together on programme distribution matters is provided in the final chapter, “The Line Interconnection System”, in the book “Sound and Television Broadcasting General Principles” by K.R. Sturley and published by Iliffe in 1961.

The changes wrought by the arrival of the GPO Tower and the microwave distribution system were covered in outline in Wireless World, including articles in 1964 December page 608ff and 1966 July page 336ff.  (See:  http://www.americanradiohistory.com/Wire...gazine.htm.)

Nothing was said in either article about high quality audio distribution by microwave, but I imagine that TV sound at least would have had to pass through the complex switching centre.  And some of the multitude of microwave telephone circuits might have been set up as music quality links.

Cheers,

Steve

[ppppenguin]

Sound in syncs didn't just guarantee audio quality in the distribution network. It also made sure that sound and vision always travelled together and never got separated. I don't think anyone ever seriously proposed it for transmission over the air as a means of adding high quality stereo. The main reason would be that most sync separators in TVs would be unable to cope with it. I also suspect it would have reduced the ability to sync accurately under poor reception conditions and it could easily have been wrecked by multipath. None of these is a problem in the distribution chain.