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Old 31st Oct 2015, 4:53 pm   #1
John_BS
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Default BBC 1940's line & telephone equipment

I've just scanned this BBC Technical Instruction, which was written during the second world war. There is reference to a "Gas-mask" jack in Figure 4!

For anyone interested in learning about hybrids, this is a good read.

John
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Old 31st Oct 2015, 9:47 pm   #2
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Default Re: BBC 1940's line & telephone equipment

I have a hard-bound copy of this: one of the original WWII issues. I hope you realise you have just contravened the Defence Regulations (Official Secrets Act!).

There's some interesting stuff in about Droitwich Programme Input Equipment, amongst other things.
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Old 1st Nov 2015, 5:14 pm   #3
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Default Re: BBC 1940's line & telephone equipment

I believe that it was also possible for any woman found using certain types of makeup to be prosecuted for witchcraft under English law until just a few years ago. Possibly the Defence Regulations have been repealed too.

Possibly at a lower level of official publication security, many Ministry Technical Instructions on AVO products were classified as "Restricted" despite almost exactly the same information being readily available commercially at the same time.

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Old 1st Nov 2015, 8:25 pm   #4
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Default Re: BBC 1940's line & telephone equipment

I suspect the aforementioned wording was appended to ALL BBC documents, training or otherwise, because 'that's how it was...'

I think we can sleep easy in our beds for discussing such matters, seeing as the likes of the Racal RA-17 EMERs - once 'Restricted' documents - are now in the public domain.

Incidentally, I was mistaken about the Droitwich info in John B. S's scanned document. My copy comes bound with Instruction TT7 (Droitwich) and a few others.
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Old 5th Nov 2015, 3:04 am   #5
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Default Re: BBC 1940's line & telephone equipment

Thanks. That BBC TI ST.6 document is certainly interesting!

I was intrigued to see that a two-wire, two-way repeater with a single hybrid and single valve was theoretically possible, although not realizable in practice because line impedance is a variable number.

In respect of the actual BBC two-way repeater, with two hybrids and two valves, one for each direction, 2400 Hz low-pass filters were used, of which it was said: "The limitation of the frequency range to 2,400 c/s increases stability by permitting greater gain before singing occurs, without impairing speech intelligibility.”

That reminded me of something I had seen in the book Sturley, “Sound and Television Broadcasting General Principles” (1961) some time back. In the chapter on the line interconnection system, bandwidths are quoted for various services, including 300 to 2300 Hz for BBC internal communication.

Given that the standard voice bandwidth (I think originally derived for the international telephone network in the 1930s) is 300 to 3400 Hz, the 2300 Hz upper limit seemed rather surprising. I understand that the basis for the standard 3400 Hz upper limit was that below this, intelligibility in the presence of noise is not improved.

In “defence” of the BBC 2300 Hz limit, Sturley said: “The BBC internal speech communication system uses a lower maximum frequency limit than the CCITT* standard because the more complete control possible allows a lower over-all loss; the reproduced signal is at a level comparable to that of the signal at the sending end and also has a good signal-to-noise ratio. Under these circumstances satisfactory intelligibility can still be achieved with the narrower band.”

In the light of the information about the 2400 Hz upper limit provided in ST.6, I’d say that the above justification was a post facto rationalization for a very restricted upper limit that was in fact required to enable the use of BBC’s own two-wire, two-way repeaters, and not because it was a “good idea” per se. (I suppose that by way of comparison, the GPO longlines would have been four-wire, with one-way repeaters.)

Incidentally, the CCITT telephone bandwidth was quoted as 300 to 3300 Hz, which shows some “shaving” of the 3400 Hz number, I suspect more for convenience (to get down to a 3.0 kHz actual bandwidth) than because of any re-evaluation of the original premise. Sometimes it is truncated to 3 kHz, as well. But 3400 Hz has an implied or derived precision that suggests that, basis whatever statistical analysis was done at the time, that it was closer to the optimum than say 3300 or 3500 Hz.

Of course, more recent work has shown that for teleconferencing, a 7 kHz upper audio frequency limit is required for good intelligibility without undue fatigue, and some suppliers provide such circuits. I suspect then that the original 3.4 kHz limit was predicated on the use of telephone handsets and one-to-one conversations, where word and phrase repeats are easily requested.

Cheers,
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Old 5th Nov 2015, 10:15 am   #6
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Default Re: BBC 1940's line & telephone equipment

in a very basic phone system for a local call over a strowger exchange and all copper connections there is loads of bandwidth available.

for long lines with 4-wire connections and unidirectional repeaters, bandwidth still doesn't cost extra.

It was when Frequency Division Multiplex came on the scene that bandwidth became expensive. Signals were converted in frequency and to single sideband form. Channel bndwidth directly set how many channels could be packed into the bandwidth of a coax cable, or the assigned bandwidth of a microwave link.

Did the BBC use its own repeaters because it wanted to place them at its own sites and so be independent of exchange supplies? So, as long as the leased lines were intact, they had a link?

Using a 2-wire connection would obviously have been priced less by the GPO. And the 2 wire line forced the hybrid-coupled amplifier scheme which forced poor bandwidth given the gain required.. Hmm?

David
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Old 5th Nov 2015, 11:28 am   #7
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Default Re: BBC 1940's line & telephone equipment

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Originally Posted by Radio Wrangler View Post
Did the BBC use its own repeaters because it wanted to place them at its own sites and so be independent of exchange supplies? So, as long as the leased lines were intact, they had a link?
GPO / BT was used to trunk the feeds to the sites. At Skelton the programme feeds, originally 'traditional' GPO copper, were replaced (1970s, I think - got it written down at home) by a 48kHz split-carrier FDM system until the late '80s as the system was to be phased out early '90s. The progs came in via the BT Harraby repeater station at Carlisle and Penrith exchange (diverse routes) and a further BT rep. station nearer the transmitting station.

In the '90s they went to BT fibre for a short time but BT charged an absolute fortune so an 'in-house' satellite global distribution system was created where BBC prog. would come in via satellite to the HF sites before rebroadcast on SW.

Of course, there were off-air programme distribution feeds (RBL / RBS) for TV outstation relays and sound-in-syncs for TV main sites - I don't believe ITV used SIS, having more money to pay for separate sound circuits.

Equalisation was carried out at BBC ends (but that would only be part of it) and prog. feeds for MF sites were distributed from main TV transmitter programme input equipment over BT / GPO circuits, and I remember doing line 'squeaks' from MF - TV sites. I didn't know what was between, but if anything, it would be BT.

Sorry to be so vague - it was a long time ago: my notes are at home and I'm not!
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Old 9th Nov 2015, 3:11 am   #8
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Default Re: BBC 1940's line & telephone equipment

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Originally Posted by Radio Wrangler View Post
in a very basic phone system for a local call over a strowger exchange and all copper connections there is loads of bandwidth available.

for long lines with 4-wire connections and unidirectional repeaters, bandwidth still doesn't cost extra.

It was when Frequency Division Multiplex came on the scene that bandwidth became expensive. Signals were converted in frequency and to single sideband form. Channel bndwidth directly set how many channels could be packed into the bandwidth of a coax cable, or the assigned bandwidth of a microwave link.
So it would appear that a definite and internationally agreed upper frequency limit became a necessity once FDM links were established. After 3400 Hz was established as the international upper frequency limit, I shouldn’t be too surprised if it then became a general target, with for example longish lines being loaded to reach 3400 Hz but not go much beyond.

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Originally Posted by Radio Wrangler View Post
Using a 2-wire connection would obviously have been priced less by the GPO. And the 2 wire line forced the hybrid-coupled amplifier scheme which forced poor bandwidth given the gain required.. Hmm?
Whilst the hybrid-coupled two-way amplifier seems to have forced the restricted BBC internal telephone bandwidth in the first place, the BBC then took advantage of it in other ways so that even if the original reason disappeared, it was still needed.

Sturley also notes that wideband (10 kHz) music circuits, when not required for program distribution, were used for speech circuits, each able to accommodate one baseband and three FDM 300-2300 Hz channels, plus three teleprinter channels in the guard bands, which looks like a real “shoehorn” job. I think that more usual for voice FDM circuits was something like 4 kHz spacing.

Cheers,
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Old 9th Nov 2015, 8:54 am   #9
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Default Re: BBC 1940's line & telephone equipment

Thanks, Russell.

The one thing I was absolutely sure about was that the Post Office and BT in turn would have charged an arm and a leg.

I started at HP designing test equipment for the FDM system and around that time there was another group in the lab designing baseband equipment to test leased lines. GPO etc charged a bomb for their services, but the customers got their own back by having a careful testing regime to ensure that the quality never fell below what they were paying so heavily for. And in turn, GPO had to have a careful testing regime to ensure that they couldn't be found to have breached contract limits.

On the whole, I think the test gear manufacturers came out best

Towards the end of HP, when the accountants were firmly in the ascendant, every department had to be a profit centre of one sort or another. The internet existed, and the www was just about to burst on the world. The IT department charged all other departments $1 for every megabyte moved onto or off of the site!

I calculated the value of a CDROM disc at this rate, then the value of a small van load of discs (without jewel cases). I wrote up a suggestion that we post guard on the entry points to make sure no IT employee drove such a thing in... the cross charge would have bankrupted us.

The BBC with its need for greater bandwidth than basic phone connections, reliable drop-free service and diversity in routing would have been seen as a cash cow and milked for all it was worth.

David
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Old 9th Nov 2015, 9:24 am   #10
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Default Re: BBC 1940's line & telephone equipment

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On the whole, I think the test gear manufacturers came out best
They do say the best way to get rich in a gold rush is to be selling shovels .....
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Old 4th Mar 2016, 3:00 am   #11
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Default Re: BBC 1940's line & telephone equipment

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Whilst the hybrid-coupled two-way amplifier seems to have forced the restricted BBC internal telephone bandwidth in the first place, the BBC then took advantage of it in other ways so that even if the original reason disappeared, it was still needed.
Making that association was in fact incorrect. The 300 to 2300 Hz BBC telephone network bandwidth came out of the planning for its post-WWII network, as described in BBC Monograph #48 of 1963 June, “The Development of BBC Internal Telecommunications”, available here: http://downloads.bbc.co.uk/rd/pubs/a...nograph_48.pdf.

To quote from that Monograph:

“The decision to use music-type lines in conjunction with a specially designed carrier system meant that the following questions had to be answered. How many telephone and telegraph channels could be accommodated on a carrier system using lines of bandwidth 50 c/s to 8,000 c/s? What maximum transmission loss was to be allowed between outlying premises? What minimum bandwidth of the speech channel could be allowed for a good grade of service?
“A series of Immediate Appreciation Tests was made, from which it was found that for good transmission a band of frequencies extending from 300 to 2,300 c/s was satisfactory provided the overall transmission loss between offices did not exceed 15 to 20 dB. These results were confirmed by a further set of Judgment Tests using artificial lines and telephones in the presence of an ambient noise level normally found in offices.”


and:

“In order to keep the overall transmission loss within the prescribed limit of 15-20 dB, four-wire circuits with amplifiers would be needed between main centres and some outlying premises.”

So the similarity of upper frequency limit between the pre-WWII hybrid-coupled two-way amplifier and the post-WWII telephone network (which used a four-wire system) was essentially coincidental.

Cheers,
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