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Old 3rd Dec 2018, 12:41 am   #1
Chris55000
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Default Hum and noise in BBC Sound Radio broadcasts?

Hi!

In my younger days I had considerable difficulties in getting extraneous hum and noise out of the valved equipment I looked at as I didn't understand the principles like I do now, which brings me to another discussion item – that is how quiet BBC sound radio broadcasts were, and still are, from the general hum and noise point of view!

What precautions did the BBC go to in the valve days?

Did they use electronic h.t. stabilisation for the early audio stages of their sound radio transmitters, or were their engineers simply so careful enough with the circuit design to be able to get good quiet backgrounds with conventional h.t. smoothing & decoupling?

We take the absence of extraneous hum and other background noise for granted in all our consumer audio gear nowadays, but Paul's humming AMG111 I helped him with recently reminds me this wasn't always the case!

Can any Member recall any BBC Sound Radio broadcast plagued by excessive hum or other unwanted background noise?

Chris Williams
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Old 3rd Dec 2018, 7:43 am   #2
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

I assume you are referring to off air broadcasts and not BBC Sound which is what they call iPlayer nowadays?

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Old 3rd Dec 2018, 9:31 am   #3
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

I can't answer your question, but I remember (from a short placement) that there were radio studios in the basement of BH from which the Northern Line was sporadically audible - at least, that's the engineers' story as I remember it.
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Old 3rd Dec 2018, 9:45 am   #4
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

I may still have somewhere a BBC television sound recording, recorded in the US in 1938 during freak propagation conditions. This was, I think, given to me by Andy E.

As Jasmine Bligh's microphone is faded up, (induction-type) background hum is really noticeable. I was rather shocked to hear this.

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Old 3rd Dec 2018, 10:14 am   #5
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

When I worked in operations and maintenance at the beeb (World Service) in late 70's and early 80's Bush House was mid way between valve and transistor broadcast equipment. The valve amplifier chains and studios built around GPA4 and C9 amplifiers all used bulk psu's in the racks with passive component filtering for HT supplies. It was the careful design of the these PSU and amplifiers that gave the low hum and noise. From memory, most hum issues arose from faulty valves and unbalanced circuits - every piece of equipment was balanced in and out using transformers so very resilient to induced hum and noise.
BT link circuits could be the cause of some additional hum and noise especially on temporary circuits set up for special event (Saturday afternoon sport feeds stick in my mind).
The transistor equipment also generally used bulk psu's with electronic voltage control and stabilisation.
I cannot really comment on how the transmitters dealt with filtering and stabilization but I'm sure someone on the forum will have experience in that area.
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Old 3rd Dec 2018, 11:12 am   #6
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

At the start of Broadcasting House all the amplifiers where run from batteries, charged overnight.
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Old 3rd Dec 2018, 11:58 am   #7
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

I was involved the re-engineering of the Bush House control room around 1981, which was a process akin to milking a porcupine, given that the old kit continued to operate whilst we built and tested the new stuff around it, only swinging the jumpers over to the new blocks on the distribution frame when programme schedules allowed. Once that bit was running smoothly, the old kit was powered down and taken out.

Most of the control room amplification was done on C9 valve amplifiers, eleven plug-in modules to a half-bay panel with the twelfth position occupied by a common power supply. These ran reliably for years on end, largely because they were never switched off, odd failures being dealt with by hot-swapping. On one occasion, the wrong batch of C9s were switched off. The absence of Green (WS) was swiftly noticed, and the rack switched on again. Seven amps of eleven came back!

The only goof which surpassed that on my watch was when we cut the wrong PSN/50 and reconnected it with choc-blocks just in time to prevent Lilliburlero fron going off the air...
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Old 3rd Dec 2018, 12:45 pm   #8
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

Quote:
Originally Posted by mark_in_manc View Post
I can't answer your question, but I remember (from a short placement) that there were radio studios in the basement of BH from which the Northern Line was sporadically audible - at least, that's the engineers' story as I remember it.
Actually the Bakerloo Line as BH sits above it between the Regents Park and Oxford St stations - I heard the trains rumbling by many times whilst working there!

There's an interesting read here http://underground-history.co.uk/bbcbh.php about an unfounded rumour of a platform for BH on the line - the article concludes with

"Update 2006: Over the last couple of years, extensive work has been done in the refurbishment of BBC Broadcasting House. In fact, "refurbishment" is rather a weak word to describe the work... As the facade is a listed building, this couldn't be changed, but essentially, the building's innards have been completely gutted and rebuilt so that the studios are now on floating foundations to attempt to reduce the noise from the Underground"
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Old 3rd Dec 2018, 5:39 pm   #9
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

I noticed on the broadcasts of radio Jersey when they carry the local parliament debates there is a considerable amount of hum noticeable, in fact when someone stops speaking you can here the hum cone up and the AGC brings up the gain.
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Old 3rd Dec 2018, 8:17 pm   #10
Chris55000
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

Hi!

Sorry – I didn't make it absolutely clear in my first question – it does refer to "off–air" broadcasting rather than the time–shifted iPlayer/BBC Sounds recordings!

To digress a bit, from a repair point of view, the use of good–quality metal or diode rectifiers together with modern electrolytics gives an unsmoothed h.t. of quite a bit more than the original set maker could get, allowing a preliminary stage of h.t. smoothing between the rectifier and the O/P transformer or main h.t. line to be added whilst still giving the set–maker's designed h.t. voltage.

Murphy Radio always quoted mains–voltages of 240–245V a.c. on most of their Radio Service Sheets, so the relatively (lower) voltages quoted for many a.c./d.c. technique chassis wasn't entirely down to lower nominal mains voltages, a point made by Spreadbury in his second volume of his TV Servicing books, who wrote "advantage cannot be taken of higher mains voltages as everything has to be scaled down to 200V DC operation" to suit those who only had that supply available at the time!

In the 1960s and 1970s more thought was given to improving performance by increasing the available h.t. line voltage, but with valves, germanium transistors, transformers and other components becoming more scarce and costly year on year, I always repair equipment to give the original maker's nominal stated voltage!

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Last edited by Chris55000; 3rd Dec 2018 at 8:36 pm.
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Old 3rd Dec 2018, 8:27 pm   #11
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

In the early days of BBC Radio London the audio suffered from dialling clicks of the Strowger type. I believe this was cured by using lower impedance land lines.

I believe that the landlines to Wrotham were affected by flooding in 1968 and the transmitters relayed MW transmissions !
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Old 3rd Dec 2018, 8:35 pm   #12
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

The GEC BRT400-series of receivers - which were used by the BBC for off-the-air monitoring of SW/MW transmissions at places like Caversham - did indeed use electronic hum-reduction rather than the more-usual lots-of-L-and-C in the HT supply.

When you listen to some of the off-air recordings of international SW broadcasts that often show up on the likes of Radio4 or BBC WS when they're discussing a historic military-coup or invasion or something, there's often quite a bit of hum! Though I guess this is more likely to be originating at the transmitter end.
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Old 3rd Dec 2018, 8:44 pm   #13
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

If 50Hz hum or 100Hz ripple was transmitted by the BBC in 1938, it makes me wonder what was the audio modulation bandwidth at that time?

Has anyone actually measured the frequency of that hum on that 1938 recording?
Could it be 60Hz hum or 120Hz ripple?

Is it possible that the hum was an artifact of the demodulation process in the American receiver?
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Old 3rd Dec 2018, 10:17 pm   #14
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

Further to Alvin's recollections above, I was a Technical Operator at Broadcasting House in the late 70s and I recall the rumble of passing tube trains being heard quite clearly in the basement studio B10 I think. I also remember instances when quiet recordings had to be stopped as the rumble become too obtrusive.

When working in what was called the Continuity Suites (home of Radio 1, 2, 3 and 4) the desk operator would listen to the studio output "off air" (Armstrong receivers) to monitor and log any problems with the transmitter - usually Wrotham.

Happy days really. Most of the audio amplifiers in the Control Room area were indeed valve - and painted in battleship grey of course!

Actually there were some great stories going around BH about other noises heard late at night during the night shift that were attributed to the resident ghost, but such things are off-topic!

Last edited by dryjoint; 3rd Dec 2018 at 10:29 pm.
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Old 4th Dec 2018, 11:21 am   #15
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

Quote:
Originally Posted by dryjoint View Post
Further to Alvin's recollections above, I was a Technical Operator at Broadcasting House in the late 70s
I was there from December 1970 to June 1977
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Old 4th Dec 2018, 11:48 am   #16
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

Ahh, but did you know or even meet each other?
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Old 4th Dec 2018, 2:58 pm   #17
Panrock
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

Quote:
Originally Posted by Silicon View Post
If 50Hz hum or 100Hz ripple was transmitted by the BBC in 1938, it makes me wonder what was the audio modulation bandwidth at that time?
Very good. Allegedly 15 KHz on the 7.2 metre sound transmitter.

Quote:
Originally Posted by Silicon View Post
Has anyone actually measured the frequency of that hum on that 1938 recording?
Could it be 60Hz hum or 120Hz ripple?

Is it possible that the hum was an artifact of the demodulation process in the American receiver?
No, it was buzzy 'pick-up' style hum, with frequencies almost certainly based on 50/100Hz. The reason: it faded up with the mic, just before the announcer spoke.

I spent yesterday looking for this old recording... I still haven't found it! If I do, I'll post it online.

Steve

Last edited by Panrock; 4th Dec 2018 at 3:10 pm. Reason: accuracy
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Old 4th Dec 2018, 5:35 pm   #18
hannahs radios
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

I visited the BBC monitoring station at Caversham/Crowsley in the mid 80's and remember that all the receivers were fed from big lead acid batteries that were on permanent float charge. The audio inputs were completely hum free.
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Old 4th Dec 2018, 7:47 pm   #19
dryjoint
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

Quote:
Originally Posted by Alvin View Post
Quote:
Originally Posted by dryjoint View Post
Further to Alvin's recollections above, I was a Technical Operator at Broadcasting House in the late 70s
I was there from December 1970 to June 1977
I think I started in 1978, so probably we just missed each other. It was a great place to work and was my first job after school. Lucky indeed.
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Old 5th Dec 2018, 2:13 pm   #20
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Default Re: Hum and noise in BBC Sound Radio broadcasts?

On H.F. transmitters (well, the Marconi B6122 / BD272 types), the a.c. valve filaments in the modulator stage were centre-tapped with the 2nd stage valve fils being fed via centre-tapped rheostats. The rheostats were adjusted for hum-balance at performance test time.
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