BBC Radio 4 long wave reception problems etc.

[linescan87]

Yes it's been very good for a long time, yesterday was obviously just a little glitch. I as a regular listener of the long wave service, be it Radio 4 or the World Service through the night I really appreciate the effort put into keeping the transmission quality to a high standard.

If indeed the people involved do look in here, my sincere thanks to you.

Regards,
John Joe.

[paulsherwin]

I wonder if there's a post-it note stuck to a rack in Droitwich somewhere, saying "In case of sibilance, switch this off and on again"

[G8HQP Dave]

Or, 'kick the Optimod here X'.

[russell_w_b]

Originally Posted by G8HQP Dave

'kick the Optimod here X'.

The Marconi BD272 transmitters at Skelton were supplied with 11kV d.c. via solid-state rectifiers but used to be supplied via AR64 excitron rectifiers. When they were converted to s/s, the cooling fan was left operational and it came in on a contactor. Failure to close would result in a 'non-urgent alarm'.

It got to the point where the contactors used to stick occasionally, and a kick behind the control cabinet would get it going. On Sender 57 someone kicked it with a muddy boot and there was a handy footprint on 'X' for ages!

[G8HQP Dave]

When I was a child, one of the responsibilities of the head of the house was knowing where and how hard to thump a radio or TV in order to restore normal operation. Nice to know that the same technique was being used at the other end!

[AC/HL]

That's the 5 guinea repair bill. 5 bob for the thump and £5 for knowing where and how hard.
"They" would be Martin, if indeed the prompt came from here.

[Refugee]

I once worked at a place that had the workshop lights on a contactor.
Sometimes it used to start buzzing loudly. Whoever was near to a switch used to flick it on and off a couple of times to get rid of the noise.

[Aubade65]

Has the R4 silibance problem returned? Sounds a bit 'shshy' today at lunchtime on a couple of sets which were fine yesterday
Could it be the snow or is it just me

Regards

[Pfraser]

No, it's not just you.

Sibilance on my Tecsun PL380, whether set to 6kHz or 4kHz bandwidth.

[Aubade65]

That's good to know-at least the ears are still working

Wonder how long it will take to get it fixed?

[paulsherwin]

Arqiva seem to know how to fix the problem, but the Droitwich site has been lights out for many years now so somebody has to drive there and reseat the board or whatever else the fix is. I'd be surprised if it was fixed before Monday now, given that it's Friday afternoon and the weather is poor.

[cathoderay57]

Hi folks, here in Bristol this afternoon I've noticed a recurrence of nasty sibilance nigh on distortion on R4 Long Wave. Unless all my valve radios are faulty, seems like an issue at the transmission end. I've encountered this phenomenon before, but not for quite a long time. Anybody else?

Cheers

Jerry

[Aubade65]

Sounds as if it has been fixed

[paulsherwin]

Shows how much I know

Maybe somebody just happened to be on site.

[Refugee]

It was on FM too and is now fixed or as good as usual here with a poor signal.

[G0HZU_JMR]

I had a listen mid afternoon (Friday) and the audio was very poor. I quickly set up one of my analysers to capture a few minutes of the transmission either side of the news. On playback I could see that the time pips had visible harmonic distortion out to about the 4th or 5th harmonic. There was also some splatter out beyond the usually sharp cutoff at +/- 5kHz when the news presenter was speaking. The sibilance was also very noticeable. I captured the pips again this evening and the harmonic distortion had virtually gone. The wanted pip tones are something like -15dBc and the 2nd harmonic distortion level is about -55dBc. The transmitter may actually be cleaner than this (when it is healthy) as I think my little active antenna causes a tiny amount of distortion.

When the transmitter is poorly, the distortion terms from the time pips jump up at least 20dB and I can see harmonics out to several kHz.

[Refugee]

The pips are as far as I can hear just a filtered square wave anyway.

[G0HZU_JMR]

Looking directly at the analyser when the pips are sent I think the transmitter sends each pip as a 1kHz sinewave with fairly low distortion rather than any form of squarewave. This is because the only significant terms I see on the analyser are the carrier and the two modulation sidebands associated with the 1kHz sinewave.

I'm using a fairly modern Tektronix spectrum analyser to monitor the pips and this analyser should be clean enough not to degrade the quality of the BBC transmission. However, my little active antenna might contribute some distortion. So the purity of the sinewave pips might be better than I'm currently measuring.

To give some idea of what I'm currently seeing I did try demodulating the signal when the transmitter was healthy and the AF output appears to show a 1kHz sinewave with about 0.7% THD. Only the second harmonic is visible. This is when the transmitter is behaving itself.

When the transmitter was poorly on Friday, the THD of the demodulated signal was about 10% with obvious harmonics out to 5kHz.

[Refugee]

There is nothing like a clean sine wave. It takes very little distortion to make the signal sound "fuzzy".

[Hartley118]

The pips sound magnificent on DAB. That 1kHz sine wave is an ideal signal for MP2 encoding and decoding.

Audio pundits could wax lyrical about the purity of the simple sinusoidal time signal heard against the 'inky black' silent background of digital radio!

Martin