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Old 1st Feb 2019, 11:19 pm   #21
Refugee
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

It was on FM too and is now fixed or as good as usual here with a poor signal.
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Old 2nd Feb 2019, 1:42 am   #22
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

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.
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Old 2nd Feb 2019, 1:04 pm   #23
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

The pips are as far as I can hear just a filtered square wave anyway.
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Old 2nd Feb 2019, 7:29 pm   #24
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

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.
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Old 2nd Feb 2019, 10:53 pm   #25
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

There is nothing like a clean sine wave. It takes very little distortion to make the signal sound "fuzzy".
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Old 2nd Feb 2019, 11:13 pm   #26
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

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!

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Old 2nd Feb 2019, 11:32 pm   #27
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

When the Droitwich transmitter is behaving itself the 1kHz pips look to be quite good on 198kHz here. I'm really just trying to describe the difference in harmonic distortion levels (of the time pips) seen on the analyser data between times when the transmitter has the sibilance issue and when it is working normally. When it is working normally the pips look fairly decent to me and I get 0.7% THD on the demodulated pips. I guess it's up to the individual to decide if this is a fairly acceptable sine wave or not. When the sibilance issue was present the THD shot up to 10%. That difference is all I'm really trying to describe

Obviously, there's not much point in the BBC trying for Hifi levels of distortion for a LW AM transmitter but I can't see any significant odd order harmonics (the components of a square wave) when I look on the analyser when the transmitter is behaving itself without the sibilance issue.

I'm only a few miles from the transmitter and I'm doing all my tests and demodulation in the digital domain using data from the spectrum analyser and I'm doing it in daylight hours to minimise propagation effects.

Obviously, it would be risky to try and assess the transmitter quality (wrt the time pips) with a low cost domestic LW AM receiver as the rapidly rising pip tones would contribute to (AF) AGC induced distortion, distortion in the IF amplifier and detector and maybe even distortion in the AF stages.
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Old 3rd Feb 2019, 1:13 am   #28
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

The pips sounded pretty clean this evening on FM so they have fixed the problem and by the sound of it the transmitter was not at fault due to it being on both FM and LW.
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Old 3rd Feb 2019, 8:06 pm   #29
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

Quote:
Originally Posted by Refugee View Post
The pips sounded pretty clean this evening on FM so they have fixed the problem and by the sound of it the transmitter was not at fault due to it being on both FM and LW.
This opens up a byway of enquiry.

I once had a lively discussion with a schoolfriend regarding the origin of the BBC Radio 2 signal for the Midlands, on 693kHz. I was robust in my assertion that it came from Droitwich; my friend was equally firm in stating Sutton Coldfield.

Now...it later transpired that he was correct, in part. For whilst the radiated signal did indeed come from Droitwich, the programme feed was routed via Sutton Coldfield.

So...has the venerable 198kHz behemoth been falsely maligned? Has there been a fault further back in the chain?
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Old 4th Feb 2019, 1:34 am   #30
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

I certainly have my doubts about there being anything wrong with either of the final transmitters due to the problem being on both 198Khz and FM.
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Old 5th Feb 2019, 12:05 am   #31
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

I had a go at filming the time pips on my analyser last night when there wasn't any obvious sibilance. See below for the video. The 1kHz pips still look quite good but I'm not convinced that the transmission is quite as clean as usual. It's a lot better than it was last Friday but there is still some spectral spreading beyond +/- 5kHz when the announcer says words with an 's' at the end. So maybe there is still a hint of sibilance? Maybe this level of spreading is insignificant, but it normally looks a bit better than this.

The pips do look like a sine wave when the captured analyser data is demodulated and you can see on the analyser that the main modulation sidebands are at 1kHz and there are no visible sidebands at odd order multiples of 1kHz. So I don't think there is any kind of filtered square wave being sent. It looks like they are at least trying for a clean sine wave at 1kHz.

If you look closely, the main carrier dips on modulation peaks and I recall that someone on here mentioned that they dip the carrier power like this to improve efficiency.

I've got about 5 minutes of captured IQ data from last Friday when the sibilance was bad and I could post up what the pips look like when the transmission has this problem?

Sorry about the dodgy quality of the short video but I hope it is interesting.

https://youtu.be/F4xJ1wX7Z4w
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Old 5th Feb 2019, 12:16 am   #32
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

The transmitted signal should stay within its transmission bandwidth even if distortion has been added earlier in the distribution chain, though I don't have an explanation and agree it's very puzzling.

To complicate things further, I understand the distribution circuit to Droitwich R4 is some arcane ISDN arrangement separate from the standard BBC Nicam circuits. This dates back to the 80s when 198kHz was still expected to have a military / civil defence role in a national emergency.
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Old 5th Feb 2019, 11:58 am   #33
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

Quote:
Originally Posted by Hartley118
The pips sound magnificent on DAB.
But always slightly late?
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Old 10th Feb 2019, 1:18 am   #34
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

Quote:
The transmitted signal should stay within its transmission bandwidth even if distortion has been added earlier in the distribution chain, though I don't have an explanation and agree it's very puzzling.
I had a bit of free time today to play with a larger (passive) antenna and this antenna works across the whole SW band. So I could compare BBC R4 against other stations.

I compared it against 1053kHz Talksport and both were received with about the same signal strength on the analyser (within a couple of dB). However, the Talksport transmission stays within its +/-5kHz bandwidth without any splashy IMD beyond this bandwidth. The BBC R4 transmission on 198kHz showed a fair bit of splashy IMD beyond +/- 5kHz today and I think it was slightly worse than my earlier video and the time pips had a bit more distortion on them today. So it is as if the signal quality has degraded a bit since it was 'fixed' last week.
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Old 10th Feb 2019, 4:09 am   #35
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

Do members of the public ever notice this sibulant fault state? I've never seen anything in the Radio Times for example!

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Old 11th Feb 2019, 12:39 am   #36
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

I think it would have been noticed by lots of listeners on the day it was sounding poor (Friday 1/2/2019?) as it didn't sound good at all on that day.

I did search the internet for problem reports for R4 on 198kHz and it had already been reported as a problem on at least one site for that day. I can't remember if it said it was noisy or if it said it was distorted but I think it was this site here that reported the problem:

http://www.radioandtvhelp.co.uk/inte...radio/am_radio

So it does look like the problem gets noticed and the time of the report appeared to be just before it was fixed on that Friday afternoon. However, I can't actually find that report today so maybe they delete them after a while.

I recorded the pips again today and demodulated them directly in the analyser and the analyser can show the modulation envelope for each pip. Each pip looks to be about 100ms long and the final pip is about 500ms long. They seem to be spaced exactly 1 second apart on the leading edge of each pip. The pip envelope has a rise and fall time of a few ms.
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Old 11th Feb 2019, 12:58 am   #37
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Default Re: BBC Radio 4 198 kHz long wave sibilance.

There was-or is, some kind of a relay TX here in Bexhill. A few years ago, I bought a Murphy set from a chap who lived nearby. He had a huge coil of telephone extension cable in the hallway and said that it was the only way that he could stop Radio 4 harmonically resonating and breaking through on his calls-a bit like Taxi's and stage amps!.

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