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Overmodulation vs over deviation on FM


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Old 22-04-2012, 08:18
Nick_G
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If an FM station overmodulates (i.e. >100%) is it not by definition also over-deviating? If so, is this not illegal?

My new Kenwood tuner has been making me think about this as it shows the modulation levels of stations. Some are in the red nearly all the time.
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Old 22-04-2012, 09:07
hanssolo
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There has been at least once breach, so I expect most stations monitor output occasionally, the FCC has a 10% tolerance so Ofcom may have the same?
http://stakeholders.ofcom.org.uk/bin...Total_Star.pdf
The transmitter was found to be deviating (that is, bleeding the signal into adjacent
channels on the FM band) in excess of 110 kHz. The maximum level of deviation
permitted by Ofcom’s Technical Code is 75kHz
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Old 22-04-2012, 09:24
martinwatkins
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Without casting aspersions on your new toy I suspect it's not giving the correct reading. Measuring deviation is a complex business, and in the late 1980s when Optimods/other processors were - sadly - arriving on FM at the BBC, various tests were undertaken on their performances. Problem was we had three modulation monitors available, and all three gave totally (and I mean hugely) different answers when we dug them out and connected them up. I don't know what the end of the story was, but one major BBC FM site threatened to install their own peak limiters as they reckoned that what was coming from LBH was overdeviating their transmitters. It was certainly causing one service to trip out, some problem in the combiner provoking trips.

Are the BBC stations showing up in the red Nick?
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Old 22-04-2012, 09:48
Nick_G
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Hi Martin/hanssolo.

The BBC nationals are all fine, apart from Radio 1 which does show the odd peak in the red. It's Capital on 95.8 that's the worst offender. The scale is in the red virtually constantly and it sounds awful.

I got this tuner (a Kenwood KT-1100SD) after reading a write-up on the FM Tuner Info site. The write-up was by an RF engineer who is seriously into FM/tuners/audiophile stuff and he has done a number of modifications to his Kenwood. I mentioned that I found the modulation display interesting as it shows when stations are over-deviating and he agreed with me, so I'm assuming it is reasonably accurate.

Kenwood's core business has always been radio so I'd guess they are more likely to get things like this right.
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Old 22-04-2012, 10:42
martinwatkins
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I don't know if it's still the case, but the BBC policy on deviation was that neither A or B or M or S (where M = A + B - 3 dB and S = A + B - 3 dB) should go above peak level; and on a lot of material this additional restriction means that the BBC don't deviate as far as other organisations (who allow A and B to peak 6 without worrying about M and S, which they take as A +/- B - 6 dB).

I'd be surprised if there really was widescale flouting of the Ofcom regs, but of course a lot of less scrupulous (ie couldn't care less about what it sounds like) stations are going to go as close as they can to the "red line".

And I entirely agree, they *do* sound utterly awful.

If I was "dictator" of radio I'd introduce an edict that gave stations a choice between no processing and full deviation, or as much processing as they like but with a 6 dB guard band imposed before the tx, That would stop all this lo-fi nonsense overnight.
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Old 22-04-2012, 10:54
Nick_G
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Thanks Martin.

Do you think the Kenwood's deviation meter display is accurate from my description? It's certainly a pretty serious effort from them - it has 4 IF bandwidth settings. AFAIK no other Kenwood tuner has this feature. It has been modded with narrower filters in the narrow path, but even in super wide (2x180kHz) the stereo lamp flicked off briefly when I was trying Capital, presumably due to the over-modulation. The tuner does seem sensitive to over modulated stations and multi-path, more so than my other ones, so it does perhaps make sonic flaws more obvious.
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Old 22-04-2012, 11:09
smorris
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It's Capital on 95.8 that's the worst offender. The scale is in the red virtually constantly and it sounds awful.
You may notice there have never been pirates (at least for more than about a week) on either 95.6 or 96.0, unlike with any other station in London - I suspect you've found the reason for that.

And yes, it's heinous sound quality, worse than many of the pirates, but it's been like that for some time now and their target audience seem to actively prefer it.
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Old 22-04-2012, 11:10
alinton
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It's impossible to accurately read deviation off air, because you'll be receiving some multipath which will make the readings higher than they should be.

The only accurate way is by taking a sample from the transmitter itself.

In the UK the rule is +/- 75kHz is the limit of total modulation including subcarriers. In the US the FCC do allow more than 100% by a factor of half of that deviation made up by subcarriers.

If your tuner has narrow bandwidth filters this could be causing your high readings. To read accurately you should have a wide front end.

A.
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Old 22-04-2012, 11:29
Nick_G
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It's impossible to accurately read deviation off air, because you'll be receiving some multipath which will make the readings higher than they should be.

The only accurate way is by taking a sample from the transmitter itself.

In the UK the rule is +/- 75kHz is the limit of total modulation including subcarriers. In the US the FCC do allow more than 100% by a factor of half of that deviation made up by subcarriers.

If your tuner has narrow bandwidth filters this could be causing your high readings. To read accurately you should have a wide front end.

A.
I suppose 180kHz is quite narrow, but stock this Kenwood has 5x180kHz. The super wide setting uses the first two filters and then a further filter gets added as you go wide/narrow/super narrow. My tuner has 180-180-150-150-110kHz filters which makes for excellent selectivity for DXing in the narrowest setting. All my modulation readings were taken in super wide mode.

But yes, I have noticed that on marginal signals the modulation readings do get exaggerated by multipath and fading. The modulation meter only kicks in when a signal is strong enough for stereo.

I'm hoping to get further mods done with a wider bandwith in the super wide mode (this should improve audio quality) and much narrower filters in the narrowest setting, for serious DXing. Perhaps having wider filters would improve the accuracy of the modulation meter.
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Old 22-04-2012, 22:53
lundavra
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I think the spec on maximum deviation was to check maximum deviation over a five or ten minute period and it was not to exceed 75 KHz. Usually left the FMA running for a bit whilst doing something else then come back and check it.

Don't know about the big sites but most others seemed to use the SBS MPX5(?) which incorporated a limiter. We had one service that had a completely unlimited feed because it was also a source for DAB so that had Optimods fitted, again just to act as limiters to ensure it did not overdeviate.
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Old 26-04-2012, 16:06
wand106
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There has been at least once breach, so I expect most stations monitor output occasionally, the FCC has a 10% tolerance so Ofcom may have the same?
Ofcom will only react if there is a complaint, there is a station near me that is over deviating, and I know Ofcom know about it, but nothing has been done, because the way they came to know about it, was not in the form of a complaint

Sorry if that appears cryptic, I appreciate this isn't Eastenders in board form, but I don't want to get anyone in trouble..
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