Civilian Wartime Radio - Further Notes

[Sparks]

Thanks Trevor,
I connected it all up again, and found the required modulation without any trouble this time. But it certainly does not show any over modulation at all, although it was distorted in the speaker. But the fact remains, that this particular station has always produced distortion no matter what radio I listen to it with.
Bob

[ppppenguin]

The obvious conclusion is that they are sending it out distorted and that your sets are blameless.

[Murphyv310]

Thanks Jeffrey.
One other thing could be excessive compression, you'll not see this on the scope but it does seem to effect some valve sets, most envelope detectors in valve sets don't like more than 80% modulation without distortion, snag is with compression some stations approach 95% modulation.

[ppppenguin]

My understanding is that envelope detectors (in other words virtually all of them until modern synchronous phase lock based detectors) will distort as modulation approaches 100%. This is certainly true in TV, where distortion is less noticeable on pictures until the advent of colour where it can cause problems demodulating the subcarrier.

[Sparks]

That was the explanation that someone else gave me some time ago, that valve sets cannot always cope with modern AM transmissions. But it is only with that particular station - the rest have good tones -
Bob

[ppppenguin]

Some stations do excessive audio processing which can cause distortion. Some stations have faulty transmitters (BBC R4 long wave has had various problems in recent years). There are features on modern transmitters that minmise electricty consumption. If overdone these can cause distortion.

Some stations really don't care about their AM output. It's legacy stuff so why bother. Are there any stations on AM only, without being on FM and/or DAB? Obviously any station can broadcast via the internet.

[Terry]

I can't find it now but I have come across a description of a compression system for AM transmitters which allows greater than 100% modulation! From memory, it works by adding a DC offset to the signal on loud passages so that the negative going peaks never quite go over the 0% level but the positive peaks drive the modulation above 100%.

Back in the 60s, Carl Thompson came into our workshop one day. I knew Carl vaguely but he was a few years older thsn me - which makes a lot of difference when you are very young, so I played with Clive Brown but his older brother, Peter, played with Carl. Over the years, I'd noticed the massive wooden mast that had gone up in his back garden, so it was obvious where his interests lay!

He'd come to us for few components which, we found out, were destined for the MV Mi Amigo - aka Radio Caroline South - when he returned the following Monday. At about this time, Caroline had played a Ted Heath track 'Poor Johnny One Note' occasionally, which had loud cymbal clashes in a few places and the volume dropped instantly on every crash, then recovered quickly, so I asked why.

The limiters were set up for 100% mod @ 1kHz, he said, so these loud sounds had to be suppressed instantly! 100% modulation? Never! But he persisted, so out came the 'scope which I clipped onto the IF strip of a Bush TR90 I'd just repaired and there was the proof - 100% mod!

Obviously,this was to help their 10kW transmitter compete with the 30% mod of the BBC's much more powerful transmitters.

I think all local radio station transmitters these days use 80% mod.

[Murphyv310]

Hi Terry.
Nice story indeed. At 80% your average radio should cope fine. My local (West sound 1035kHz) is about 4 miles from me and the site can be seen with binoculars from a bedroom window, It appears to have a modulation depth of around that magical 80%. I cant say I have any issues on any valve set here. Funnily enough one of the SDR programs actually tells you the peak modulation (AM) on the screen and it remains till another higher peak comes along, I'll need to try this on the local soon and see if it is.

[Terry]

My local (West sound 1035kHz) is about 4 miles from me ...

Ah, Trevor, 1035kHz, what would have been 1034kHz before 1978 - yet another use of one of those frequencies which those naughty pirates used to jam legitimate broadcasters!

   

Radio City, the Station on the Sticks!

(Taken from the chart issued by the EBU showing all stations in Europe operating on the 1st November 1965. Authorised stations at the top, unauthorised in the shaded portion at the bottom.)

[Alistair D]

I can't find it now but I have come across a description of a compression system for AM transmitters which allows greater than 100% modulation! 

The system you describe is called asymmetric modulation. It was discussed over on VRAT a while back.

https://www.radios-tv.co.uk/community/ra...post-23221
 
Basically the -ve going peaks of the audio are clipped/limited so that the transmitter PA does not saturate. A higher mod level can then be applied to the PA.

Two problems occur when doing this:-

Firstly; The audio signal is now distorted so the sound quality will fall.

Secondly; The receiver's AGC is confused into thinking the signal is smaller than it really so it may not turn the gain enough.  

Al