16-11-2018, 07:58 PM
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.
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.
