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Old 27th Mar 2012, 8:36 pm   #5
Ian - G4JQT
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Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

I used to work at the BBC's technical measuring site at Crowsley Park, now long gone.

All BBC MW synchronised network transmitters, (at the time Radios 1, 3, 5) were all kept within 0.05 Hz. (R4 on 198 was a different matter.)

This 0.05 Hz was deliberately chosen as the 20-second fade for those unfortunately living in the 'mush zone' without an alternative frequency was considered least worst.

There was also audio phase correction at each transmitter so the mush zone was as small as possible. This meant that not only were the RF carriers in phase as much as could be sensibly achieved, but also the phase of the audio modulation was the same at each transmitter.

The echo you hear has nothing to do with the time differences it takes RF from different tx sites to reach your receiver, these are infinitesimally small. It's to do with the lack of anyone now bothering to keep the audio of each transmitter in phase. In fact it's MUCH worse that, as whoever now runs the BBC MF sites has not only abandoned the audio phase-matching, but it arrives at different tx sites now by various digital routes with differences measured in (I'd guess) milliseconds.

But hey, why bother with all that techie stuff that makes that old AM sound reasonable. Doing all that costs too much and everyone who knows how to do it was sacked or retired. No one will notice, and even if they do they won't know who to complain to! Listeners should be all be tuned to that new-fangled DAB thing anyway.

(I'm not a bitter and twisted old BBC engineer, honestly!)

Regards,

Ian
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