[AMRadio] AM Broadcast Quality


Jim Tonne Tonne at Comcast.Net
Wed Dec 18 18:49:51 EST 2013


I have been watching this thread and nodding in agreement with W2XJ on his
statements.   But now I feel it is time to comment.

<soapbox>

I have this awful feeling that some of the tube rigs that had marginal 
iron in
the modulation path did so allegedly because "that's the way it's 
supposed to be"
but in fact the reason was cost-cutting.

If you use low-level speech processing (clipping, to be specific) then 
the low end
must be as flat as you can get it.   It might be of interest to know 
that the shortwave
broadcast transmitters (nowadays) themselves have a low-end response 
that goes
down way below sub-audible.   The contract to the manufacturer commonly says
that the modulator must handle a 50 Hz sinusoid that has been clipped 10 
dB to
have no more than 5% "tilt" in the output.   I know this because I have 
been
involved with the design of those rigs (at Continental Electronics).

This means that the low-end (bass) response must go down to way below *one*
Hz.

With those transmitters now using PWM, this is not a problem at all.
But with the Viking II and its bretheren it is a serious problem.

So, fellows, make the transmitter audio frequency response as flat as 
you can and
then use a "speech processor" to shape the amplitude and frequency 
responses.
I am toying around with writing an article for QEX about how to do this.

</soapbox>

- JimT W4ENE




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