[AMRadio] AM Broadcast Quality


W2xj w2xj at w2xj.net
Wed Dec 18 19:36:44 EST 2013


Jim

If I had to deal with a typical amateur TX I would use a Hilbert Transform clipper to avoid extreme frequency response requirements. That is not the easiest thing to build, though. An old Orban 9105 would do the job as would a software processor that is free to download but I know a number on this list are opposed to using computers in the transmission chain. 

Regarding low end response, Bob Orban did as paper stating the TX should be flat to .9 Hertz for best square wave response. 

Sent from my iPhone

> On Dec 18, 2013, at 6:49 PM, Jim Tonne <Tonne at Comcast.Net> wrote:
> 
> 
> 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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