Re: [Amps] OT: RF Speech Processor Kits - Final Final

[Gary Schafer <garyschafer@comcast.net>]

Steve Thompson wrote:
> Gary Schafer wrote:
>
> > Steve Thompson wrote:
> >
> > > Ken G3WCS wrote:
> > >
> > >
> > > > Thank you to everyone who contacted me both on and off list.
> > > >
> > > > Quite a lot of folk pointed me to audio clippers rather than RF
> > > > Processors. Just for the information of the guys who don't know,
> > > > there is a difference between the two.
> > > >
> > > > An audio clipper simply either compresses or clips (or both) the
> > > > audio signal. This can (and often does) result in 'whiskers' or
> > > > splatter which make the transmitted signal wider. This was not the
> > > > design I was looking for.
> > >
> > > Just for the record, can I mention that the split band audio clipper
> > > I referred to isn't a simple compressor/clipper that generates lots
> > > of harmonic distortion and splatter. By splitting the audio into
> > > sub-octave bands you can clip each one, and then remove the
> > > harmonics, before recombining so you end up with the same effect as
> > > rf clipping.
> > >
> > > Steve
> >
> > The problem with audio clipping for an ssb transmitter is that the
> > audio wave form is not the same as the ssb wave form like it is on AM.
> >
> > A square wave fed to an AM transmitter produces a square wave out.
> > A square wave fed to an ssb transmitter produces an infinite spike in
> > its output.
>
> I'm sorry, but I don't understand what you're getting at here. An infinite
> spike of what? Am I wrong in thinking that, in a SSB tx, an infinite spike
> in amplitude out needs an infinite spike of amplitude in? A square wave
> doesn't give you that. In any 'regular' SSB tx, you can't get an infinite
> spike in the frequency domain because there's band limiting.
>
>
> > When you clip audio you produce square waves. The more clipping the
> > more near to ideal square waves you produce.
> >
> > At high audio frequencies you can filter out the squared components
> > and round them off somewhat and get rid of some of the high frequency
> > component in the square wave. At the lower audio frequencies more of
> > the harmonics fall in the audio band that you can't filter out.
> >
> > The split band processors help in that they can filter out more of the
> > unwanted components.
>
>
> The unwanted components can be filtered out to any degree you like,
> depending on how complex you choose to make the filters.
>
>
> > But you still have the square wave problem being
> > fed to the ssb transmitter although not as bad as with conventional
> > audio clipping.
>
> Surely not, if the filtering is good enough.
>
>
> > The reason that most audio clipping systems cut the low frequencies
> > before clipping is to eliminate a lot of the trash from clipping as
> > the low frequencies give the biggest problem when you are clipping.
>
>
> You'd certainly want to do this in systems where harmonics from clipping low
> frequencies aren't filtered out.
>
> > Rf clippers solve those problems because all of the harmonics from
> > clipping fall outside the bandpass of the rf filter.
>
>
> It's late, and alcohol is clouding my brain. If I think about what happens
> to any given audio frequency when passed through a split band or rf clipper,
> I can't see where there's a difference.
>
> When searching for references to the srticle I remembered, I was struck by
> how many top end broadcast audio processors use split band clipping and
> limiting.
>
> 73, Steve

Hi Steve,

The output of an ssb transmitter is different than from a double side 
band transmitter. A double sideband transmitter produces a replica of 
the input wave form. An ssb transmitter does not.

A double side band transmitter modulated with a sin wave will produce an 
envelope with a replica of that sin wave.

An ssb transmitter modulated with a sin wave will only produce an rf 
carrier out. No replica of the modulation signal.

A square wave fed to a double side band transmitter produces a square 
wave out.

A square wave fed to an ssb transmitter produces only an infinite spike 
in amplitude out, not a square wave.
That is the problem with audio clipping for an ssb transmitter.

Clipping at rf produces the same replica of the output as that of the 
input to the rf clipper.

Split band audio clippers are better than base band clippers but you 
still can not filter out all the harmonics generated by clipping.

73
Gary  K4FMX




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