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
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