[BC] Re Oscopes Bessel Null and FM

[Robert Orban]

At 12:26 PM 7/21/2006, you wrote:
>From: "Mark Humphrey" <mark3xy at gmail.com>
>Subject: Re: [BC] Re Oscopes Bessel Null and FM
>To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
>Message-ID:
>         <74b029b80607210902x140ba9a1w13b60e03ed0ffc1c at mail.gmail.com>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>On 7/21/06, Xmitters at aol.com <Xmitters at aol.com> wrote:
>
> > Then kick in a Carson's filter that limits the occupied bandwith 
> according to
> > his well known formula. I would be willing to bet a round of beers that the
> > overmod flasher would either stop winking or it would not wink as often 
> as it
> > would without the Carson filter. It seems that the lopping off of those 
> higher
> > order sideband pairs (that last 2% of occupied bandwidth) would do two 
> things.
> > One would be to very slightly reduce the amplitude of the recovered 
> audio at
> > the receiver. The other would be that distortion products would be created.
>
>This is an interesting question.  If the audio were "aggressively
>processed", it would likely be clipped, thus having lots of odd-order
>harmonics.  But wouldn't the Carson filter remove the sidebands that
>represent the higher (harmonic) modulating frequencies -- which extend
>farther from the carrier --  to a larger extent than the the sidebands
>generated by lower (fundamental) fundamental frequencies?   If so, it
>might act as a low-pass filter on the demodulated audio.

Speaking for my company, our audio processors strictly limit their output 
bandwidth regardless of how aggressive the processing is. Realistically, to 
get "square waves" on FM, you would have to have one of the old analog 
composite clippers on the air. The most popular of these did not band-limit 
its output.

Orban did not implement composite limiting until we had engineered a 
solution that fully protected the pilot tone and subcarrier regions of 
baseband regardless of the amount of limiting being used.

Bob Orban