[BC] processing way back when

[Milton R. Holladay Jr.]

Dept. of the obscure: Back around 1970, Pye TVT in England made a limiter
that worked vaguely on the same principle as the Rf job, except it was
somehow pulse modulated, limited, and demodulated......
M
----- Original Message ----- 
From: "Robert Orban" <rorban at earthlink.net>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Sent: Wednesday, December 20, 2006 6:47 PM
Subject: Re: [BC] processing way back when


> At 06:35 PM 12/19/2006, Dave Dunsmoor wrote:
> > > .... IIRC, the
> > > audio was modulated as RF to permit gain control in the RF domain
without
> > > thumps.
> >
> >
> >Bob, I'm going to show some ignorance here now, what exactly are you
telling
> >us here? I don't think I'm following your description, or I don't see
what
> >you're
> >describing.
> >
> >thanks for a circuit operation lesson,
>
> Thump in a tube-type limiter is leakage of the gain-control signal into
the
> audio. In conventional tube-type limiters, the audio is always in
push-pull
> form and gain reduction is done by varying the bias voltage of two matched
> variable-mu tubes that also pass the audio signal. When perfectly
balanced,
> this push-pull arrangement nulls out the thump, because the thump is a
> common-mode signal.
>
> When you modulate the audio on RF in the form of double sideband
suppressed
> carrier, you can now use a single variable-mu tube to change gain. The
gain
> control signal will appear as low frequency thumps at the output of the
> tube. However, when the DSB suppressed carrier signal is changed back to
> audio via a product detector, the thumps get frequency-shifted outside the
> audio band (to approximately the carrier frequency) and can be readily
> filtered out.
>
> As an interesting side-effect, this process does not add harmonic
> distortion even though the variable-mu tubes, being nonlinear devices,
> always add nonlinear distortion to the audio. If the carrier is at 1 MHz,
> for example, the second harmonic caused by nonlinearity in the gain-change
> tube will appear at 2 MHz. After product detection, this harmonic will
> appear at 1 MHz and can be readily filtered out. This also happens to be
> the operating principle of a RF clipper, which is another audio processing
> technique that produces no harmonic distortion. (Some of our processors
> have used a "computed RF clipper" using Hilbert transforms to exploit this
> fact. See US Patent 4,495,643.)
>
> Both techniques, however, produce in-band IM distortion. So there is still
> a requirement to minimize the audio signal level on the variable-mu tube
in
> order to have the audio on an approximately linear part of the tube's
> transfer characteristic. Moreover, in the RF limiter there is still
benefit
> to using two tubes in push-pull because this eliminates even-order IM
> distortion.
>
> Bob Orban