Re: [IRCA] 738 & SAH's from Michigan - Friday morning 9/7/2012

[Russ Edmunds <wb2bjh@xxxxxxxxx>]

Indeed, I meant Hz. Thanks for pointing that out. I expect it might be difficult to locate a working or even repairable Dumont 304 today...

Russ Edmunds
15 mi NNW of Philadelphia  
Grid FN20id
<wb2bjh@xxxxxxxxx>
FM: Yamaha T-80 & Onkyo T-450RDS w/ APS9B @15'; Grundig G8
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--- On Fri, 9/7/12, rfoxwor1@xxxxxxxxxxxxxxx <rfoxwor1@xxxxxxxxxxxxxxx> wrote:

> From: rfoxwor1@xxxxxxxxxxxxxxx <rfoxwor1@xxxxxxxxxxxxxxx>
> Subject: Re: [IRCA] 738 & SAH's from Michigan - Friday morning 9/7/2012
> To: irca@xxxxxxxxxxxxxxxx
> Date: Friday, September 7, 2012, 1:57 PM
> Russ wrote,
> 
> > An audible heterodyne was often not present with
> differences of a few kHz, thus 'subaudible'. 
> 
> I think you meant to say a difference of a few _Hertz_
> 
> >Visually, you could 'see' the offset, and sometimes the
> beat between the two signals which might or more likely not
> be audible. So you saw the SAH on the scope. You used the
> same equipment to determine the actual offset value.
> > 
> > You could also manually count the beats - audibly or
> visually, as applicable - to determine the offset.
> > 
> > Russ Edmunds
> 
> 
> Another important factor is that the input to the vertical
> amplifier in the
> scope is taken from the AGC buss of the receiver, and not
> from the audio
> section. This is for the same reason, the audio response of
> the receiver
> at frequencies of 15 Hz or less is poor, just as your
> hearing is poor at such
> low frequencies. It is _recommended_ to use a DC-coupled
> oscilloscope. The
> original published work on SAH's was done (ca. 1965) by
> Gordon Nelson using Dumont
> 304 scopes with DC vert coupling and with scopes using P7
> phosphor, so the
> horizontal sweep rate could be reduced to maybe 1 or 2
> sweeps/second, if needed.
> This technique worked best when there were just 2 signals
> present, so that
> the difference would be clearly seen as a single "beat"
> wave. Also the AGC time
> constant could be modified for better LF response. Fourier
> analysis could be
> used to identify multiple sine waves present.
> 
> If you were looking at two signals, a few Hz apart, and at
> similar strengths,
> you could hear a "whoosh-whoosh-whoosh" as the two signals
> beat together.
> Nelson had discovered that two signals, a few Hz apart, *and
> differing by
> maybe => 30 dB*, could both be detected by looking for
> the very small displacement
> in the AGC voltage, when it was too small to be detected
> audibly. Since Nelson
> spent time looking for secondary European signals fading in
> underneath
> much stronger big signals, this was useful to him. He used
> published data
> from the EBU monitoring stations that listed European
> carrier freq's to 1 Hz
> resolution as a guide to identifying signals too weak to
> produce audio. Note,
> too that freq tolerances back then were much looser than
> today.
> 
> The existance of a weak "beat" was a precursor indicator of
> the possibility
> of hearing audio as the signal hopefully faded up to
> readability. Getting an
> audible signal was always the goal. This was a guide to
> where to spend your
> DXing effort, as a SAH display of an audible SAH beat is
> sort of superfluous info.
> 
> There is no reason this technique couldn't be done today,
> except for strong
> artifacts from sideband splash from signals 2 to 5 kHz away,
> masking
> the weak SAH trace, being so much worse today than we had in
> the 1960's.
> 
> Bob Foxworth k2euh
> 
> 
> 
> 
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