[TenTec] AM Receiver distortion in Orion II

[Ron Castro]

The interesting thing about all of this is that on the Orion II AM stations 
sound substantially better in SSB (same 3 kHz audio BW) than they do in AM! 
I still think there is a DSP problem in the AM detector algorithm.

      Ron N6IE
 www.N6IE.com
(Formerly N6AHA)

----- Original Message ----- 
From: "Paul Christensen" <w9ac at arrl.net>
To: "Carl Moreschi" <n4py at arrl.net>; "Discussion of Ten-Tec Equipment" 
<tentec at contesting.com>
Sent: Wednesday, January 24, 2007 3:49 AM
Subject: Re: [TenTec] AM Receiver distortion in Orion II


>> Number 2, There is clearly a difference between the two recordings, so
>> maybe
>> someone in Tennessee has a tin ear?
>
> I just ran the two audio clips through SpectraPlus FFT software.  The .MP3
> output is read directly by SpectraPlus internal to the sound card with no
> audio cabling necessary.
>
> The O2 has a sharp AM audio response cutoff at exactly 3 kHz (6 kHz of 
> BW),
> as expected.  On the FFT display, it is clearly a very sharp DSP cut-off
> with no detectable skirt.  Audio response cuts off at 3 kHz and nothing
> escapes beyond that point.  The O2 low-end response extends to below 80 
> Hz,
> but rolls off smoothly below 90 Hz.
>
> On the TS-870, the AM response is also quite sharp, but cuts off at
> approximately 3.7 kHz.  What we are all hearing is the significance of the
> speech intelligibility in the area between 3 and 4 kHz.  That region does
> more to add speech intelligibility than any other area of the audible
> spectrum.  This should come as no surprise when one looks at the
> Fletcher-Munson family of loudness curves.  At approximately 3.5 kHz, the
> ear is most sensitive to absolute sound pressure level.  Why?  Because the
> ear canal terminates into the tympanic membrane and forms a closed
> acoustical pipe whose average resonant frequency is -- about 3.5 kHz.  The
> exact point of resonance is a function of the length and diameter of our
> individual ear canals.  So, a mere 700 Hz of added bandwidth makes a
> significant difference in intelligibility and is one reason why ESSB
> proponents have been advocating the use of extended-bandwidth SSB when
> conditions permit.
>
> Although things start sounding even better beyond 4 kHz, diminishing 
> returns
> sets in quickly.  In the interest of allowing operators the use of maximum
> speech intelligibility versus spectrum conservation, 4 kHz for SSB and 8 
> kHz
> for AM is an excellent choice for maximum occupied bandwidth when 
> operating
> conditions allow it.
>
> Paul, W9AC
>
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