I agree that it's around 70 dB below the normal signal level.
The original poster believes it is the source of his problem.
My ears are not good enough to hear 12 KHz.
Jack K8ZOA
Joe Subich, W4TV wrote:
The 12 KHz artifact appears to be the audio output DAC clock,
with the demodulated audio forming upper and lower sidebands
with it. It's down considerably, but still is apparently
causing issues with those who have good high frequency hearing
response.
I'm confused ... my measurements show the "12 KHz artifact" is
more than 70 dB below a 1V peak signal. The artifact is not
measurable when the headphone level is below -30 dBV. This
level is comparable to the in band receiver noise floor with a
-73 dBm RF signal and below the in band noise level with -100
dBM (1 uV) or -130 dBm sources. I do not understand why an
out of band signal some 70 dB down should be an issue.
The only reason the 12 KHz artifact can even be noticed with
test equipment is that the K3's out of band noise floor (noise
above the DSP and IF cut-off) is so exceptional. Even with AF
gain, the audio noise outside the passband is no more than -100
to -110 dBV (I can't measure it).
To complete the record, the 12 KHz artifact is also present
on the Line Out DAC about 70 dB below the desired signal.
73,
... Joe, W4TV
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Jack Smith
Sent: Tuesday, September 09, 2008 6:09 AM
To: Dave G4AON
Cc: elecraft@mailman.qth.net
Subject: Re: [Elecraft] Re: Heil Proset & K3 Audio fixed, sort of.
The 12 KHz artifact appears to be the audio output DAC clock,
with the
demodulated audio forming upper and lower sidebands with it.
It's down
considerably, but still is apparently causing issues with
those who have
good high frequency hearing response.
The K3's audio seems to have way too much high frequency
response, with
white noise visible up to 20 KHz and beyond in the headphone
and speaker
output ports. Most communications receivers roll off the
audio response
at a few KHz.
A bit of roll off in the K3 would be beneficial, I think,
either RC or LC.
Jack K8ZOA
Dave G4AON wrote:
Using Spectrum Lab with my "on motherboard" sound card shows a
persistent artefact at 12 KHz with sometimes one either side of it.
This doesn't appear when listening to typical speech on SSB
but tuning
to CW shows it, the signals either side of the 12 KHz one
are +/- the
tone of the CW signal, i.e. typically +/- 700 Hz.
Audibly, there is more hiss and high frequency "roughness" when
listening to CW signals with the K3 than listening to the
same signal
on a parallel connected K2 with it's AF DSP filtering, I would have
hoped the RX equaliser would cut the high frequency signals but it
doesn't.
Looking at the circuit tends to suggest the place for a low pass
filter would be the output of the DAC prior to the audio
amplfier at
"LHPOUT/RHPOUT" on the K3 DSP IF: DAC Audio board. Looking at the
audio spectrum from my K3 (without FM) shows nothing above
4 KHz, so a
low pass filter to cut everything above perhaps 5 KHz would do no
harm. I am reluctant to modify my K3 unless it's an
official Elecraft
modification, but there is certainly some high frequency
audio present
that could do with removing.
73 Dave, G4AON
K3/100 #80
--------------------
It suggests that perhaps you were hearing the artifacts > 4
KHz, such
as the 12 KHz with modulation sidebands or the one at 3.9
KHz. Maybe
you were not hearing the artifacts directly but beats amongst them?
Jack K8ZOA
Paul Fletcher wrote:
/ In order to try and eliminate the objectionable "noise" (careful
avoidance of
/>/ distortion, harmonics etc deliberate) in my headset
I've fitted an
LC filter
/>/ with a 4kHz cut off. This would still pass odd order
harmonics for
lower
/>/ audio frequencies. This filter has totally eliminated
the annoying
"noise" I
/>/ was hearing with the downside that I have to run more
AF gain to
compensate.
/>/ This kind of backs up previous measurements made by
others that the
/>/ harmonics are well down, and not noticeable. It's a bit
disappointing that I
/>/ had to do this but the headphone audio is now very nice indeed.
/>/ /
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