[App_rpt-users] usb radio config file

[David Powell-Williams]

> <snip>
> Second issue:  The loudest playback sounds cause the URI to go into
> saturation and distort. I may not have calibrated the levels very
carefully,
> but even if I manually lower the txlevel setting from 800 down to, say,
300
> it still distorts - it's just a quieter distortion. What can cause this?
> <snip>
> 
> I have the same problem with a modified CM108 usb installation. 
> 
> After setting all the 'radio tune' parameters, there is serious distortion
> on voice modulation peaks, irrespective of the source being Iaxrpt or
> another node.  
> 
> However, the 'radio tune txvoice' generates a perfect sine wave at 3 KHz
> deviation, as seen on an oscilloscope at the exciter modulator or on a
> service monitor off-air.  I am using duplex=0 in rpt.conf since the node
> operates simplex.
> 
> Dave
> VE7MQ
> 2140
> 
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> 


I have not seen this on a properly configured system. Chances are it has to
do with de-emphasis configuration on the receive side, or
pre-emphasis/limiting (prelim) in the transmit side)

Where is the audio getting distorted? Transmit or receive?
so you see the distortion on the TX line or RX line if you look with a
scope? 

You might be able to determine where it is getting distored by connecting
your node to a working node you can monitor,  then send audio both ways.

Disclaimer: I offer URI support and suggestions as a courtesy only as I do
not derive any income from sales of the URI. I am in no way obligated to
provide support at any level.

Steve
WA6ZFT

Steve:

Thank you for your response. Much appreciated.

This distortion also appears when accessing the transmitter digitally via
Iaxrpt with no radio receivers involved.  Ditto for when my node is
connected elsewhere, such as Calnet.  The problem is entirely on transmit,
somewhere in the route from apt_rpt internals to the MICOR exciter audio
input.  The receive audio is perfect.

I have two independent nodes on separate computers. I followed your
suggestion and linked them together.  Using a service monitor, I fed 1 KHz
tones at various deviations into the receiver of Node A and monitored the
transmit output of problem Node B.  There is no distortion until reaching 4
KHz.  At 5 KHz deviation, there is serious distortion of the sine wave.

I want to emphasize that this problem remains constant no matter if the
modulation source originates in a radio receiver, another node or Iaxrpt. As
Ken mentioned earlier, adjusting txvoice moves the deviation up and down but
has no effect upon the distortion.  FWIW, the distortion (on voice) sounds
like listening to a loudspeaker with a ripped cone.

After reading Ken's last post, I tried 'txprelim=yes' and it was a great
improvement.  It would be nice if the limiting could be selected
independently.

To summarize: it seems that with 'txprelim=no', audio from a receiver seeing
5 KHz peaks or the equivalent from Iaxrpt or from other nodes, is going to
be distorted by the URI being overdriven.  Unfortunately, the distortion
strikes right at the deviation level that most users hit regularly.

I am not using the DMK product. I understand it's a very professional unit.
I rolled my own some time ago, following the original construction details.

Thanks again for a great system.

73,
Dave
VE7MQ.