[App_rpt-users] usb radio config file

[Stephen Rodgers]

David Powell-Williams wrote:
>> <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.
> 
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> App_rpt-users at qrvc.com
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> 

This probably explains why I never experienced this. I never use the mic input with a URI as I like to send the
pre-emphasized and limited audio directly to the modulator. This works quite well as long as you have a a transmitter
using direct FM and not phase modulation.

Steve
WA6ZFT