[op25-dev] Allstar integration? [1 Attachment]

Graham Norbury gnorbury at bondcar.com
Fri Dec 28 14:15:23 UTC 2018

I've often thought about attempting to write an enhanced version of 
sockaudio.py with AGC functionality.  I'm not well versed on the various 
algorithms though, and haven't yet found any suitable code samples 
online that can be readily repurposed.  If anyone has any pointers I'd 
be open to looking at it.


On 12/28/18 8:52 AM, wllmbecks at gmail.com [op25-dev] wrote:
> [Attachment(s) <#TopText> from wllmbecks at gmail.com included below]
> Here are a couple of additional thoughts/comments…..
> You don’t necessarily have to run Icecast on the RPi.  You can just a 
> easily configure darkice to push the
> audio stream to a preconfigured mount point on a remote server.
> One of the unfortunate aspects with digital audio (p25) is that few 
> radio technicians understand how to setup proper microphone or console 
> audio levels.  On analog system they were used to looking at the 
> deviation scope/meter on their service monitor and cranking the mic 
> gain to achieve rated system deviation.   However, this is not the 
> case in P25 or other digital modes.  The ones and zeros hit specific 
> deviation targets (symbols) that don’t care how high or how low the 
> actual audio is.
> Graham Norbury has added a feature to his boatbod fork of op25 that 
> give you the option on the rx.py command line to adjust the recovered 
> audio level.   I’ve found that in my location that on some systems 
> some additional gain is helpful and have specified *-x 2* on my rx.py 
> command line to increase the gain.    You may want to vary this value 
> experimentally as needed to compensate for local conditions.
> I don’t know what the limits are but at some point you are likely to 
> start clipping and then you’ll know the upper limit.  Perhaps Graham 
> will chime in with addition information on this subject.
> On the subject of P25 audio.   I won’t go into methods I’ve used to 
> set and measuring digital audio levels, but I am attaching 
> *apco_ref.wav*, an audio test tone file crated and calibrated for use 
> with the op25 on the Raspberry Pi to produce a test tone equivalent in 
> level to that reproduced if you were to connect a service monitor up 
>  to your Pi’s SDR modulated with a standard P25 1011 (tone) test 
> pattern.  I did this because of a need to setup audio levels with 
> external devices.   One such example is where I break out the audio 
> from the RPi and send it through an automatic gain control (AGC) 
> amplifier before sending it on to darkice for conversion to an mp3 
> stream to level out p25 system/user levels.
> Bill, WA8WG

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