[arm-allstar] Audio processing, was Re: cdm1250 transmit audio

[Kevin Custer]

On 3/3/2020 4:39 AM, "Patrick Perdue via ARM-allstar" wrote:
> I'm mostly trying to fill out the bottom end from radio TX audio more 
> than increasing the top. I realize attempting to increase high end 
> response from RF to ASL would be generally pointless due to FM 
> bandwidth restrictions, niquist theorem, 3.5 k brick wall low pass 
> filter, etc. But, if I can extend the bottom just a little past where 
> the filters live, all without going into distortion and saturation, or 
> getting into frequencies around where PL tones live, even just 10 or 
> 20 hz makes a pretty significant difference with some radios, and not 
> at all with others. Listening to my radios on simplex vs unfiltered 
> through my SM50 from the other side of Allstar, this can be achieved, 
> but I admit that I am picky about audio. I have processes for 
> maximizing intelligibility of digital modes as well. And, of course, 
> every time I hear an Allstar node where GSM is the preferred codec, I 
> die a little inside.

Patrick,

You mentioned that some of your installations required amplification.  
If the amplifier you're using doesn't have the ability to drive the 
input impedance of the microphone circuit, low end roll off will occur, 
as well as distortion due to loading that amplifier below its capable 
current handling ability due to the low impedance.  This is a concern 
with the amplifier component used in the URI, as the amplifiers 
capability is 2000 ohms, but many radios are 600 ohms, some are lower.

There are other, more capable radio adapter interfaces that factor that, 
as well as having enough voltage swing to modulate just about any radio 
directly at the FM modulator.  Just Google RA-40.

Kevin W3KKC