Re: Audio volume level passed to apps lower than expected


jdow
 

The VERY common technique for AGC in SDRs turns the whole chain around backwards. It measures the input, calculates what is needed to get the desired output, and applies that to the signal leaving the AGC process. When everything is "float" type digits it does not matter if you run filters and other "IF" processing at levels of 0.0001 rms or 1.0 rms. And the gain can be anything within the range of "float" type digits for AGC. (I did find a bug in FM stereo demodulators that were running at a level of 1.0 getting a value in a digital feedback loop, the phase lock loop for the stereo demodulator, that dropped one of the terms down below the range of a 32 bit floating point (float) number. The loop did not work the way I "knew" it should work so I spent time tracking it down and figuring out a somewhat rude but very effective fix.)

{^_^}

On 20210915 14:46:31, Tony_AD0VC wrote:
Frank,

The waterfall and spectrum are upstream from the AGC function in the DSP panel. I'm not the one to ask for the particulars but I would guess that the AGC is part of the demodulator function. Some SDRs do have a hardware AGC function as well but that is not what is being controlled in the DSP panel of SDRC.

Tony


From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io> on behalf of Frank O'Donnell K6FOD <gio@...>
Sent: Wednesday, September 15, 2021 12:31 PM
To: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io>
Subject: Re: [SDR-Radio] Audio volume level passed to apps lower than expected
 
Thanks, Tony. I see that when I have AGC off, the gain slider in that section had been set to minimum. Moving that slider up and down doesn't affect the RF signal level as seen in the waterfall or dBm meter. However, it does significantly affect the audio output level, and by moving it up I get a strong input level in the apps receiving audio from SDR Console. So problem solved -- thanks again!

Just so I understand this, am I right in thinking that the AGC slider acts at the IF stage? Thus it doesn't affect RF levels reported, but will affect the audio output since that's downstream? Also -- it appears that it doesn't affect levels as recorded in I/Q .wav files, which I gather are taken before the IF stage?

Frank

Join main@SDR-Radio.groups.io to automatically receive all group messages.