I’m not an audio engineer or anything like. But I would like to understand how the audio chain works (a little)
I notice that when I select Channel L+R in the VARA FM soundcard settings, the audio bumps up about 6 dB compared to just having the L channel selected.
From the schematic, I see that there is just one input (MICIN) to the CM108 chip. I also note that MODE is pulled low, which I think means that the CM108 is put into headset configuration - and with that, the audio goes through a mixer. I notice that the same thing happens when using a DRA board that has the CM119 chip aboard.
Now forgive me, but does this configuration cause the mono input audio from the radio is split into two output digital channels: Left and Right (i.e. the same signal is output on both channels?) So there’s always audio on both channels. Also, this isn’t bidirectional because if Channel R is selected in VARA FM, no audio is output (I’ve tested this empirically and appears to be the case.)
like the stereo headphone jack on a pc or hi-fi.
so you can choose to send just one channel to the radio or both (‘downmixed’ depending on the interface)
some interfaces use one of the channels to ‘trigger’ ptt.
so the option is there for whatever works best for your installation.
the mic in that you’re referencing is the audio from the radio into the interface.
and yes, that is a single channel signal. so if you had a dualband radio that had the ability to send band A to one side and band B to another, the interface wouldn’t keep them discrete.
Thanks for the response @webmonkey - much appreciated. Now this is weird, but, what I have observed conflicts a bit with your explanation.
For example, if I set a single channel - let’s say L since that is what Digirig uses, then audio from the radio is converted to a digital stream by the CM108 and ends up being processed by VARA FM. VARA FM says the VU audio strength is, say -17 dB.
Now, if I set VARA FM to L+R, I see a 3 to 6 db bump in the audio - and the VU meter reports a signal somewhere between -14 to -11 dB. That is a doubling or quadrupling of the audio signal!
The only reason I can think of for this to be the case is that the channels must carry identical signals - perhaps one more attenuated than the other, or not. If they were different signals, then their superposition would end up being pretty noisy.
I’m guessing that VARA FM is simply adding the channels together. My speculation makes sense since there is only one mic input pin on the CM108 - not two. However, this doesn’t mean I’m not missing something! I wish VARA FM was less opaque with respect to it’s workings (i.e. open source and better documented).
As @webmonkey pointed out the right and left channel options in VaraFM refer to the output from the computer’s perspective - the audio from the computer to the radio. Digirigs use CM108B with stereo output capability. Only left channel is used for audio payload that reaches radio. In some Digirig models (Digirig Lite, Digirig DR-891) the right channel is connected to the internal VOX circuit and will trigger radio’s PTT if constant tone or payload sent there.
Now when it comes to audio from radio to the computer, the codec only has a single channel so what you get on the computer is a mono signal. There should be no difference in the level of the received audio when you change channel selections for transmitted audio unless VaraFM does some magic of its own. Can’t imagine the reason that would make sense though.
Yes, this is what I would expected too looking at your schematics and the CM108B datasheet.
So, based on your input, then VARA FM must be doing something odd since the VU meter does report a 3 dB to 6 dB audio gain when L+R is selected over L. I see this when using a Masters Communications DRA board too - which has the CM119 chip.
Either VARA FM may be doubling the signal by multiplying the input values by 2 or 4 for reasons beyond our comprehension or there’s a bug. This could be either in the meter visualisation or somewhere else in the audio processing chain.
So frustrating not knowing what’s going on and no easy way of finding out!
Thanks - as always - I love Digirigs and own quite a few now and am always promoting them for their ease of setup and use (especially the Lite version - it’s amazing!)
