[sdiy] Pan laws - logarithmic or linear? (SSM2164 stereo panning)

[Tom Wiltshire]

Hi all,

I wonder if someone can help me get something clear in my head. I'm  
getting muddled up between logarithmic and linear scales.

I'm trying to implement a sensible-sounding pan on my dual stereo  
SSM2164 VCA. This is considerably complicated by the fact that the  
SSM2164 has a log response on its control input. The effect of this  
is that if you simply fade down the left channel linearly whilst  
fading up the right channel linearly, you get a massive hole in the  
middle. The sound moves a little towards the centre, but disappears.  
Finally it re-emerges over by the right speaker.

I understand that the classic pan law is 3dB down in the middle. In  
theory, it should need to be 6dB down, but in practice 3dB is usually  
enough.
This reduction is achieved by using something that looks like a  
cosine curve from 0 to Pi/2 on the left channel, and the same  
reversed on the right. In the centre, both channels are at about  
2/3rds or 3/4 of full output.

Now what I don't understand is what the Y-scale is on this cosine  
graph - is it logarithmic or linear?

I'm controlling the two VCA channels from software using a 12-bit  
DAC, so what I'm going to finish up doing is using a lookup table to  
compensate for the log control response and give me something more  
like the function I'm after. clues as to what this table should look  
like appreciated.

I think I might be suffering from Sunday night burnout. I've been  
developing this circuit all weekend, but now I've got a 'Pan'  
variable and a 'Volume' variable, and I've got to convert them into a  
'Left CV' and a 'Right CV' and the response of those two CVs is  
logarithmic and it's all just muddling up in my head.

The 'design process' wisdom was to go and take a break, right? I'm on  
it....

Thanks,
Tom