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

[Oscar Salas]

Maybe is useful to you the THAT application note 120: "VCAs in a Pan Potentiometer Application" This is made with the VCAs 2180 that are also logarithmic. 

www.thatcorp.com/datashts/dn120.pdf

Regards,
Oscar.



--- On Sun, 2/14/10, Tom Wiltshire <tom at electricdruid.net> wrote:

> From: Tom Wiltshire <tom at electricdruid.net>
> Subject: [sdiy] Pan laws - logarithmic or linear? (SSM2164 stereo panning)
> To: "synth diy" <synth-diy at dropmix.xs4all.nl>
> Date: Sunday, February 14, 2010, 10:16 PM
> 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
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