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

[Tom Wiltshire]

Thanks Oscar, that is useful.

I've been experimenting with various functions to try and get  
something that sounds good, and I'm getting closer. I've tried cos/ 
sine curves, and various quadratics. My current function is similar  
to the soft-knee they show in this datasheet, but I put the knee in  
the centre, whereas they have it further over to one side.
I'll try adjusting it like they show and see if that helps.

At least with a lookup table, I'm not struggling to find a hardware  
implementation for any given function. If I can plot it, I can do it  
- simple. Digital does have it's uses.

Thanks again,
Tom


On 15 Feb 2010, at 12:12, Oscar Salas wrote:

> 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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