Combining PAN LAW and VOLUME

NuttyMonk · Joined: Jun 30, 2020 Posts: 63 Location: UK

Hi all,

i am in the process of designing a stereo Eurorack mixer. I am currently using an SSI2162 for the panning and another SSI2162 for the overall volume level. Both of these stages, first panning, then volume require an TL072 op-amp after them to convert the current output of the SSI2162's into voltage.

What i would like to do is combine the functions of panning and volume control into a single SSI2162 for each channel (L and R). I am controlling the voltage going into the SSI2162's using a microcontroller. This way i can apply a proper pan law. I've already done the calculations and code in excel and am happy with the results.

The problem i have is trying to figure out if i can combine the two functions of panning and volume control into one SSI2162. This way i can get rid of two IC's from my design.

If the panning is either directly in the middle or panned hard left or right, then i can do the calculations myself. The problem is when the panning control isn't in one of these positions. How do i go about keeping the relationship between the left and right channel while increasing or decreasing the volume control.

I think i should probably do the calculations in the dB space and then convert that back to a digital value to control the DAC which controls the SSI2162's. I know that -3dB is half of the perceived volume as far as our ears are concerned. So if i figure out what the relationship is between the left and right channel (e.g. right is -3dB of left) then i can reduce or increase the two stereo channels, keeping the same -3dB difference between them.

Firstly, would that simplistic way of calculating the levels for the channels work? If so, what about when the volume control is pushed to the top or the bottom? Do i just put in a hard limit at the top so neither channel can go beyond the maximum value allowed by the SSI2162's and then keep the other channel at a lower value as i calculated above? And when the volume control is way down at the bottom do i just ignore any extra calculations or limits since we can barely hear anything anyway?

Any advice from someone who is trained or knowledgeable about this issue would be greatly appreciated. I could spend a lot of time using trial and error but i reckon i might save time and provide a useful resource for others by asking this question here instead.

I've included a spreadsheet with my pan law calculations in case anyone else would like to look at them. This is where i got the formulas from to do the calculations. https://courses.ideate.cmu.edu/15-322/f2019/wp-content/uploads/2019/08/Loudness-Concepts-Panning-Laws.pdf

Thanks for your time.

Cheers

NM

PANNING LAWS.xls

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