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Do I need to change gain if merging from stereo to mono?

February 24, 2013 | 4:35 pm

I have a patcher that takes two separate inputs, by default goes through 1.0 gain (allowing me to adjust gain if I want) and then goes to two outputs of a dac. Here’s the section of the patcher in question

– Pasted Max Patch, click to expand. –

Now suppose I need to merge those two inputs (left and right) so as to go out through a single channel rather than two channels. Do I have to reduce the initial gain from each input before I connect them to a single input port on a dac?


February 24, 2013 | 8:04 pm

If both signals are scaled to 1.0, then there is a strong likelihood that the combined signals will produce an output above 1.0 when their peaks line up. Most DAWs that allow you to combine a stereo signal into mono give you the option of reducing the signal by 3dB in the process to prevent clipping. I’ve always made it a practice to do the same thing in Max by using [*~ 0.5] on either the two channels before summing, or on the summed signal before it gets to the [dac~].


February 24, 2013 | 9:48 pm

Just a note: [*~ 0.5] would be -6 dB. For -3 dB (which is the typical adjustment) you need [*~ 0.71]. Ask a pocket calculator for the exact value of sqrt(2) if you want more precision (don’t forget, about 6 digits is as good as it gets for Max).

The -3 dB guideline may depend a little bit on your material, but it’s a standard rule-of-thumb.


February 24, 2013 | 10:58 pm

Oh, I always thought the DAW was doing a -3dBm conversion, which would be [*~ 0.5]. Is that not the case? Is it dB SPL?


February 25, 2013 | 6:07 pm

Thanks for the responses — the goal is that I am sending a stereo pair to channels 1/2 at FoH such that the two faders are at 0db (say) and then I temporarily sum my stereo pair to a single channel that I now send out on channel 1, the FoH engineer should not have to adjust the channel 1 fader at all.


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