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mixxxdj / mixxx

Mixxx is Free DJ software that gives you everything you need to perform live mixes.
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Ability to use a stereo DAC output, to drive balanced mono signal (ie, invert a single channel) #12838

Open SephReed opened 8 months ago

SephReed commented 8 months ago

Feature Description

I've gotten used to this issue at open mics, though I'm usually using Bitwig which has the ability to invert any channel (creating a balanced line) and I can manage the problem that way.

Currently, MIxxx has no way to output through many different types of setups. Basically anything where an unbalanced line comes out of a laptop (eg like the headphones jack) and then plugs into a balanced input (eg 1/4" converter or even RCA sometimes). There are more ways of connecting to a mixer where it won't work than ones where it will. From what I can tell, the expectation is for people to buy specialized equipment to do what amounts to flipping the sign bit on a buffer of floats (though it's much harder in analog realm).

Just tonight, I was DJing my friends going away party. I almost couldn't do it because every cord I had to go from my laptop to the mixer had the same problem: I was unbalanced, and the mixer expected a balanced input. It almost killed the party not having this functionality... there would have been no music. No phone could connect, no laptop, nothing.

The request is simple: A toggle for balanced/unbalanced next to each output.

image

By adding this feature, DJs could use any cord setup whatsoever to connect to any input on a mixer. They could even connect directly to an amp. As long as there's something that will plug into both sides, there will be a way. Without this tool.. I dunno. I'm frankly confused as to what other people do. There's no way everyone thinks it's normal to use a special tool just to invert a channel, when that's a thing software can do so easily.

Maybe most people just don't know how balanced lines work? I'm very confused tbh.

SephReed commented 8 months ago

FWIW, also searching for a system wide MacOS solution:

And I've found something to do. It's listed in the stackexchange link.

JoergAtGithub commented 8 months ago

Just inverting would generate a huge DC bias. I think you have to add an offset of 0.5x full output amplitude, to have 0V DC at silence.

SephReed commented 8 months ago

I've tested this plenty of times and it works fine. And by inverting one channel, both should pick up similar hardware level DC Off, which would then cancel out when re-inverted at the mixer. So, this should do the opposite of what you said and cancel out any DC offset as well as any line noise picked up along the way.

Also, even if it is was doubling DC bias, that just means turning up the sound to makeup for the lost headroom. The speakers operating slightly off center is far better than them not being usable at all.

And his is all assuming that my built in DAC is significantly worse than an external one would be. On a M1 Pro, it seems the built in DAC is totally solid compared to external ones.

Finally, if DC bias doubled to the point of being a problem, the easiest solution would not involve multiplying the signal (volume), but adding/subtracting from it (bias). Assuming a somewhat consistent hardware bias, all you'd have to do is turn a knob at silence until sound check shows it being close enough to 0V.

daschuer commented 8 months ago

Can you make a GUI mockup? I can imagine to add "Channel 1-2 balanced" or "Channel 1-4 balanced" However this will be confusing if the sound-card has already a balanced output.

Be-ing commented 8 months ago

I don't think I've ever come across a mixer that has no unbalanced inputs. Many DJ mixers only have unbalanced inputs (except maybe the microphone inputs). Most other mixers have a stereo 1/8" TRS or dual RCA inputs for unbalanced stereo inputs from laptops and phones. Many amplifiers have unbalanced inputs as well, for example home theater amplifiers and some active studio monitors.

balanced input (eg 1/4" converter or even RCA sometimes)

1/4" (or any size) phone jacks can be balanced or unbalanced. 1/4" TRS phone jacks are typically used for balanced mono inputs, but a TRS jack could just as well be unbalanced stereo. RCA only has two conductors so it's always unbalanced.

The typical way to connect an unbalanced output to a balanced input is with a "pseudobalanced" connection where the "negative" pin is shorted to ground or left floating. Balanced inputs don't need balanced signals; you can also send the positive signal on one conductor and zero on the other and the difference between them will still be the signal, though it'll be half the voltage of a true balanced signal:

Balanced signal: (V + noise) - (-V + noise) = 2V Pseudobalanced signal: (V + noise) - (0 + noise) = V

This is probably why software doesn't provide options for inverting one channel of a dual mono pair; it's not necessary with an appropriate adapter. You don't need to invert the phase in software or hardware. Adding that to Mixxx's preferences would surely confuse plenty of users.

Your screenshot shows you have a Scarlett Solo which has balanced 1/4" outputs... so what's the problem? Or do you have a first or second generation Scarlett Solo with RCA outputs?

atskler commented 8 months ago

I was unbalanced, and the mixer expected a balanced input. It almost killed the party not having this functionality... there would have been no music.

You can always connect an unbalanced output to a balanced input. You will get the music, and probably some ground loop and additional noise.

Here is a great guide for how you connect unbalanced to balanced: https://www.ranecommercial.com/legacy/pdf/ranenotes/Sound_System_Interconnection.pdf