Ask Hackaday: The Ten Dollar Digital Mixing Desk?

Ask Hackaday: The Ten Dollar Digital Mixing Desk?

There comes a point in every engineer’s life at which they need a mixing desk, and for me that point is now. But the marketplace for a cheap small mixer just ain’t what it used to be. Where once there were bedroom musicians with a four-track cassette recorder if they were lucky, now everything’s on the computer. Lay down as many tracks as you like, edit and post-process them digitally without much need for a physical mixer, isn’t it great to be living in the future!

This means that those bedroom musicians no longer need cheap mixers, so the models I was looking for have disappeared. In their place are models aimed at podcasters and DJs. If I want a bunch of silly digital effects or a two-channel desk with a crossfader I can fill my boots, but for a conventiaonal mixer I have to look somewhat upmarket. Around the three figure mark are several models, but I am both a cheapscate and an engineer. Surely I can come up with an alternative.

Cheap And Nasty Sound Cards To The Rescue!

An analogue mixer is an extremely simple device at heart, it simply sums a series of audio signals each of which has its own volume control fader. It’s so simple that one can be made with passive components only, and indeed there are extremely affordable mixers that do just that. Most small mixers however use  straightforward op-amp gain stages and buffers, with adjustable ones for each channel. It’s possible to make one without too much bother, and indeed I considered exactly that. The problem was that the budget climbs with each successive channel towards the point at which I’d be better off spending a bit more and buying one. I’m not pricing for the most expensive faders on the market, but a reasonable quality linear potentiometer adds quite a bit per channel to the BoM.

The USB sound card, with its case removed.
They claim this thing has a TI PCM2902 chip inside, and who am I to dispute that!

At this point it occurred to me, can I use the PC as a live mixer with multiple sound cards? I can order a heap of very cheap and nasty USB sound cards for under ten dollars, so it won’t cost me much to try. I placed the order, and when they arrived I plugged them in and instantly had a computer with five audio jacks. Unfortunately I can’t just fire up Audacity expecting an awesome multi-channel experience. I have a load of sound cards to choose from, but I can only record from one of them at any one time. It’s time for a dive into Linux audio, to a level I’ve never needed to do before because, well, it’s always just worked, hasn’t it?

Who Knew There Was So Much To Linux Audio!

A screenshot of Alsa Mixer, showing a list of sound cards
So near and yet so far, I can see them but not touch them!

In the beginning, there was the Open Sound System, or OSS. My Linux in the 1990s was all about setting up web servers, so the first Linux sound subsystem passed me by. Instead like probably most of you, I’m used to ALSA, the Advanced Linux Sound System. This sits at kernel level and provides an interface to the disparate pieces of sound hardware there may be connected to the system. On top of that lie sound servers providing a further interface layer such as PulseAudio or Jack,and in many distributions the whole lot has been replaced by PipeWire.

All these promise mixing and multiple card support as their killer feature, so somewhere in that lot it should be possible to find what I want, right? Unfortunately not, because while they can all see a load of soundcards, none of the various machine configurations I tried could make applications see more than one of them at once. Perhaps a solution could be found in binding several cards together as a virtual ALSA card. But here yet again there’s no reward, because as the instructions point out, the real hardware will drift out of sync over time. I wonder whether my live mixer application would find this less problematic than a simultaneous multi-track recorder, but something tells me if it did, everybody would be doing it.

So I’ve conspicuously failed to make a cheap live mixing desk out of a thousand-dollar laptop and ten dollars’ worth of cheap sound cards. Plenty of you will be no doubt be queueing up to berate me for my less-than-1337 level of Linux wizardry, but the truth is I’ve never really concerned myself with the multimedia features before. I’m still curious though, can this be done? Answer me below in the comments!


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