Notices by 🇳🇴 Thor — backup account (thorthenorseman@octodon.social), page 103

@ctrlaltdog This

@ctrlaltdog 👍👍⭐️⭐️⭐️❤️❤️👌

Everyone who checks out my SoundCloud listens to maybe 2-3 tracks and conclude what type of music I make based on that. The thing is: I have an extremely varied back catalogue. There's adult contemporary, disco, jazz, electro, orchestral, ragtime, hip hop, funk, electro swing, synth pop, heavy rock and more. To give people an idea of the variety, I think I'd have to make a sort of condensed DJ mix of the best tracks in either genre.

My roomie is a gamer and has two computers. The Synergy software enables him to focus input seamlessly between them, but the same isn't true of their audio. His 5.1 speakers from Logitech only have one set of analog inputs, and SPDIF has its issues.

I have drawn up a schematic for a USB-powered 5.1 summing box with 2 simultaneous surround sources. I'm going to get some PCBs made and he's going to pay me for it.

Anyone else want one?

@rsolva It's less clunky and more fun to just design a PCB...

@Siphonay ?

@rsolva That's a bit overkill, though. I mean, I can order the parts to build this for $20-30 and a PCB for $10-15. He already has an XLR mic preamp. This should have a simple, low-cost solution.

@rsolva SPDIF is out of the question, as I said, so that leaves analog 5.1, and AFAIK, no affordable consumer product will perform the simple task of mixing two 5.1 signals coming off triple mini-jack connectors into a single triple mini-jack 5.1 signal.

@rsolva Oh, but the sound card part is 100% fine. I don't think you understand the problem yet. The problem is when you have TWO 5.1 sources going into 5.1 active PC speakers with ONE set of 5.1 inputs. My roomie already has the sound sources. The problem is to mix them so you can hear them simultaneously.

@rsolva 12 + 6 = 18 op amps. You'd need five 4-in-1 op amp chips to do this, with two op amps to spare. To actually create your virtual ground, you could use a 1:2 voltage divider (2 resistors in series between Gnd and Vcc) and feed the centre signal of that into one of your spare op amps.

@rsolva If you want to operate this off a 5V DC supply, you simply create a virtual ground at 2.5V, feed your 10kOhm resistor into this, and place a coupling capacitor tuned for a 10kOhm load ahead of that, and boom, you have your buffered input. Now repeat this 12 times on your circuit board, mix the 12 signals into 6 with resistor Y-junctions, and you've got your mixed signal. To drive your 6 line inputs at the other end with this, use another 6 op amp buffers.

@rsolva This is why, when you attempt to plug a 600 Ohm headphone into a line output, the audio will sound thin. Thus, it's crucial to have the right load.

Next solution? You still want your Y-junction with the resistors in order to do the mixing, but you need to maintain a correct load. The simplest way of doing this is to use an op amp buffer. Op amps have inputs with virtually no load. If you connect a 10kOhm resistor to ground with a T-junction at the op amp's input, you're in business.

@rsolva Your typical DC powered audio device uses a virtual ground centred between the power rails, so a typical 5V device would have a virtual ground of 2.5V. To remove this DC bias on the outputs, coupling capacitors are used. These capacitors filter out frequencies between a threshold that is determined by the capacitor's value and the load, which is presumed to be 10k. If a different load is seen, the filter will be wrong. If the load changes, the filter will be unstable.

@rsolva The way to avoid a short circuit is to add resistors to the inputs to the Y-junction, but this creates another problem: Line-level outputs expect to drive a 10kOhm load, and line-level inputs exert such a load. The resistors will interfere with that, and the interaction of the different voltage levels from each source will create a wrong and inconsistent load for each source.

@rsolva The answer would be some kind of box with 3+3 input jacks (6+6 channels) and 3 output jacks (6 channels) that mixes the 12 channels into 6. Using 3 Y-cables in reverse might seem like a good idea, but it's actually a very bad idea, because you're driving one set of signal lines with two conflicting signal levels. In electronics parlance, this is what we call a short circuit, and it has a nasty tendency to cause the magic smoke to seep out of your components.

@rsolva This is probably the case for most gamers who own 5.1 systems. The easiest solution is to use the 3 analog minijack connectors. Games that output 5.1 in PCM format will handle these just fine, but if you, like many other gamers who like to play full-screen games while they stream on Twitch or surf the web, have two computers, how do you connect them to your typical Logitech 5.1 system, which only has one set of analog inputs?