in a vast echoing space

DIY Audio Snake, Part 2

(Alternate title: “As If I Didn’t Hate Myself Enough Already.”)

All the bits showed up, a heaping pile of small bubblepack envelopes with mixed Mandarin and English labeling. I’m sure I’m on at least two watchlists now due to the cumulative volume. I didn’t really track arrival dates, but the pieces trickling in at a rate of one or two per day over most of a week. After writing the last post, I revisited the project box dimensions and ultimately decided I’d better preemptively order larger ones. It’s a good thing I did, too, because the ones I originally got were far too small.

So it all got here, and then I put it on a shelf for a couple more weeks while I worked up the nerve to start the build. On December 7th I finally got sufficiently brave – foolhardy? – to make a thing. It all went pretty much as previously described: HDMI to DB-25 in the boxes, XLR to DB-25 as pigtails, and pins as I thought they ought to go. What I wasn’t prepared for was the amount of time taken building the pigtails. The microphone cable ended up being much smaller in diameter than I thought it was which meant space wasn’t an issue but stripping it sure was. In order to give myself enough spare that I could afford to make mistakes, I used 16cm lengths between the DB-25 and the XLR connectors. Good thing, too, as I ended up with only around 50cm left (of the original ~300cm – ordering in Imperial units then measuring in metric is hard). If I ever do anything like this again, I’ll figure out how to strip shielded wire without accidentally cutting through half the shielding. (Really, the big problem was that the largest hole on my wire strippers was still too small. By “figure out”, I really mean “buy a more appropriate tool”.)

I used 10cm lengths inside the box because unlike the mic cable, I’d overestimated how much I’d need for that. The boxes I ended up using were more shallow than wide (which was perfect) and I could’ve physically gotten away with 8cm or less – I just don’t know that I’d have been able to have worked with a more constrained length. Stripping short wire is a bit difficult because you don’t have much to hold onto. As for mounting the connectors onto the end plates of the box, Dad cut (drilled, really) appropriately-sized holes, then I mounted both the HDMI and DB-25 connectors with threaded anchors. The HDMI connector looks a bit funny from the outside as there’s no such thing as a screw-down HDMI cable but I used one anchor through the board (with a nut on the backside) as a spacer then ran another anchor through the end plate into the first one. The threading in the anchors isn’t as deep as their screws are long so there’s almost a millimetre of slack between the end plate and the anchor head. It doesn’t matter, though, as the mounting holes were drilled sufficiently small that the screws won’t move without rotational torque.

And then I tested the boxes with a multimeter, labeled the connectors, and took them to Team Night without ever having run audio through it. Yeah, I live life on the edge alright. More like, after seven hours of assembly, I needed some affirmation of my life decisions regardless of how the final product turned out. And the response was as positive as I’d hoped. A surprising number of people thought I was somehow running a real HDMI signal, not just using an HDMI cable as an analogue carrier. But once we got that sorted out, we found a speaker and a 50′ HDMI cable and took them for a spin. We didn’t actually try with microphone-level signals, but with line-level signals they worked admirably. Signal loss was barely audible for the first four channels, and got progressively worse (but not by much) for the last two.

There’s a couple other things I’d still like to do. The pigtails are especially fragile as the mic cable is too small in diameter to fill the holes in the plastic sheathes of the XLR connectors, and even six of them don’t fill the hole in the plastic DB-25 hood. This means the connections on both ends are supported only by the shortest wire in the cable. I made sure in all cases that it’s the ground connector (physically, the cable shielding) carrying the stress but that’s only marginally better. I’d like to add some better stress relief before (if ever) using the units “for realsies”.

I’d also like to get some hard numbers on signal loss. Probably just straightfoward as connecting each line to the ins and outs of an audio interface, generating some signal, and comparing what comes back to the interface against what was sent out. But it would be nice to do the test with a variety of lengths of HDMI cables to see how much the cable plays in versus shoddy soldering.

Up next: probably nothing. Even if the signal test does say nice things, the product is still kind of pointless. Six channels isn’t worth much, and HDMI is ultimately pretty fragile. Even with reinforcement, I’d still be wary of throwing something as half-baked as this onto a stage. It certainly wouldn’t be an important stage. Still, it was a neat project in a lot of other ways: I got to try some 100% Chineseum XLR connectors (terrible), solder a lot of wires (backbreaking until you get your table set up correctly), and learn about how HDMI electrically works under the hood (before promptly ignoring that and doing something else). A+ would recommend.

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