Mixing signals

What turning on an amplifier sounds like

I was very frustrated today by power-saving technology. I thought it would be a nice idea to hook up my TV, sound system, and selected other electrical macguffins to a master/slave powerboard. That way, I turn on the TV, everything comes on at once. I turn off the TV, everything turns back off again. More convenient and super environmentally conscious/dolphin-friendly. How could I lose?

Well, it turns out that all this environmental friendliness is starting to trip over itself, because most devices now start up in standby mode. So you can't just power them on to power them on, you have to power them on and hit a button on the remote. Some devices, by pure dumb luck I assume, will accept the switch already being pressed in when they start. If that's the case, you can hack the behaviour you want if you figure out a way to hold the button in permanently. I may or may not have a G-clamp affixed to the sound system in the loungeroom.

However, the amplifier in my room was less cooperative. I spent some time searching for a way to trick it into powering on when it powered on, to no avail. Everything seemed hopeless. In a gesture akin to burning a goat carcass, I even emailed the manufacturer's product support address. I have not yet received the bland non-reply I know is my due, but when it arrives I'll be sure to assume a suitable pose of supplication, head towards the Philippines, and pray that I never sink so low again. However, while drowning my sorrows in the depths of old product manuals, I made a curious discovery.

The amplifier has two remote-control ports, one out and one in, so that you can synchronise it with other hardware made by the same company. Of course, I don't have any hardware made by the same company, but, hey, a control port is a control port. The ports were standard 3.5mm audio jacks, so I figured I could break that out to a couple of wires, and then... learn enough signal processing to reverse-engineer it, I guess? I was actually at a bit of a loss about how to proceed. The public transport-themed Bus Pirate seemed like a pretty good bet, but I'd have to order it and I still wasn't confident I'd necessarily get it to work.

But then it suddenly hit me. If it looks like an audio jack, maybe it sounds like an audio jack! I don't need to understand the protocol, I just need to be able to replay it. So I plugged some earphones into the IR-out port and damn near blew my eardrums. Turns out digital signals are not designed for casual listening. Still, that's definite progress! I eventually managed to hook up a USB sound card and a line cable, recorded the "sound" of me turning the amplifier off and on, swapped the cable to the IR-in port and played it back.

And, would you believe it? It worked! I don't think in my entire life I've ever had something deserve to work less and still work. I am gobsmacked by my own hillbilly ingenuity. I am now controlling my amplifier by playing sounds at it from a tiny ARM box that detects when the TV is turned on via HDMI-CEC. I put the remote control audio files up on Github. So if you have an HK-series amplifier, or you just want to hear what a remote control sounds like, that might be useful to you.

But, uh, maybe turn your volume down first. Remote controls are loud.