As you may have noticed, there’s not much audio content here yet. That’s because I’m currently not only bubbling with ideas, but also working without good tools. So before we design anything, we need to have the right tools in order.
Priority one was a useful desk scope / multimeter: check.
Priority two is a spectrum / audio analyzer.
Now I actually do own one, I never threw away my trusty Spectral Dynamics SD375 . But as with my big Tektronix o’scope, it’s a bit of a boat anchor to keep around the house. So rest assured both of these are safely in storage, and I’m busy training my son to appreciate them so he can inherit and care for them one day hopefully far from today.
I’ve been scouting for an Audio Precision on eBay a bit, but without much success. I’m not ready to shell out thousands of bucks for something that is extremely cool, but still flawed. You see, the one thing an AP has going for it is credibility. You show any old weird curve and as long as it has the AP logo on the right hand top corner, you’re golden. And let’s not forget the bragging rights. “Yeah sure, I’ve got an AP at home” – gets all the girls!
But foregoing that, it has to be admitted that especially the system one machines are showing their age. The user interface runs under DOS or windows 98. Wow. forget attaching an internet enabled network cable to those beasts.
AP themselves do not recommend buying one. Of course if I was selling system twos, I’d also recommend against buying a machine I’m no longer selling. But they do have a point: there is some custom hardware in there and when that breaks, you’re done for. No support, no parts.
Reflecting on all of that for a bit, I pondered what the alternative could be. A good sound card with a PC sounds like the logical choice. A quick inventory actually turned up some quite decent parts:
- M-audio Delta 1010. One of my favourite sound cards. I can safely say that now that I have gathered at least enough of them for my own use. I scoured them in parts, buying the cards and the break-out boxes separately, and built internal power supplies in each of them. SNR is around 110 dB which is very good but of course not stellar. However for distortion it’s more than enough, and if it is about finding noise floors all I have to add is a low noise preamp. The Delta range is excellently supported under both windows and linux, and built for heavy duty use (some clearly showing signs of it). At least a 15 year production run with minimal changes shows that this was a very good basic design. There are some potential tweaks that I’m sure will show up here one day, but as a rule of thumb you cannot go wrong with this card and for the (used) price, it’s simply a winner.
- VIA EVCM-F mini-ITX motherboard with a 1 GHz CPU and a VT8235M chipset. It seems they were made for POS (point-of-sale aka cash registers) and I do agree about the POS part. I bought this off eBay hundreds of years ago. Unfortunately I could only find a 256MB stick of RAM but bigger ones should cost pennies if I’m feeling frisky to supercharge this beast. I don’t even know the original purpose I had for it anymore (I think it did have to do something with a Delta card), but the great thing is it runs warm but not hot with just a few small heat sinks. Together with a 12V power supply adapter fed from a laptop brick and a compact flash to PATA adapter with a 4GB Sandisk extreme III card it is quiet. Quiet as in SILENT. Your move, Audio Precision.
That should do it! As slow as this junk hardware is, it probably runs circles around the machines that AP had to work with. The downside is of course that this box will have to do all the heavy analysis (FFT, deconvolution, averaging, etc) lifting. Well, it’s an experiment.
Even though there are a few very nice audio analysis programs for windows, this box is going to run linux. No discussion. Hobby is supposed to be fun, not torture.
Due to the limited hardware capabilities it would have to be a pretty small foot print version, but at the same time it would have to be able to run some exotic software. That led me to Archlinux. Their modular design and custom package manager ticks the right boxes, and the support on their wiki is just stellar.
Getting Arch to run was easy enough. One hiccup was that the disk was formatted as a GPT partition and syslinux is not a good bootloader for that. At least I could not get it to run. GRUB2 did just fine and in just a few hours I had a running machine. Less time than a fresh windows install needs to pull in its 8,000 updates.
The window manager is LXDE, but mostly I’ll be logging in over SSH with X forwarding so just X11 would have been enough. The screenshots below are made on an Ubuntu machine using X11 forwarding.
The first analysis programs I installed are Jaaa and baudline. More on that in a subsequent post. It’s not exactly APone style functionality yet, but at least jaaa and japa are open source and that means what you don’t have, you can build. In this case it is unfortunately true that I only trust AP to do a proper implementation, and all others have to be open source and if not, I guess I have little choice but to lead the way. This story is just beginning…
All in all, spending a day or so installing some very nice software and re-using dust-gathering equipment now means I can do some pretty impressive and reproducible measurements. As soon as I’ve cleared my desk and built this all into some enclosure…