One of the attractions of buying at the bottom end of the electronics market by mail order from China is that you never quite know what will come your way. Sometimes it’s a diamond in the rough, while with others it’s a mess. Occasionally along comes something which should work but doesn’t, and that’s the moment when you wonder if you could fix it. [Nyanpasu64] had just such a device, an HDMI to VGA converter with audio that didn’t work. What could be wrong?

The HDMI to VGA chip has an onboard audio digital-to-analog converter (DAC), and it’s a delta-sigma design. This type of DAC is frequently used in audio applications because it works by shifting its switching frequency many times higher than the input sample rate, thus reducing considerably the distortion. This one wasn’t performing as advertised though, and the problem turned out to be that switching frequency being all over the output. Clearly the filter wasn’t working, which led to the design of a new filter. The write-up is therefore an extensive dive into filter design, and in part also a discovery of the effect of impedance on them.

For a super-cheap module to cause so much work, one might ask why not simply spend a few more dollars and get a better one. But had they done that we wouldn’t have seen this write-up, so we’re sticking with team cheap.

We’ve looked at audio DACs, in the past.

There are many experimenters who have had a go at a mechanical television, and though there are a few challenges, it’s a relatively straightforward project in 2026. A hundred years ago though it was still beyond the cutting edge of technology, and that’s where [Paul Kocyla] is placing his build. It’s a mechanical TV system, using only parts that would have been available in the 1920s. The project isn’t finished yet, but we suggest following along for some fascinating insights into developments in early electronics.

As it stands he has a wooden chassis, a period power supply and amplifier, a synchronous motor, and of course the Nipkow disk that makes it all possible. The electronics aren’t quite finished, and he’s yet to source a neon lamp. This last party may be particularly tricky, as there were specific flat-plate neon lamps made for this application. It’s interesting to find that the motor would synchronize to the grid frequency and would need to be restarted a few times for the frame to be in the right place.

His last posting contains a particularly interesting nugget of information for anyone using tubes. The amplifier carries a 120 Hz hum, something difficult to trace. The culprit is the early tubes with directly heated cathodes formed from the heaters themselves; they had such a low thermal mass that they would “blink” at 120 Hz if fed with AC. A set of period copper oxide rectifiers solve this by feeding DC to the heaters. There’s a YouTube series to follow, and we’ve placed the most recent one in which he fixes the power supply, below the break.

Back in January, we marked the hundredth anniversary of mechanical TV’s invention. Meanwhile, some of us have been known to experiment in this direction too.