A 38-Year-Old Vocoder Project

It is hard to remember that scant decades ago, electronic magazines — the pre-Internet equivalent of blogs — featured lots of audio circuits based on analog processing. Music synthesizers were popular for example, because microcontrollers were expensive and unable to perform digital signal processing tasks in the way you would use them today. [Julian] has been trying to build a vocoder from that era from ETI magazine. Along the way, he’s making videos documenting what he’s found and how’s he resolving issues.

The circuit generates levels for particular input frequencies. It does so with a two-op-amp bandpass filter, a two-op-amp rectifier, and then an op-amp lowpass filter. That’s five op-amps for each band (there are 14 bands) plus the support circuitry. And that’s just the input section! Today, you would simply sample the signal and do a fast Fourier transform (FFT) to get the same kind of data.

We can sympathize with [Julian] as it looks like he’s worked on this off and on for a long time. The playlist has some preparatory videos going back to 2015. Part of what’s taking so long is probably that he’s making different subsections and testing them in various ways. So while the project isn’t quite finished, it is interesting to watch the description of the pieces, the testing, and the issues he’s encountering. While we’d be more likely to simulate and use a scope on the real hardware, there’s something charming about building real circuits and using VU meter boards to visualize their outputs.

Building something from old plans is bound to turn up some oddities, and this project is no exception. [Julian] notes that in the 14 bands, all but three use a pretty standard bipolar op-amp. But three of the bands use a FET-based op-amp. If you’d guess it was for frequency response, you’d guess wrong. The three bands are in the higher range, but there are two higher bands that use the bipolar devices. [Julian] was soliciting opinions about why this should be. We immediately recognized the need for the extra resistor on the bipolar op-amp was to counter input bias error. But it wasn’t clear to us why the three bands were “special” to start with. There are a few theories about that in the YouTube comments.

One thing we don’t miss is the lack of a microcontroller to handle a user interface. Look at all the controls on that front panel! Not only is that a lot of wiring, the panel work is substantial. Today, you’d have an LCD, a few buttons for a menu, and a rotary encoder.

If you want to brush up on your filter design [Elliot Williams] did a whole series on filters, as did [Bil Herd]. Both of these can give you some background to further appreciate this project.

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