Nowadays, if you want to delay an audio signal for, say, an echo or a reverb, you’d probably just do it digitally. But it wasn’t long ago that wasn’t a realistic option. Some devices used mechanical means, but there were also ICs like the TCA350 “bucket brigade” device that [10maurycy10] shows us in a recent post.

In this case, bucket brigade is a euphemism calling to mind how firemen would pass buckets down the line to put out a fire. It’s a bit of an analog analogy. The “bucket” is a MOSFET and capacitor. The “water” is electrical charge stored in the cap.  All those charges are tiny snippets of an analog signal.

In practice, the chip has two clock signals that do not overlap. The first one gates the signal to a small capacitor which follows the input signal voltage. Then, when that gate clock closes, the second clock gates that output to another identical capacitor. The second capacitor discharges the first one and the whole process repeats, sometimes for hundreds of times.

In addition to a test circuit and some signals going in and out, the post also shows photomicrographs of the chip’s insides. As you might expect, all those identical gates make for a very regular layout on the die.

You might think these devices are obsolete, and that’s true. However, the basic idea is still in use for CCD camera sensors.

Sometimes, those old delay lines were actually columns of mercury or coiled-up transmission lines. You could even use a garden hose or build your own delay line memory.