Post : Storing 32 bytes of data in a piece of glass
URL : http://hackaday.com/2012/11/09/storing-32-bytes-of-data-in-a-piece-of-glass/
Posted : November 9, 2012 at 6:00 am
Author : Brian Benchoff
Tags : delay line, delay line memory, mercury delay line, piezoelectric
Categories : classic hacks

http://hackadaycom.files.wordpress.com/2012/11/mike.png

After finding an old piezoelectric delay line in an old TV, [Mike] decided to figure out how it works (http://www.youtube.com/watch?v=-qerYLM-eEg) and in the process stored his name in sound waves reflecting inside a piece of glass

[Mike] was intrigued by these old-fashioned delay lines after watching [Dave] from EEVblog's teardown of an circa 1985 camcorder (http://www.youtube.com/watch?v=tQyX3F4ggM8) . [Dave] found a piezoelectric delay line in his camcorder - a device that is able to store digital data by sending a sound wave into a glass plate, letting the sound wave bounce through the plate. and picking up the sound on the other end. It's actually not too dissimilar to a mercury delay line (http://en.wikipedia.org/wiki/Delay_line_memory#Mercury_delay_lines) used in the earliest computers.

After sending a pulse through his piezoelectric delay line, [Mike] picked up an echo almost exactly 64 microseconds later. After hooking up a simple circuit constructed out of a 74-series chip, [Mike] found he could 'loop' the delay line and keep a pulse going for up to 3 milliseconds.

Three milliseconds isn't much, but by injecting serial data into the delay line, [Mike] was able to spell out his name in binary, as seen above. It's just 32 bits stored for a fraction of a second, making it a very volatile, low-capacity memory, but functionally equivalent to the old mercury delay lines of yore.

It's certainly not what [Mike] or [Dave]'s delay line was designed to do; these video delay lines were used to hold the previous line of video for a form of error correction. Outside [Mike]'s workbench and a few museums, though, you won't see a delay line used as a form of computer memory. A very cool build and an awesome history lesson, and we thank [Mike] for that.

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