Recycling used punched cards

Posted September 6, 2005

You remember punched cards for storing data? Unfortunately they were a bit before my time, though the MITSFS still uses them a lot to write on instead of buying new index cards. Once you punch a hole in them, the hole is there for good, of course, and basically you can store one bit per hole (or per absence of a hole). A curious mind might wonder, "Well, even after I've stored my data on there, there are still some places left over where I could punch new holes. Can I store more than one bit per bit if I reuse a card?"

It turns out that you can. Granted, punching new holes tends to destroy the original data, but you can reuse a card that someone else has already punched by just encoding your own data with sufficient error-correction to be able to ignore the "noise" that the previous punches represent.

In the limiting case of someone using a card as a communications channel (i.e. I punch just a couple of holes to encode a little data, mail it to my friend, then he punches a couple of new holes and mails it back, etc. etc.), you can get a limiting usage of 2.37 bits of data transfer per bit of storage. Neat!

The full paper has some other interesting results, and additionally makes the point that these sorts of questions and techniques may become important again for nanotech purposes: replace "punched card" with "atomic-scale data storage" and "card readers" with "autonomous nanobots" and you can get a very inspiring picture of how many devices can (and will have to!) share data storage and transmission in the presence of competing devices as well as natural environmental noise on that scale.