Tuesday, 6 October 2009

iPres 2009: Guttenbrunner on Digital Archaeology, recovering digital objects from audio waveforms

Early home computers often used audio cassettes as data media. Quite a bit of such data still exist in audio tapes in various archives, getting in worse and worse condition. Can they migrate the data without the original system in the future?

The system they used is the Philips Videopac+ G7400, basically a video game system released in 1983… and another one (!).

Data are encoded in bitstreams, which in turn are encoded in analogue waveforms (via a microphone/headphone socket pair and an audio cassette system!). They worked out how the waveforms responded to changes in the data (basically reverse-engineering the data encodings; would not have been so easy without a working computer). As a result, they were able to write a migration tool from the audio streams to non-obsolete formats.

It turned out there was already a solution that worked where there was a good signal from the tape, but these were often very old tapes in poor condition, so they implemented a different approach, which worked better.

Using old tapes, the other system recognised found no files. The actual system recovered 6 out of 23. Their new implementation recovered 22 out of 23 files, in some cases with errors. They checked by re-encoding the recovered files (on new tapes) and reloading to the actual system; most had minor errors that could be fixed if you knew what you were doing.

They think their findings are valid for all systems that use audio encodings, although there will be wide variations in encodings and file types, but it’s not extensible to other media types.


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