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Somebody invented a system where the biasing frequency was detected on playback of the tape, and that was used as a reference to pitch the whole thing back to original speed automatically. Don't know if any system has filtered down to an affordable level from the hallowed halls of the restoration agencies (eg CEDAR), but if you recorded a piece at 24/96, it would be interesting to see if you can detect anything in the 18-19kHz and upwards range on a spectrum analyser plug-in. Not an expert on biasing frequencies for different systems, so can't be of any help beyond that.

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PS 8.3kHz for ferric cassette, 14.3kHz for chrome cassette, 19kHz for 15 IPS reel from memory, research needed for anything else...

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Edit: Scrub that, I've got it completely round my neck -it's 30kHz upwards (some are 150kHz) not much audio frequency recording equipment will be capable of recording that, so it will be serious restoration setups that can do it...

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That's why it puts this methodogy firmly in the realms of restoration agancies, as it needs specialised, purpose-built detection equipment which can then modulate the recording or pitch-control process.

Tends to be used for uneven wow rather than consistent tape stretch, I'd guess, but a wonderful piece of lateral thinking and development on somone's part...

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