I was asked by a younger person how many giga bytes does a (magnetic) tape hold?
I had to decribe the concept of analogue/ digital, A2D process, error rate, even the home computers of the 80's. all this whilst completely and utterly drunk
the conversation latest for ages - no idea whether he fully understood or if I was talking rubbish.
But it it made me realise how much life has changed
my older brother had a weekend job where he would go and change backup tapes for some company on the weekends. he'd bring his Amiga and we'd play video games while we waited for the tapes to do their thing. I was pretty young so I don't remember the details, but they had to rewind/seek/? which took a long time.
Those were most likely backup tapes made specifically for data like DLT or LTO, Travan, etc. DAT and 8mm (Data8) could legitimately be used for backup. VHS was never a widespread backup medium especially when affordable more reliable options were readily available in the 90s - DVHS itself flopped and backup onto consumer VHS was always a gimmick.
Tape is still widely used for backup, it’s not quite dead.
While never popular, Metrum/MountainGate VHS drives and libraries saw significant use by at least a few large customers for a number of years, so were presumably more than a gimmick (source: personal tech support experience).
Note that, in spite of the recording media (S-VHS, IIRC, not D-VHS) used, these drives were rugged, expensive, rack-mount devices and were never marketed as consumer products.
I wouldn’t really call that a standard, more like an inexpensive consumer offering - there a bunch during the 90s it’s sort of an enticing offering - just use the vhs recorder you already have. You’d be crazy to use that for mission critical backup.
I used my 16-bit 192 kHz sound card to determine how much data can fit on a cassette by "prolonging" the length of each bit until I got an acceptable error rate. I found that the absolute best you can do with a decent old Technics tape deck is 8-bit at 12kHz which means for a 90-min Tape you can fit about 60 MB on one tape.
I noticed however that switching between bits produced a "ringing artifact" oscillating at about 60 kHz. If this is caused by my particular setup I don't know. With a better modulation than just PCM this could be compensated and you could pull probably more data out of the stream.
Thanks for the hint. The next thing to try would be to connect the tape head directly to the IO-Pins of a Raspberry PI and then use a simple manchester encoded signal ramping up the frequency. A Raspberry PI can push up to 1 MHz and has an internal Schmitt-Trigger which would be perfect for reading the data back.
The absolute ceiling for a high quality Type IV tape is probably around 600MB, assuming a dynamic range in the low 60s and low 20s frequency response - ie that is how much it would take to sample losslessly. Though storing digital data losslessly is another story, it will be some fraction of that for forward error correction, but with sophisticated encoding you could probably get a few hundred megs on a high quality tape. A type I tape will be well under 100MB.
I had to decribe the concept of analogue/ digital, A2D process, error rate, even the home computers of the 80's. all this whilst completely and utterly drunk
the conversation latest for ages - no idea whether he fully understood or if I was talking rubbish.
But it it made me realise how much life has changed