There used to be special ISA cards that allowed for connecting a VCR to a PC in order to use the VCR as a tape-backup.

I always thought that was "cool" from a technological standpoint, but a bit gimmicky. I mean, sure, you could do the same with a bunch of audio cassettes if you're patient.

...

But, actually, I've done some quick research/math and think it may not have been so ridiculous, after-all.

In fact, it was probably *much* faster than most tape-backup drives at the time, due to its helical heads. And certainly a single VHS tape could store far more data. (nevermind their being cheap, back then).

In fact, the numbers suggest videocassettes were pretty-much on-par with spinning platters from half a decade later in many ways.

https://en.m.wikipedia.org/wiki/VHS

https://en.m.wikipedia.org/wiki/ST506/ST412

I somehow was under the impression the head on a VHS scans one *line* of the picture each time it passes the tape. But, apparently it actually scans an entire frame. At 60Hz!

In Spinning-platter-terms, it'd be the equivalent of a hard disk spinning at 3600RPM. Sure, not blisteringly fast, but not the order-of-magnitude difference I was expecting.

The video bandwidth is 3MHz, which may again seem slow, but again, consider that hard drives from half a decade later were limited to 5MHz, and probably didn't reach that before IDE replaced them...

"The limited bandwidth of the data cable was not an issue at the time and is not the factor that limited the performance of the system."

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I'm losing steam.

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But this came up as a result of thinking about how to archive old hard drives' data without having a functioning computer/OS/interface-card to do-so.

"The ST506 interface between the controller and drive was derived from the Shugart Associates SA1000 interface,^[5] which was in turn based upon the floppy disk drive interface,^[6] thereby making disk controller design relatively easy."

First-off, it wouldn't be too difficult to nearly directly interface two drives of these sorts to each other, with a simple microcontroller (or, frankly, a handful of TTL logic) inbetween to watch index pulses and control stepping, head-selects, and write-gates. After that, the original drive's "read data" output could be wired directly to the destination's "write data" input. Thus copying from an older/smaller drive to a newer/larger one with no host inbetween.

It's got its caveats... E.G. The new drive would only be usable in place of the original, on the original's controller, *as though* it was the original drive.

The new drive has to have equal or more cylinders and heads. It has to spin at exactly the same rate, or slower. Its magnetic coating has to handle the recording density (especially, again, if it spins slower).

BUT: it could be done. Which might be good reason to keep a later-model drive like these around if you're into retro stuff. I'm betting some of the drives from that era can even adjust their spin-rate with a potentiometer.

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Anyhow, again, I've a lot to cover, but I'm really losing steam.

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Enter the thoughts on using VHS as an only slightly more difficult-to-interface direct-track-copying method... as long as the drive (maybe Shugart or early MFM? is within the VHS's abilities, which it seems there may very well be such drives in retro-gurus' hands.

So, herein, say you've got a 5MB mini-fridge-sized drive with an AC spindle-motor (which just happens to spin at 3600RPM, hmm). Heh...

Anyhow, I guess it's ridiculous these days, each track could be recorded directly to a PC via a USB logic analyzer, right? (certainly with #sdramThingZero - 133MS/s 32-bit Logic Analyzer) 

Or isn't there something like that already for floppies ("something-flux").

Anyhow, I guess the main point is that storing/transferring the flux-transitions in "analog" [wherein I mean *temporally*] has the benefit of not needing to know details like the exact bit/sample-rate, or the encoding format (RLL/MFM/FM], nor anything about higher-level details like the OS's choice of sector-size or hex values for padding bytes.

I guess this has all been done. 

I just thought it interesting that VCRs are in many ways on-par with hard disks from the era. I knew the helical-scan thing was a clever solution, but had no idea it was *that* clever. Heck, cassettes aren't cleanrooms.

...

I guess maybe part of the idea was something along the lines of backing up the original minifridge drive onto tape, then dumping that back to a 5.25in drive of higher capacity (once a suitable one is acquired); their interfaces are basically compatible, and unlike IDE/SCSI, no knowledge of the actual *data*/format would be necessary in the transfer, nor would the new drive's actual parameters need to be identical to the old one's. Yet the old controller card would think it the same. I think that's intriguing. (oh, bad sectors in different locations might prevent this idea, heh). 

It's not dissimilar to my finding out that throwing a 1.44M 3.5in floppy drive in place of a 400K 5.25in drive was perfectly doable in #OMNI 4 - a Kaypro 2x Logic Analyzer .

Not unlike using a duplicate of your favorite video-game floppy-disk to play from, rather than wearing-out the original... Except, instead of disks, we're talking drives, heh!

...

12/16/22:

The HaD blog just had a relevant article...

https://hackaday.com/2022/12/13/vhs-decode-project-could-help-archival-efforts/#more-567080

 In the comments was this, which i'll peruse later: https://www.spiraltechinc.com/otis/IRIG_Files/IRIG_Chap6F.htm