Storing data on videotape (1975) | Hacker News Reader

Storing data on videotape (1975)

cerncourier.com

38 pointssohkamyung23 comments

On a similar subject, you may have noticed that the two most common sampling frequencies for digital audio are 44.1 kHz and 48 kHz (along with their multiples, such as 88.2 kHz and 96 kHz). Why is that? Specifically, where did 44.1 kHz come from?

It's because early digital audio, before compact discs existed, was done by recording data onto video cassettes by the use of an adapter that converted analog audio to digital, then generated a signal that looked (to the videotape recorder) like a video signal.

In non-interlaced NTSC video, there are 30 frames per second, and 525 scan lines. But not all 525 are usable because of the vertical blanking interval. (Essentially that interval is just the time it takes for the CRT to move the electron beam back up to the top of the display.) So these adapters just encoded 3 samples' worth of data into a single row of video, and used 490 out of 525 lines because of the vertical blanking interval. And 3 * 490 * 30 == 44100, so 44100 samples per second.

NTSC also has an interlaced mode, which is 60 fields per second (two interlaced fields equal one frame), but each field is half as many lines, so the math works out the same.

PAL video is similar except that it is 25 frames per second instead of 60, so to compensate, when encoding audio into PAL video, there are 6/5 as many lines used, and 3 * 588 * 25 = 44100 as well.

More info:

https://en.wikipedia.org/wiki/44,100_Hz#Recording_on_video_e...

How many frequencies have their own Wikipedia page?

https://en.wikipedia.org/wiki/Crystal_oscillator_frequencies

Just a nitpick, but NTSC and PAL are always interlaced. They were designed for CRTs, and so there's no provision in the standards for indicating progressive frames because it didn't make a practical difference on those display systems.

It became a headache for digital systems, but not until the early to mid 1990s — before that digital video was usually stored at a half-frame resolution anyway because the processing power wasn't there for full-frame 480/576 video.

Oh, valid nitpick. My reference point for NTSC is whatever my old Amiga and Commodore 1084s monitor were capable of together, which is based closely on NTSC/PAL but not exactly the same.

https://en.wikipedia.org/wiki/ArVid this was rather popular in certain parts of the world in the early-mid nineties not just in the former USSR but the former USSR satellite states as well. Pirated software arrived from Austria weekly on QIC 80 tapes, got copied to ArVid/VHS in Hungary and sent on to Ukraine and it spread from there. The gigabytes capacity looked utterly ridiculous at a time when Lands of Lore with its eight floppies taking a whopping 11 megabytes was a crazy big game.

Later the pirates switched to 2GB DAT tapes and even later hard drives. By the time I left that world (early 00s), FTP existed, it sure did but there were quite a bit of problems with both storing and transferring enough so the weekly packages didn't stop.

Wow. 2GB of uncompressed data per tape is really impressive. That's more than most of the data-backup tape systems at the time.

I'm surprised/disappointed it never caught on in the US; I would have loved a backup system like that. But instead we got DAT/DDS, where the cheap recorders were barely interoperable and the expensive ones cost more than most individuals could afford. (And I think DDS tapes were under 2GB, in terms of true capacity and not the shifty advertised capacities.)

I suppose there wasn't enough money in a VHS-based tape system to get any company interested. Everybody wanted their own proprietary tape format so they could do the obnoxious razors-and-blades business model, I guess.

In the 90's I traded warez on Iomega ditto 250MB tapes with a small group of friends who were active in the BBS warez scene. They would get all the good stuff, trade, upload and get priority for being a good "upper" (or some such slang). Every month or so they would release a list. You made a wish list from that list and handed it in along with a blank tape and a few bucks for chips and soda. Few days later my friend would collect the tapes and rollerblade to my place to drop them off on his way home.

> QIC 80

Thanks for the flashback. My QIC80 neurons haven’t fired in 25 years.

I bought a little peripheral for my amiga 500 in the 90s that could use a vhs machine as a data backup. I could store about 100 megabytes on it. Unfortunately I don’t remember what it was called. It did work ok and it was really inexpensive I think I paid about $100 for it. The interesting thing to me was how simple the electronic guts of it were. No flat packs or long dips.

PCs got Danmere Backer, which got Linux drivers: http://linbacker.sourceforge.net/

It could store up to 5.3 gigabytes per tape.

I'm surprised the Amiga required extra hardware; I thought it had video capture and output built-in?

All the Amiga models could generate NTSC and PAL video, though some models needed adapters for composite output to connect to a VCR. None of the Amiga models had built-in video capture. Commodore and others made genlocks (to overlay graphics on video) and I think those contributed to the machines' reputation of being able to capture video.

Probably Video Backup System. It displayed black and white patterns on the screen.

https://www.bigbookofamigahardware.com/bboah/product.aspx?id...

yep that’s it in all it’s hand soldered glory!

That does appear to have a single 8 pin DIP in there.

What the VHS backups would look like if you watched them:

Amiga Video Backup System: https://youtube.com/watch?v=1jrLz__PlM0

Danmere Backer: https://youtube.com/watch?v=TUS0Zv2APjU&t=13m30s

Interesting that these all look black-and-white. I'm guessing using VHS's low-bandwidth chroma channels for additional capacity wasn't worth the added complexity.

Indeed ntsc does not have a lot of bandwidth devoted to chroma. Color on broadcast tv was tacked on about a decade after it was established. If you look at the signal on an oscilloscope only a small part is for chroma. I think it is called chroma burst.

Alpha Micro (a 68K platform with a strong DEC influence common in certain vertical markets in the 80s and 90s) systems did this as well. Some of their systems could even boot from VHS.

https://ampm.floodgap.com/

On a slightly related note, I used to use a hi-fi stereo equipped VHS (used for recording NICAM digital stereo transmissions) to listen to music, e.g. transferring my vinyl albums to VHS tape. Advantages (beyond protecting the original vinyl) included:

- Quality - the audio format used for recording NICAM audio was vastly superior to that of audio cassettes.

- Capacity - you could easily fit a double concept album on one tape so no interruptions flipping the record over, or even in many cases have an artist's entire back-catalogue on a single tape.

- Indexing - on my video recorder you could even record index points on the tape, so when you had several albums on one tape you could forward/rewind to the start of the album relatively easily.

That's super neat. What kind of VCRs support NICAM audio?

Like most fancy VCR features NICAM decoders started out as an expensive high end thing in the late 80s and gradually trickled down to lower price points. Note that there were no VCRs that recorded audio in the NICAM format, only decoded it.

The actual recording mechanism was VHS Hi-Fi, which uses helical scanning to record a relatively high quality analog stereo track onto the tape vs the normal not-very-good VHS mono audio track that uses a stationary read/write head.

In the late 90s D-VHS came out which recorded digital MPEG video and audio onto VHS tapes (and supported high definition 720p/1080i!) but it was an extremely expensive niche thing that never took off in any meaningful way.

Interesting; I'd not heard of NICAM, but there were several other audio formats that used video tape as a storage mechanism.

The Sony PCM-F1 recorder was probably the most common, and could be attached to either a VHS or a Betamax recorder.

My personal favorite, and one that I still own, was the dbx 700 (https://en.wikipedia.org/wiki/Dbx_Model_700_Digital_Audio_Pr...) which used delta-sigma rather than PCM encoding, and produced an NTSC output suitable for tape. (For reasons I don't really understand, 3/4" U-Matic was the dominant recording format, at least that I saw. But it works fine onto a good VHS deck.) The digital format was ahead of its time, although it was a bit of a dead-end technologically unless you count SACD (arguably also a dead-end).

Compared to the F1 it was a joy to use, because you didn't have to leave the headroom of a PCM recorder (it doesn't "brickwall" if you run a hot signal into it). In the days when most audio engineers were used to analog tape and recording very hot, this led to much better-sounding recordings.

You could also take the video signal and send it around a video distribution system (closed-circuit TV, cable, satellite, etc.) and have a very high quality audio feed on a point-to-point link.

Occasionally the boxes turn up on the used market... I still have one, and largely just keep it in the signal loop because of all the neat 1980s blinkenlights and digital VU meters.

Original (PDF) article from CERN News can be read at [1]

[1] https://cds.cern.ch/record/1729841

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