You might be surprised at how the signals are actually recorded on tapes. For VHS tapes for example, it's actually a series of stacked "diagonal" lines of signal. Imagine a sequence of symbols like \\\\\\\\\\\, except at any given horizontal point along the tape there are about 5 stacked traces of signal. They're also diagonal on audio cassettes, but much less "slanted" relative to the tape's length.
I don't remember the precise terms for the various concepts, but hopefully that shows how specific to a given format the reader needs to be.
Audiotape is - or at least, Phillips compact cassettes are - recorded linearly. The recording method you're talking about for videotape is called helical scan, because the heads trace out a helix pattern. You might be confusing that with the head azimuth, which does alternate between fields on VHS to reduce cross-talk or something.
Some other bits of jank in the VHS spec:
- Because the tape is moving while it's being written, it stretches the signal out on the tape. This is perfectly fine for normal playback. But when the tape is not moving, the signal's now too wide for the playback head, and you can only read about half the picture. That's why your VCR had bars of static whenever you paused (unless you sprung for the four-head model)
- Audio is still recorded linearly, and you can't exactly chuck a linear head in an angled, spinning drum. So you have to put the audio head further away from the tape. And that distance is fixed; changing it means your machine is now playing audio out of sync with the video.
Also, there is an extension to VHS that lets you record audio along the video in the helical area, it's called VHS Hi-Fi and it improves the audio dramatically with the trade-off that any minor video glitches will add pops to the audio. Humans are way more sensitive to gaps in audio than video, after all.
I dumped a bunch of albums, and a few CD's, to Hi-Fi VHS. A while back, I stumbled onto a compatible VCR at a yard sale for a couple bucks.
Yes, those gaps are annoying as all get out. Tracking has to be dialed right in to recover the sound properly.
But, once you do that?
It's really great! Frequency response goes almost to zero, and up to 22Khz, and it's flat for most of that range.
One of the albums I recorded had a warp in it. When the actual vinyl was played on a good stereo system it was possible to see the speaker cones actually move in time with the warp. It's essentially a very low frequency signal.
Cassette does not reproduce that. I bet a good reel-to-reel system would.
Hi-Fi VHS reproduced it pretty much bang on perfect!
The best part was being able to add index marks! While recording, one could press a button and get one of those written to the tape. I had several tapes made with music I really like and could access pretty much anything on it quickly.
Yeah, that channel is where I learned it as well. I probably should have linked it.
Basically every video he does scratches that itch of learning a new thing about the world. The analog video series is indeed especially fantastic (laserdisc, VHS, B&W vs color TV, blu-ray, etc).
If a high resolution reader could “recreate” the entire tape signal in memory, you could do the format specific parsing in software. You could even identify the tape type based on the signal patterns
I've heard of someone doing this by connecting an ADC directly off the heads of a VHS player, but I can't find it now. The bandwidth of such a system is only about 6MHz, putting it well within modern SDR capability.
If you "scan the magnetism" for the whole width of the tape -- which such a generic reader for magnetic tapes would have to do anyway -- you can parse out such things after the fact.
adg.
.beh
..cf
Even if your input is "a..db.gec.hf", that should be able to postprocess out later, for any possible angle, given enough resolution.
It's a lot like decoding music from vinyl grooves based on a 2D image scan of the vinyl.
That's going to take an extremely high resolution to avoid cross-talk. For a linear tape with multiple tracks you just need to have a few small heads at different heights.
That sounds like a similar problem to reading unknown floppy formats. I'll bet you that actually scanning the flux (I think that's the term) image of the tape without shredding it is a more difficult mechanical problem.
I think one major network in the US is 720p60, but the vast majority of broadcast transmissions globally is 1080i25 or 1080i30*
Of course sadly so much content is produced at 1080p24 nowadays for that “film” effect it’s rather meaningless for display purposes.
Online of course it tends to be p50 or more commonly p60 (even in 50hz countries). YouTubeers tend to be able to afford more lights and clearer microphones than big budget productions too.
Right, and the fact that there were two different signal standards and VHS was a leaky abstraction that mostly just encoded the signal as-is all adds to the complication of any "generic magnetic tape reader" and hoping to get useful digital data back from that.
That's a backronym. VHS originally was an abbreviation for "Video Home System".
Edit: also, the helical scan on a VHS tape can't really be said to be "vertical". A helical scan already implies a direction along the length of the helix, and "vertical helix" doesn't really make sense in the context of scanning down the length of a magnetic tape. Adding "vertical" to the actual term "helical scan" can really only be an attempt to back-fit an acronym to "VHS".
I am at least aware of the diagonal recording on VHS tapes, that is why I assumed it might be hard do build a generic reader as you could not necessarily take advantage of the geometry or the speed of the tape or things like that. There is probably a good reason for the complexity inside an old VHS recorder.
Other tidbits I recall from what my Dad told me years ago. He repaired VCRs for a while.
The beta machines would phase shift the signal 180˚ every other frame when writing, and they use that for noise cancellation. VHS did four frames at 90˚ each, because of patents.
The read heads were on a spinning drum. Beta machines had an L loading system that would grab the tape and wrap it almost all the way around the drum with the read heads, exposing more tape at once to the reading mechanism. VHS had an M loading system that grabbed the tape on either side and pulled it up, covering maybe half of the drum, because of patents.
And supposedly VHS won out because Sony was begging a lot of money for patent licensing. But tape capacity was probably a factor, too.
I don't remember the precise terms for the various concepts, but hopefully that shows how specific to a given format the reader needs to be.