For my new crime-novel I am in need of some more specific info on hiding digital data on analog VCR or VHS cassettes.
It's important to keep in mind that VHS is not only notorious bad, but even too bad to really capture full TV quality (only 3MHz and reduced Chroma). It's one of these barely good enough technologies that allow cheap devices that are better than not being able to do the job, but not really doing a good job.
This is also the reason why any attempt to use the blanking interval (teletext) for data storage. while it might be nice and even would allow decoding with any equipped TV, VCS is simply not able to record them in sufficient quality.
In principal there are distinct recordings on a VHS tape that can be used: Video and Audio. Recording Audio was another another way to lower bandwidth needed and thus reduce cost. Essentially there is a linear audio track, much like from a compact cassette, on the lower end of the video tape.
For such a project using the Audio track might be the most plausible way (unless we're presenting a super high tech James Bond type villain) as simple manipulation can be done already with kitchen level tools, while semi professional equipment (in this case mainly a computer and some self made analogue electronics) would already allow high quality (within limits) results.
The drawback is that the audio channel is about 10 kHz - which is even below what a lousy kids cassette deck in 1980 could do (12-15 kHz). In theory the 10 kHz might be good for some 5 kBit/s, except we want to hide it in existing audio, so I guess there isn't room for more than 50-100 bit/s, that's 10 bytes per second, or 36 kByte/hour.
Sounds not much by today's standard, but it's still ~10 pages cramped with text (or more like if not all lines are filled all the way). Quite room for a lot of lists and secret information. So if your intended usage fits that, using the audio track would be not only quite possible, but even doable with reasonable effort.
It would need be two (good) VHS recorders, two tape machines and the encoding hardware.
Essentially this would involve recording the existing audio from an existing cassette (*1) - best on a good tape machine - then that recording this thru whatever soft/hardware setup is used to insert the signal and record the result on the second tape machine. This has the advantage that replay and second recording could be made as often as needed without stressing the tapes. The first tape machine could be saved if the modification can be done in real time. Still, I would record the intermediate result on tape, not directly onto the second VCR. But even that could be saved - if the equipment used is fast and reliable enough. When the new sound track is ready, the original tape is copied onto a new one, directly from one VCR to the next, but only the video track, audio track to be replaced by the new one. The only hurdle is to press play at the right time.
All of this can be done with unmodified VCRs and no special hardware except what is needed to encode the message.
Of course, if this is set in the mid to late 1990s, it would as well be possible to store and modify the audio data in digital form. But requirements would be much higher - including a lot of software that can't be found on the internet. Using a few analogue components is way less effort - and at that time, more people would still carry the knowledge to do so.
One issue would be the selection of the right tape to record on. Best would be a new cassette, except if it's supposed to look like a prerecorded one (for cover) it might be hard to find a fitting one. Using a second prerecorded could be a way out, except rerecording on VHS is a mess. Quality will drop visibly.
A solution might be using a modified recorder, one that only overwrites the audio track. This would require a bit of tinkering, but is entirely possible. With an additional, dedicated erase head this could deliver a genuine cassette with a modified audio track. Best cover possible.
Oh, and one more thing: This doesn't work with Extended or Long Play cassettes. Their signal is even worse.
Now, doing the same for video is several magnitude more complex. Some modifications could be made using analogue tools, still, it's be a lot of work at least. And forget about any digital manipulation. It would be rather visible with afordable technology at that time. Unless the villain is Lex Luthor, he might have the means to build a computer capable of doing so without easy noticeable artefacts.
Using later HIFI-VHS
Around 1985 JVC introduced HIFI recording by adding a rather complex encoding below the video signal. Now not only full 20 kHz and an extreme good noise ratio, but also full stereo was available. By using the same method as above a 2-8 times higher data rate would be possible, delivering maybe 100 kByte per hour of video.
The real effect is if we fast forward until a few years later when essentially all premade cassettes were recorded in this mode ... in addition(!) to classic linear recording. By 1990 even cheap VCR were capable to decode the new standard, wich meant, if they found an embedded HIFI signal, the linear track was simply ignored.
Long story short, if the story is set after 1990, then the 'old' linear track will (almost) never show up when the cassette is played in a regular VCR. But the linear track is still there and it still offers it's 10 kHz channel. Good for maybe 0.5-1 KiB/s of data to be hidden. That's some 2-3 MiB per hour of recording. Not much, but for sure en extreme amount for 1990 - the equivalent of a stack of more than 20 diskettes.
Using MPEG-1 as defined for Video-CD in 1993 (prior to DVD), this is good for at least 20 seconds of video in TV quality - much more by reducing the frame rate. But serious, I would drop the video idea here - rather go for pictures and/or text.
The whole elegance of that part would be that the data is hidden complete without any influence to the carrying flick. 100% side band storage. Only someone using a more than 5-10 year old recorder would be noting that signal.
Also, while recording may still need a bit of knowledge, the whole process will be simplified as there is no signal to be modified, just the linear audio track replaced by the data track, which can be generated without any regard to the video/audio content of the cassette. Hardware can be less complicate and be a single modified VCR, equipped with an additional audio erase head (the tricky part) and a separate input for the write head of the linear track.
On the decoding side it's even less effort, as each modern (HIFI) VCR still contains the separate audio head for reading the linear track for compatibility with old recordings. So it's two wires and maybe an amplifier to break out the signal to be feed again onto the decoding computer. The whole modification can be made invisible from the outside, maybe with a hidden switch to activate, otherwise hidden in plain sight.
Any DCI may find this by chance only. Like viewing the film on such a modified VCR were the villain had forgotten to switch it back, or if his office is only has a real old VCR, or that VCR's HIFI detection is somehow broken.
*1 - After all, a prerecorded tape will be least suspicious, who would look on an original packed copy of 'Earthquake' or 'Godfather' ? Or even better, some genuine Kids show? 'Why you ask officer? I bought that at the Disney store for my kids'