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For my new crime-novel I am in need of some more specific info on hiding digital data on analog VCR or VHS cassettes.

I have already read a lot online and seen the LCR oddware video on that case (Danmere backer), so I know it's possible with specific hardware, but would it be possible to "hide"/ obscure digital data on a VHS or VCR tape that already contains analog data (a whole film) but leaves space to spare on the tape?

If so, would the data be randomly spread out over the length of the tape, so when watching the original content (the film), an uninformed third party (i.e. police) would encounter merely some weird pixels at random intervals throughout the film, but would not necessarily suspect any digital data is on that same tape?

Or is there some way to store the digital data only in free space on the tape, so it is not visible at all when playing the tape normally?

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    It is called video steganography. The experiment has already been done: upperclassmonroe.blogs.wm.edu/2019/05/29/… – Brian Tompsett - 汤莱恩 Feb 26 at 13:02
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    Welcome! I suspect your question is too broad. I’m not sure that there’s relevant published research on your idea in particular, but you will find related research and implementations by looking up steganography. On VHS cassettes you’d have to deal with the unreliability of the medium, so purely digital techniques might not be applicable. – Stephen Kitt Feb 26 at 13:03
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    You should also pay attention to the state of technology at the time your crime-novel takes place. Today it's easy to manipulate VHS signals; at the time when VHS was introduced, it would have been a major effort. Also keep in mind that there's something called the "vertical blanking interval" (VBI); that is not show on a TV, but it's comparatively easy to hide data in it, digital or even analog. – dirkt Feb 26 at 14:16
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    @dirkt; @wizzwizz4 in my story the analog videos containing hidden video-files are being produced around 1992-1995 and used by a criminal network that has access to specialist equipment. – Asia Feb 26 at 16:18
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I can think of a couple of ways to do it. The "teletext" method would work, hiding data in the blanking lines off the top of the frame. Because of the way video tapes work these can get mangled, but if you used a low enough data rate and repeated patterns over several frames you could reconstruct it.

This was used to record VITC (Vertical Interval Time Code) on a single scan line, which over 90 bits of data carried hours (often used as "reel number"), minutes, seconds and frames, with 32 bits of user data often used for labelling up tapes.

Too obvious? You could add bursts of digital tones to the soundtrack. Some high-end VCRs used "Hifi Stereo" audio encoded as FM along with the video signal on diagonal video tracks. The linear audio tracks held the same information in lower quality - because the tape speed was low the bandwidth was low. You could hide digital tones on the linear audio track, or the hifi tracks.

Still too obvious? You could use subaudible tones on the soundtrack. The bandwidth for audio was very limited - the upper range was maybe 10kHz, about half the quality of CD, and it was noisy. If you encoded a digital message as a very low frequency tone (think something like Morse code but instead of on/off keying you shifted from maybe 20Hz to 25Hz - too low to hear but high enough to detect) then you could bury a signal.

You could also use amateur radio digital modes like PSK31 which work by having a quiet droning noise from two signals 31Hz apart where you shift the phase of one (hence Phase Shift Keying 31) which can send data at about four characters per second.

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If you want to add digital data to an already existing tape, I imagine you'd need some rather very special equipment, probably purpose-built - the recording heads are not multitrack.

However, if you need to record "hidden" digital data simultaneously while recording the analogue video, then there was a way - and it was even sometimes used. And that was information hidden in blank lines, i.e. the Teletext. It was often said that VHS is no good for teletext, because the bandwidth is not good enough, but people with very good cassettes and high quality recorders managed to read back the data. (I had just an ordinary VHS and sometimes, but only sometimes, I could read teletext from the recording, usually with a lot of errors).

If you want really obscure data, you can use custom format of your data (with lower frequency) and nobody would be any wiser...

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    Maybe it is possible to overwrite on the linear audio head without altering the video content. – Paul Ghobril Feb 26 at 14:18
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    FWIW, there were at least two different data rates used for teletext-like services. I don't remember the numbers, but the "slow" service only put something like three bytes-worth of data into each scan line, while the "fast" service put several tens of bytes. – Solomon Slow Feb 26 at 14:19
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    @Asia well, you asked for digital content :-) Teletext is really limited for video, and if we are speaking about the '80s or the '90s, forget about digital video. But... analogue transmission is all about frequencies, and you can hide the second video quite easily either somewhere above the colour subcarrier (you might get a faint ghost image and that might alert the adversary to look closer, and the frequency is too high for the VHS, but maybe enough for S-VHS), or, with a more professional equipment, into the blanking interval (this is what PAL+ did). – Radovan Garabík Feb 26 at 14:34
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    To be contrary: the audio track is separate from the video track on VHS; you could therefore surely apply some steganography to add hidden digital data into the audio track on an existing tape after the fact? Probably very low data rate for the purposes of the steganography, but a decent amount across an entire tape? – Tommy Feb 26 at 17:11
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    If your CRT TV vertical size was poorly adjusted, you could see the digital data at the bottom of the screen. It would normally be beyond the bottom of the screen where you couldn't see it. – Mattman944 Feb 26 at 22:08
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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.

Using Audio

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.

Using Video

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'

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  • Cool! I think I'll opt for the MPEG-1 / VCR method since I had indeed hoped this might be realistic. Perfect, thanks a lot! – Asia Mar 1 at 7:49
  • @Asia Well, keep in mind that this is only a realistic thing if the novel is set after ca. 1996. The critical part is not the VCR modification, but the availability of digital video. While decoding will work on an (upper end) PC, encoding needs more. Not at least digitizers. It might be hard to imagine from today's POV, but what we deem basic today was barely possible back then - and barely possible is hard to believable. Back then everybody would have used the data bandwitdh to store text, or at maximum pictures. The choice is (all with an access time of an hour) or 500 pics or a whole book. – Raffzahn Mar 1 at 11:06
  • @Asia In fact, thinking of it - and the time frame - with the neccessary technology at hand, it's way more believable to use Video-CD technology to hide the track. Either by inserting one or more video tracks with what seems to be bad data, but is only obfuscated (nor even encrypted) video. CR-R with pirated videos were a big thing, especially in Asia, at the time. Or even hide it in what seems to be a music CD. Some tracks with recordings from 'your kids kindergarden singing' and after that obfuscated video-CD tracks - now one can store not just 20 seconds but half an hour of ok-ish video. – Raffzahn Mar 1 at 11:27
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You could encode binary using index marks in the control track:

The control track is also used to hold index marks, which were normally written at the beginning of each recording session, and can be found using the VCR's index search function: this will fast-wind forward or backward to the nth specified index mark, and resume playback from there. At times, higher-end VCRs provided functions for the user to manually add and remove these marks — so that, for example, they coincide with the actual start of the television program — but this feature later became hard to find.

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By modulating the brightness of alternating fields you can encode binary data. There was a physical game that used this, I think it was Wheel of Fortune. You could play at home and your game knew how to respond because the "answer" was encoded in the video, and it picked it up with a simple photoelectric sensor. But humans watching the same show didn't notice.

You could also use video or audio steganography much like images are watermarked to detect improper use or distribution. You'd want something that could stand up to the limited bandwidth of VHS, but it is very possible without casual detection.

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When is the novel set?

My first thought is that in the 80s it was common for home computers to use audio cassette tapes to store programs and data. I would imagine that the audio track on a VHS cassette could also be used the same way. When played back normally you'd simply hear a bunch of noise, although it would be recognizable to anyone familiar with such things.

However, if it was used to store an actual program, which in turn displayed the data, you'd need to know what type of computer to connect it to the actually get the program to run.

However, getting a bit creative, and remembering that these things were all analog and usually had the standard red, white and yellow RCA connectors, I'm wondering what would happen if the analog audio was sent to the video channel and recorded as an image? The result would probably just look like a bunch of noise onscreen... However, if that cassette was played back with the video out routed back to the audio input of a computer... hmmm... would it be good enough to preserve the program and actually allow it to be loaded? I have NO idea - perhaps with suitable amps and EQs to regulate the levels in and out?

Its a crazy speculative idea and I lack the hardware required to test it.

I did find this information:

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

So apparently the Russians were doing a more high tech version of it in the 90s on PCs with a special ISA board. I'd imagine the results would still be noise if played back on a standard VCR. So some data could be added to the end of a movie and most people would dismiss it as nothing. But if you KNEW, and had the right hardware, well - bingo!

I was gonna mention Danmere too but looks like you know that one.

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