The DECO Cassette System was an old arcade game hardware platform released by Data East way back in 1981.
The story
This system was designed as a way to quickly swap games in an arcade cabinet. It had a four part motherboard and an audio tape drive. The games were stored on audio cassettes, and the system would load them into memory when it was turned on. This was a great idea, but audio cassettes rapidly decay after repeated usage, and tape failures were common, which was why this platform was discontinued in 1984 (it enjoyed its height of popularity in 1982 right before the first batch of tapes started failing left and right).
The game
There were a lot of games released for this system, many of them (Burger Time, Bump 'n' Jump, etc), were also available on dedicated hardware from other distributors. I have listed the titles I was able to verify, but there are probably a lot more of them than this.
List of DECO Cassette System Games
(Probably incomplete)
The Machine
DECO systems shipped in two different dedicated cabinets. Both were nondescript, and had generic four color control panel overlays, and monitor bezels. The marquees were unique to each game, and were supposed to be changed when you changed the cassette inside the cabinet (but they weren't always, as I encountered one of these that was mislabeled a long time ago).
Unfortunately Data East shipped a hardware key with each cassette, which must be present before the game will load (to prevent widespread piracy). But I have examined the documentation carefully, and I believe I have figured out how to get around this entirely. The tape contains 2 "files" one is the game, and the other is the "key". This value in the "key" file is checked against the value stored in the hardware key, if it matches, then the system will load the game. So in theory you could record a new cassette, and swap the data in the "key" file to match the value stored in your hardware key. That would have been rather difficult for the average arcade operator to do back in the early 80s (you would have to do it perfectly), but today (with a modern computer) it is a simple matter of editing the binary images, and then recording them back as audio data (that way you can do it perfectly).
The "key" files are very small, here is the one for Astro Fantasia as viewed in notepad, ÁtsÀMÌ9å2ŸQ¾˜7Ä}Š¼Iöcb»Hõ.
But wait, theres more.
If this works, than it is a simple matter to build a DECO multigame by swapping the cassette drive out for an audio CD player. With each game stored as an audio track (they are only a few minutes long). The cassette drive hooks up to the motherboard with an 8 pin interface, two of which are audio (binary data stored in audio format actually), 2 are unused, and the other four are "stop", "play", "ground", and "rewind". The documentation does not infer that the tape drive reports back to the motherboard at all, so in theory it could simply be replaced with a CD player hooked up to the two audio pins. If the tape drive does report back, then it could simply be disconnected from the audio (data) pins, and the CD player could be attached to them instead (the motherboard would think the data was coming from the tape drive, but in reality it would be coming from the CD player).
But I don't have a system to test this on, so this is all just theory right now (it can definitely be done one way or another, a similar CD loader has already been made for the Atari 2600 to replace the old tape loader that Starpath used to sell).
Where to play
You probably aren't going to bump into one of these systems down at your local pizza shop, but many of the titles have been ported to various consoles, and MAME supports about half of them.
I would only recommend buying one of these for your arcade game collection if you are planning on trying the multigame hack I have described above. The original tapes are unreliable at best. (Although don't pass up a good deal on a system with a lot of cassettes included, just make sure they have the hardware keys to go with them).