Looking at my lossy-compressed music library, the smallest album I have is 15MB and the largest (single disk) album is 70MB. Using LP-sized cards (12"x12"), a binary image would end up being about 1100-2500 DPI respectively for those two albums. Much too small to be scanned effectively. If you reduce the requirements to a single rather than an album, then you are looking at 1-10 MB per code, which is 300-900 DPI, still too dense.

The camera would also be a limiting factor. The iPhone 12 has a 12 mega-pixel sensor, so even if you can get exact pixel-for-pixel alignment of the sensor and coded data, you would need to represent 10-50 bits of information in each pixel (eg via color/brightness), for an album, or 1-8 bits for a single song. In practice, you need several pixels per bit, so you would have to scan the code in sections - you couldn't image it at once. At that point, perhaps a tape would be more appropriate than a card.

That is with fairly transparent compression though. You could be a lot more aggressive, and might get something that would work with a single photo.

You can just about squeeze an album onto a floppy disk - see https://shkspr.mobi/blog/2020/09/a-floppy-disk-mp3-player-us...

Using Opus at a ridiculously low quality, you can probably fit a standard 3 minute pop song in ~200KB.

A QR code, with minimum error correction, can fit a maximum of about 4KB. So a square of 7x7 codes would be enough.

The largest type 40 codes are 177x177px. So that's about 1,200px square.

Depending on the resolution of your camera, and how far away it is, you might be able to squeeze that onto an A5 print out.

I also thought about this, but I did neither find an RFID chip that was capable of storing enough data for this, nor the space for encoded data in printable form.

There is a commercial product for children, called Tonies (Or Toniebox) - see https://us.tonies.com/. They have something called "Kreativ-Tonies" (german), where you can store up to 90min of audio content, but I did not find out, how they are working technically. I think that they just store it on a cloud server and stream / download the content on usage, but I'm not sure about this.

BTW: I don't think, that storing the music on a card directly would be the best approach, because if the card breaks, you have to rebuild the whole card. If the music is stored on the device or cloud, you just have to print one.

I'll take it a step further and suggest that the original idea is super bleh, but "music actually on the card" is genuinely interesting. Possession of the thing is deeply and fundamentally different from "glorified QR code."

So big that you cannot call it "card' anymore :)

Jokes apart here you can find an insight about qrcodes capacity: https://github.com/dtonon/papiro