How did the old portable cd player car cassette adaptors convert audio from a CD to be played over a cassette?
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A cassette tape will have a magnetic field that is different over time and a read head will detect the change when the tape moves. The head will produce an electrical signal that is the sound. The electrical sound signal is the same as what a headphone outlet produces.
A cassette tape is recorded but has a write head that produces a magnetic field from an external signal and is changes the magnetization of the tape.
So the tape is a way to store the output of a write head that you later can read with a read head.
But what if we skipped the tape and put the write head next to the read head?
The result is that the sound is transmitted between them. So make a case with a write head and wires to a sound source. Put the write head in the right location so the read head can read it. You can now send an electrical sound signal to a device by using the cassette slot.
That is how the cassette adaptors worked they look like [https://guide-images.cdn.ifixit.com/igi/DgKGWjIPbWe1Fd6H.large](https://guide-images.cdn.ifixit.com/igi/DgKGWjIPbWe1Fd6H.large) inside. The cogs are there so the auto-reverse direction mechanism is not engaged.
Read and write head is technically the same part used in different ways, I just called that for simplicity. It is not different from how a electric motor is an electric generator used in reverse
Look at https://www.youtube.com/watch?v=dH4n8fUjtLQ for more info
They had a small electromagnet that would be right next to the tape player head and reproduce the analog signal that would be on a cassette tape.
In a cassette player, a magnetized tape runs past a detector which converts the magnetic signal into an electrical signal, which gets fed straight into the speaker to produce sound.
A CD player does normal CD player stuff, but instead of a speaker it has a small electromagnet right by the cassette reader, so the electrical signal turns into a magnetic signal which the reader then picks up normally.
Cassette tapes work by taking a plastic tape and coating it with something that can be magnetized. By changing the direction/strength of the magnetization, you can encode an analog signal on it. When that tape moves across a coil (the read head) the changing magnetic field induces a current in it. That creates the signal that is amplified and played as music.
The adaptor mimics this process by simply replacing the magnetic tape with a little electromagnet (coil) that the audio signal can be sent through. That looks to the read head just like a moving magnetic tape.
Cassettes are just magnetic signals imprinted onto the magnetic tape. So the device just needed to modulate those same magnetic signals on the fly. It you take the signal from the CD, and output it as the proper magnetic signals.
You know the read-heads that all tape players have? The half-dome metal piece that makes contact with the tape and converts the magnetic signal on the tape into electrical signals?
The adaptors have ANOTHER one of those, “wired backwards”, inside a fake tape – that converts the electrical signal from the CD player into a magnetic signal.
So the player is basically one of those heads reading the “tape” normally, but in this case it’s actually reading ANOTHER one of those heads that’s pressed up against it, and that’s inside a fake tape doing the opposite job to normal.
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