Today I bought a cassette tape from Oxfam on Cambridge highstreet: “Mozart Symphony No. 41 in C Major”. I reckoned that I could probably put up with listening to it a lot while debugging the recorder. The slight trouble was that the cassette was write-protected so I couldn’t simply record over the old music. As it turns out, it’s not particularly hard to get around this. The only protection is a small hole in the top of the cassette which tells the cassette recorder that this is read only. Cover the hole with a bit of tape and the cassette player now has no idea.
It’s one thing for the tape player to work, it’s another thing to be able to replicate the signals myself and record what I want. So I got out my oscilloscope and took some measurements.
As you can see, during recording, the signal across the tape head is approximately 100mV in amplitude, and the maximum I saw it reach was about 200mV. Because I had my oscilloscope set to AC mode, what you don’t see here is that the signal is actually DC biased by about 50mV. The reason this is done is the ensure that the electromagnet in the tape head is in it’s linear region resulting in a less distorted wave form. While DC biasing improves the fidelity of the wave form, it can be improved further by AC biasing as I intend to do in my design. You can read more about biasing here.
I also measured the signal across the tape head during playback, however the signal was so small it was barely visible and isn’t worth me uploading a picture. Hopefully given some amplification it will become more visible.
Anyway to finish off the post here is a video of the mechanism in action. In this clip you can see the tape head (centre) being engaged during playback and also the erase head (to the left of the tape head) being engaged during recording.