Using an iPhone as a cassette recorder for 80's home computer

Question

I've got an old 80's home computer (a Microbee) that I've just got running again. It uses cassette to save/load programs and I thought I'd see if I could use my iPhone instead of an actual cassette recorder/player (since I don't have one).

I made up a cable with the 4 pole iPhone style 3.5mm plug and for loading (playing from the iPhone) it works perfectly. For saving (recording into the iPhone), the iPhone doesn't seem to switch over to the external microphone input and continues to record from the internal mic.

I've wired up the plug as follows (based on this: https://discussions.apple.com/thread/1114891?start=0&tstart=0)

• Computer Tape Out -> 3.5mm plugin sleeve (iPhone Microphone In)
• Computer Common Ground -> 3.5mm next up from sleeve (iPhone Ground)
• Nothing -> 3.5mm next down from tip (iPhone Right Out)
• Computer Tape In -> 3.5mm plugin tip tip. (iPhone Left Out)

Would welcome any ideas/suggestions.

Answer 1

The link in Michael Kohne's comment is the answer:

The iDevice needs to see a resistance in the neighborhood of 5k between the microphone conductor and ground. That tells it that a microphone has been plugged in. If it is a direct short, it thinks a headphone was plugged in. Open circuit means nothing was plugged in. The exact value is not important, from experimenting I found that 4.7k seems to work. This resistor is put in series with the isolation transformer.

I found that by putting a 10k resistor across "computer tape in" and ground the iPhone switches to the external mic. The record level is a fraction high - I don't know if that's related to the resistor or simply due to the signal from the Microbee.

Disclaimer: I have no idea what I'm doing, use at own risk!