A few of us have been experimenting with SSTV using HTs and Android phones. Acoustic coupling gets us P3 or P4 at best. Is it possible to build or buy a simple cable to connect the phone to the HT (I am using a Yaesu FT60,) or do I need something to isolate/ match input/output levels?
The wiring would be identical to a cable for APRS purposes (software TNC on the phone). I haven't found anyone doing this but I don't see why it wouldn't be possible.
You want an interface circuit to protect both devices, but it can be entirely passive. The two relevant protections are reducing the audio output to match the microphone input (series resistors or L-pads), and preventing any DC bias voltages from passing to the other device (series capacitors or audio transformers).
I don't have the time right now to write up a full schematic.
Here's some official information on Android headset electrical specifications: jack, plug. These pages are mostly focused on the button functionality but they do have some useful specifications for the mic and speaker audio.