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App seems to no longer be on Google Play, but is available here.

Updated version available here.

To get audio data into the tablet or phone, the microphone input on the device is used. This is usually a 4 pole combined headphones and mic connector, and so a special cable is needed, detailed below

Parts needed (parts available from most places, not just maplin):

- 3.5mm 4-pole plug

- 3.5mm socket

- Some wire or shielded audio cable

- Some resistors (eg 1K & 22K - see below)

This page shows the pinout for the most common layout of 4 pole connector, along with a list of phones for which it applies, but is not a complete list. If you can buy a headphone/mic headset accessory for your phone that has a 3.5mm connector then it will support audio in via its input (it is possible that some devices will switch the mic and gnd connections - if you connect them the wrong way round the resistors will prevent damage to the radio or phone)

The basic layout for the cable is shown below. The connector layouts shown are for the connectors linked to above. If you have different connectors use the continuity setting on a multimeter to be sure of the layout of the connector. The colouring on the 4 pole connector shows what is connected to what

The resistors form a potential divider as the radio output is typically designed to drive headphones, and so the resistors reduce the amplitude to something expected by a mic input. Some devices also need a low (1K-2K) resistance between mic and gnd to detect the microphone

Typical values of resistor are: R1 = 22K and R2 = 1K, although some experimentation may be needed if these attenuate the signal too much, or if the audio input is not detected by the device.

[Note, Kevin Walton, 04/08/13, Samsung Galaxy S2 did not recognise the mic with a R2 as 1K, but did recognise a mic had been plugged in with R2 as 1.5K, it also required a ferrite filter to reduce the noise.]

Some phones appear to throw out a lot of RF noise on the audio cable which can cause interference with the radio. Adding a ferrite ring on the cable at the phone side should solve the issue. (Thanks to Jon G8KNN for pointing this out)

Source code is available here:

https://github.com/mattbrejza/rtty_modem