Timecode Cables

Headphone jack at the iOS end of the cable

On the Apple iOS device end of the cable, this jack must be one with four distinct silver bands (as pictured at right). Without those four bands, an iPad/iPhone/iPod touch won't be able to accept audio input.
Denecke has also posted a PDF with the specifications for connecting LTC devices to the iPhone / iPad / iPod Touch.

Jack at other end of the cable

This jack must fit the sound source (such as your TC generator or sound recorder). Your sound source’s manual is a good place to find the type of plug required. Failing that, you could Google for that information.

Attenuation

decreases the volume of the audio that the Apple iOS device receives. Depending upon your sound source, you may not need any attenuation. However, most pro-level sound equipment outputs at LINE level (very loud). All Apple iOS devices receive audio at MIC level (very quiet). If your sound source outputs LINE level audio, then it will easily overpower the poor Apple iOS device. So the audio must be attenuated (quieted). Attenuation may require a bit of trial and error. Not all sound sources (cameras, TC generators, and sound recorders) output audio at the same volume levels. If your sound source has an output volume control, then it is easy to test from MovieSlate’s Settings > Timecode Syncing > Test Headphone Connection. While viewing MovieSlate’s test screen, simply adjust the soure source’s volume until the test screen levels are green and timecode appears in the test screen. Some sound sources don't have a way to control their output volume. In these cases, if you find that the volume is too high for the Apple iOS device, then you'll need to also purchase an attenuating cable.