SMPTE LTC timecode reader and generator for iPhone and iPad
4:20 reads and generates timecode over a wired audio connection: a class-compliant USB audio interface, or a headset-jack adapter with a line input. There is no microphone mode. This version works with LTC only; MIDI timecode is not supported.
Timecode arrives as audio, and iOS treats every audio input, wired included, as the microphone. If the permission is declined, the reader cannot run; you can grant it later in Settings. What happens to the audio is covered in the privacy policy: decoded on the device, never recorded, never sent anywhere.
Check the chain in this order: the source is actually running code (some gear goes quiet with the transport stopped, some does not), the cable is seated at both ends, and the level is healthy. LTC decodes best at a solid line level, neither barely moving nor clipping.
If lock comes and goes on a slow cycle, check Voice Isolation. iOS applies it per app and remembers the choice across launches, and it destroys timecode decoding. While 4:20's reader is running, open Control Center, tap Mic Mode, and set it to Standard. One fix sticks, because iOS remembers that per app too.
Email support@jbertuch.net. The single most useful thing to attach is the diagnostics export from the ⓘ button in the app; it records what a written description usually misses. Add the interface or adapter in the chain and what the readout showed.