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I recently found the PiFm project which sends a FM signal through a GPIO pin to effectively create a FM transmitter out of the Pi.

A problem arises when you do a normal playback of a .wav audio file with mplayer or aplay. After you did this the PiFm only gives out a "hum" on the chosen frequency, instead of the chosen audio.

What kind of consequences has the playback of an audio file on the GPIO ports ?

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There are various versions of PiFM. Some use timed DMA to transfer the data. Timed DMA will use either the PWM or PCM peripheral (probably PWM by default). The peripheral will be configured specially for PiFM. Using mplayer or aplay will reset the PWM peripheral so you will need to restart PiFM afterwards.

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    Well that explains a lot. I'm just unclear with what you exactly mean with restart PiFm. I am probably killing it only partially so the PWM is not reset. I rebooted the Pi until now to solve the problem. How should I reset it without a reboot? – WalyKu Apr 13 '14 at 17:43

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