Hi Sylvain,
What do you mean by PTT ?
(I mean I know what PTT means in the context of a walkie talkie, but I don't really see how it applies to the red pitaya or gr-osmosdr. Or rather I see several different things it could mean .. ).
In case of the Red Pitaya SDR transceiver, it's a command to enable or disable the transmitter output and eventually to drive a relay that switches the antenna between the RX path or TX path and/or to control an external amplifier.
I've seen that some SDR transceivers (Elad FDM-DUO, FLEX, ANAN) have an output called PTT-Out and graphical user interfaces of some SDR programs (Quisk, PowerSDR, QtRadio) have a button called PTT or MOX that controls the PTT output of the SDR transceivers. So, I thought that it's a common name for this kind of command.
Cheers,
Pavel