Using the C920 Webcam With the Portsdown
The Logitech C920 Webcam can be used with the Portsdown transmitter for both video and sound. It can be used in 3 modes: as a normal webcam providing images for MPEG-2 transmission with sound (either SD or 720p), as a normal webcam with H264 SD pictures (but no sound) being encoded by the Raspberry Pi, or using its internal H264 encoder to transmit pictures at SD, 720p or 1080p with sound. For internal H264 encoding modes, the bitrate is fixed and the Portsdown MUST be selected to 2 MS or more, 7/8 FEC (or DVB-S2 16APSK 7/8 FEC 1 MS) for this to work.
In Portsdown Stretch (the current latest software), the use of a Webcam with the RPi Camera connected requires that the webcam is unplugged at boot, and only plugged in once the Portsdown has booted up. This is because of an incompatibility between the way that the RPi Camera drivers are loaded and the way that the Webcam drivers are loaded. This problem could be corrected if anyone is prepared to develop suitable udev rules to sort it out. It is beyond the capability of the current Portsdown team - any volunteers?
There is no issue if the RPi Camera is not connected.
The C920 is quite power-hungry and can be the straw that breaks the Camel's back in the RPi power budget, leading to under-voltage events with unpredictable consequences. For best results, use a powered USB Hub, or at least make sure that your RPi PSU is up to the job. Remember that you can check if there have been any power events from the Info screen on Menu 2.