Can I count on being able to share a DirectShow capture filter between filter graphs?

1
votes

I ran a test today with a DirectShow graph I assembled that had a Capture Filter assigned to my VOIP phone at the top of the graph. The app takes the audio from the capture filter and writes a WAV file, as part of the filter graph's operations. Out of curiosity I ran two copies of the program, fully expecting one of them to throw an error complaining that the capture device was "in use". Much to my surprise both copies of the program worked fine and each created its own WAV file of the recorded audio. The audio in both files was smooth and without problem and twins of each other in regards to the contained audio data.

Can I count on all DirectShow capture filters to exhibit the ability to be shared between multiple filter graphs? Or is every device/driver different?

1
filterdirectshowaudio-capture

1 Answers

2
votes

If the filter instances don't share internally any exclusive access resources (such as hardware, specific TCP ports etc), you are free to duplicate them within a process, or in multiple processes. There are no implications as for specific filter to be only active in a single instance throughout the system.

Important example include:

  • USB video capture: a video capture device is normally intended to be used by one application only, so as soon as it is active it is locked no other application or filter instance can capture from it

  • Audio playback: popular user mode API for audio is a layer on top of actual playback implementation, internally a driver mixes audio from mutliple audio-enabled applications; so when you play audio, there is no exclusive lock involved because actualy device is shared between applications and there is code running around which combines audio from the applications transparently.