1.5. Audio Inputs and Outputs¶
Audio inputs and outputs are physical connectors of a device. Video capture devices have inputs, output devices have outputs, zero or more each. Radio devices have no audio inputs or outputs. They have exactly one tuner which in fact is an audio source, but this API associates tuners with video inputs or outputs only, and radio devices have none of these. [1] A connector on a TV card to loop back the received audio signal to a sound card is not considered an audio output.
Audio and video inputs and outputs are associated. Selecting a video
source also selects an audio source. This is most evident when the video
and audio source is a tuner. Further audio connectors can combine with
more than one video input or output. Assumed two composite video inputs
and two audio inputs exist, there may be up to four valid combinations.
The relation of video and audio connectors is defined in the
audioset
field of the respective struct
v4l2_input or struct
v4l2_output, where each bit represents the index
number, starting at zero, of one audio input or output.
To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the ioctl VIDIOC_ENUMAUDIO and VIDIOC_ENUMAUDOUT ioctl, respectively. The struct v4l2_audio returned by the ioctl VIDIOC_ENUMAUDIO ioctl also contains signal :status information applicable when the current audio input is queried.
The VIDIOC_G_AUDIO and VIDIOC_G_AUDOUT ioctls report the current audio input and output, respectively.
Note
Note that, unlike VIDIOC_G_INPUT and VIDIOC_G_OUTPUT these ioctls return a structure as ioctl VIDIOC_ENUMAUDIO and VIDIOC_ENUMAUDOUT do, not just an index.
To select an audio input and change its properties applications call the VIDIOC_S_AUDIO ioctl. To select an audio output (which presently has no changeable properties) applications call the VIDIOC_S_AUDOUT ioctl.
Drivers must implement all audio input ioctls when the device has
multiple selectable audio inputs, all audio output ioctls when the
device has multiple selectable audio outputs. When the device has any
audio inputs or outputs the driver must set the V4L2_CAP_AUDIO
flag
in the struct v4l2_capability returned by
the ioctl VIDIOC_QUERYCAP ioctl.
1.5.1. Example: Information about the current audio input¶
struct v4l2_audio audio;
memset(&audio, 0, sizeof(audio));
if (-1 == ioctl(fd, VIDIOC_G_AUDIO, &audio)) {
perror("VIDIOC_G_AUDIO");
exit(EXIT_FAILURE);
}
printf("Current input: %s\\n", audio.name);
1.5.2. Example: Switching to the first audio input¶
struct v4l2_audio audio;
memset(&audio, 0, sizeof(audio)); /* clear audio.mode, audio.reserved */
audio.index = 0;
if (-1 == ioctl(fd, VIDIOC_S_AUDIO, &audio)) {
perror("VIDIOC_S_AUDIO");
exit(EXIT_FAILURE);
}
[1] | Actually struct v4l2_audio ought to have a
tuner field like struct v4l2_input, not
only making the API more consistent but also permitting radio devices
with multiple tuners. |