1.6. Tuners and Modulators¶
Video input devices can have one or more tuners demodulating a RF
signal. Each tuner is associated with one or more video inputs,
depending on the number of RF connectors on the tuner. The
field of the respective struct
returned by the ioctl VIDIOC_ENUMINPUT ioctl is
V4L2_INPUT_TYPE_TUNER and its
tuner field contains the
index number of the tuner.
Radio input devices have exactly one tuner with index zero, no video inputs.
To query and change tuner properties applications use the
VIDIOC_S_TUNER ioctls, respectively. The
v4l2_tuner returned by VIDIOC_G_TUNER
also contains signal status information applicable when the tuner of the
current video or radio input is queried.
VIDIOC_S_TUNER does not switch the
current tuner, when there is more than one at all. The tuner is solely
determined by the current video input. Drivers must support both ioctls
and set the
V4L2_CAP_TUNER flag in the struct
returned by the ioctl VIDIOC_QUERYCAP ioctl when the
device has one or more tuners.
Video output devices can have one or more modulators, uh, modulating a
video signal for radiation or connection to the antenna input of a TV
set or video recorder. Each modulator is associated with one or more
video outputs, depending on the number of RF connectors on the
type field of the respective struct
v4l2_output returned by the
ioctl VIDIOC_ENUMOUTPUT ioctl is set to
V4L2_OUTPUT_TYPE_MODULATOR and its
modulator field contains the
index number of the modulator.
Radio output devices have exactly one modulator with index zero, no video outputs.
A video or radio device cannot support both a tuner and a modulator. Two separate device nodes will have to be used for such hardware, one that supports the tuner functionality and one that supports the modulator functionality. The reason is a limitation with the VIDIOC_S_FREQUENCY ioctl where you cannot specify whether the frequency is for a tuner or a modulator.
To query and change modulator properties applications use the
VIDIOC_S_MODULATOR ioctl. Note that
VIDIOC_S_MODULATOR does not switch the current modulator, when there
is more than one at all. The modulator is solely determined by the
current video output. Drivers must support both ioctls and set the
V4L2_CAP_MODULATOR flag in the struct
v4l2_capability returned by the
ioctl VIDIOC_QUERYCAP ioctl when the device has
one or more modulators.
1.6.3. Radio Frequency¶
To get and set the tuner or modulator radio frequency applications use
the VIDIOC_G_FREQUENCY and
VIDIOC_S_FREQUENCY ioctl which both take
a pointer to a struct
ioctls are used for TV and radio devices alike. Drivers must support
both ioctls when the tuner or modulator ioctls are supported, or when
the device is a radio device.