To query the attributes of a modulator applications initialize
the index field and zero out the
reserved array of a struct v4l2_modulator and
call the VIDIOC_G_MODULATOR ioctl with a pointer
to this structure. Drivers fill the rest of the structure or return an
EINVAL error code when the index is out of bounds. To enumerate all modulators
applications shall begin at index zero, incrementing by one until the
driver returns EINVAL.
Modulators have two writable properties, an audio
modulation set and the radio frequency. To change the modulated audio
subprograms, applications initialize the index and txsubchans fields and the
reserved array and call the
VIDIOC_S_MODULATOR ioctl. Drivers may choose a
different audio modulation if the request cannot be satisfied. However
this is a write-only ioctl, it does not return the actual audio
modulation selected.
To change the radio frequency the VIDIOC_S_FREQUENCY ioctl
is available.
Table 1. struct v4l2_modulator
| __u32 | index | Identifies the modulator, set by the application. |
| __u8 | name[32] | Name of the modulator, a NUL-terminated ASCII string. This information is intended for the user. |
| __u32 | capability | Modulator capability flags. No flags are defined for this field, the tuner flags in struct v4l2_tuner are used accordingly. The audio flags indicate the ability to encode audio subprograms. They will not change for example with the current video standard. |
| __u32 | rangelow | The lowest tunable frequency in units of 62.5
KHz, or if the capability flag
V4L2_TUNER_CAP_LOW is set, in units of 62.5
Hz. |
| __u32 | rangehigh | The highest tunable frequency in units of 62.5
KHz, or if the capability flag
V4L2_TUNER_CAP_LOW is set, in units of 62.5
Hz. |
| __u32 | txsubchans | With this field applications can determine how
audio sub-carriers shall be modulated. It contains a set of flags as
defined in Table 2. Note the tuner
rxsubchans flags are reused, but the
semantics are different. Video output devices are assumed to have an
analog or PCM audio input with 1-3 channels. The
txsubchans flags select one or more
channels for modulation, together with some audio subprogram
indicator, for example a stereo pilot tone. |
| __u32 | reserved[4] | Reserved for future extensions. Drivers and applications must set the array to zero. |
Table 2. Modulator Audio Transmission Flags
V4L2_TUNER_SUB_MONO | 0x0001 | Modulate channel 1 as mono audio, when the input
has more channels, a down-mix of channel 1 and 2. This flag does not
combine with V4L2_TUNER_SUB_STEREO or
V4L2_TUNER_SUB_LANG1. |
V4L2_TUNER_SUB_STEREO | 0x0002 | Modulate channel 1 and 2 as left and right
channel of a stereo audio signal. When the input has only one channel
or two channels and V4L2_TUNER_SUB_SAP is also
set, channel 1 is encoded as left and right channel. This flag does
not combine with V4L2_TUNER_SUB_MONO or
V4L2_TUNER_SUB_LANG1. When the driver does not
support stereo audio it shall fall back to mono. |
V4L2_TUNER_SUB_LANG1 | 0x0008 | Modulate channel 1 and 2 as primary and secondary
language of a bilingual audio signal. When the input has only one
channel it is used for both languages. It is not possible to encode
the primary or secondary language only. This flag does not combine
with V4L2_TUNER_SUB_MONO or
V4L2_TUNER_SUB_STEREO. If the hardware does not
support the respective audio matrix, or the current video standard
does not permit bilingual audio the
VIDIOC_S_MODULATOR ioctl shall return an EINVAL error code
and the driver shall fall back to mono or stereo mode. |
V4L2_TUNER_SUB_LANG2 | 0x0004 | Same effect as
V4L2_TUNER_SUB_LANG1. |
V4L2_TUNER_SUB_SAP | 0x0004 | When combined with V4L2_TUNER_SUB_MONO the first channel is encoded as mono audio, the last
channel as Second Audio Program. When the input has only one channel
it is used for both audio tracks. When the input has three channels
the mono track is a down-mix of channel 1 and 2. When combined with
V4L2_TUNER_SUB_STEREO channel 1 and 2 are
encoded as left and right stereo audio, channel 3 as Second Audio
Program. When the input has only two channels, the first is encoded as
left and right channel and the second as SAP. When the input has only
one channel it is used for all audio tracks. It is not possible to
encode a Second Audio Program only. This flag must combine with
V4L2_TUNER_SUB_MONO or
V4L2_TUNER_SUB_STEREO. If the hardware does not
support the respective audio matrix, or the current video standard
does not permit SAP the VIDIOC_S_MODULATOR ioctl
shall return an EINVAL error code and driver shall fall back to mono or stereo
mode. |
On success 0 is returned, on error -1 and the errno variable is set appropriately:
The struct v4l2_modulator
index is out of bounds.