VIDIOC_G_MODULATOR - VIDIOC_S_MODULATOR - Get or set modulator attributes
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.
SDR specific modulator types are V4L2_TUNER_SDR and V4L2_TUNER_RF. For SDR devices txsubchans field must be initialized to zero. The term ‘modulator’ means SDR transmitter in this context.
To change the radio frequency the VIDIOC_S_FREQUENCY ioctl is available.
| __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. |
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| __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, or if the capability flag V4L2_TUNER_CAP_1HZ is set, in units of 1 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, or if the capability flag V4L2_TUNER_CAP_1HZ is set, in units of 1 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 Modulator Audio Transmission Flags. 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. |
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| __u32 | type | Type of the modulator, see v4l2_tuner_type. | ||
| __u32 | reserved[3] | Reserved for future extensions. Drivers and applications must set the array to zero. |
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| 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, V4L2_TUNER_SUB_STEREO or V4L2_TUNER_SUB_SAP. 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_SAP. |
| 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. |
| V4L2_TUNER_SUB_RDS | 0x0010 | Enable the RDS encoder for a radio FM transmitter. |
On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.