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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _fm-tx-controls:
+
+********************************
+FM Transmitter Control Reference
+********************************
+
+The FM Transmitter (FM_TX) class includes controls for common features
+of FM transmissions capable devices. Currently this class includes
+parameters for audio compression, pilot tone generation, audio deviation
+limiter, RDS transmission and tuning power features.
+
+
+.. _fm-tx-control-id:
+
+FM_TX Control IDs
+=================
+
+``V4L2_CID_FM_TX_CLASS (class)``
+ The FM_TX class descriptor. Calling
+ :ref:`VIDIOC_QUERYCTRL` for this control will
+ return a description of this control class.
+
+``V4L2_CID_RDS_TX_DEVIATION (integer)``
+ Configures RDS signal frequency deviation level in Hz. The range and
+ step are driver-specific.
+
+``V4L2_CID_RDS_TX_PI (integer)``
+ Sets the RDS Programme Identification field for transmission.
+
+``V4L2_CID_RDS_TX_PTY (integer)``
+ Sets the RDS Programme Type field for transmission. This encodes up
+ to 31 pre-defined programme types.
+
+``V4L2_CID_RDS_TX_PS_NAME (string)``
+ Sets the Programme Service name (PS_NAME) for transmission. It is
+ intended for static display on a receiver. It is the primary aid to
+ listeners in programme service identification and selection. In
+ Annex E of :ref:`iec62106`, the RDS specification, there is a full
+ description of the correct character encoding for Programme Service
+ name strings. Also from RDS specification, PS is usually a single
+ eight character text. However, it is also possible to find receivers
+ which can scroll strings sized as 8 x N characters. So, this control
+ must be configured with steps of 8 characters. The result is it must
+ always contain a string with size multiple of 8.
+
+``V4L2_CID_RDS_TX_RADIO_TEXT (string)``
+ Sets the Radio Text info for transmission. It is a textual
+ description of what is being broadcasted. RDS Radio Text can be
+ applied when broadcaster wishes to transmit longer PS names,
+ programme-related information or any other text. In these cases,
+ RadioText should be used in addition to ``V4L2_CID_RDS_TX_PS_NAME``.
+ The encoding for Radio Text strings is also fully described in Annex
+ E of :ref:`iec62106`. The length of Radio Text strings depends on
+ which RDS Block is being used to transmit it, either 32 (2A block)
+ or 64 (2B block). However, it is also possible to find receivers
+ which can scroll strings sized as 32 x N or 64 x N characters. So,
+ this control must be configured with steps of 32 or 64 characters.
+ The result is it must always contain a string with size multiple of
+ 32 or 64.
+
+``V4L2_CID_RDS_TX_MONO_STEREO (boolean)``
+ Sets the Mono/Stereo bit of the Decoder Identification code. If set,
+ then the audio was recorded as stereo.
+
+``V4L2_CID_RDS_TX_ARTIFICIAL_HEAD (boolean)``
+ Sets the
+ `Artificial Head <http://en.wikipedia.org/wiki/Artificial_head>`__
+ bit of the Decoder Identification code. If set, then the audio was
+ recorded using an artificial head.
+
+``V4L2_CID_RDS_TX_COMPRESSED (boolean)``
+ Sets the Compressed bit of the Decoder Identification code. If set,
+ then the audio is compressed.
+
+``V4L2_CID_RDS_TX_DYNAMIC_PTY (boolean)``
+ Sets the Dynamic PTY bit of the Decoder Identification code. If set,
+ then the PTY code is dynamically switched.
+
+``V4L2_CID_RDS_TX_TRAFFIC_ANNOUNCEMENT (boolean)``
+ If set, then a traffic announcement is in progress.
+
+``V4L2_CID_RDS_TX_TRAFFIC_PROGRAM (boolean)``
+ If set, then the tuned programme carries traffic announcements.
+
+``V4L2_CID_RDS_TX_MUSIC_SPEECH (boolean)``
+ If set, then this channel broadcasts music. If cleared, then it
+ broadcasts speech. If the transmitter doesn't make this distinction,
+ then it should be set.
+
+``V4L2_CID_RDS_TX_ALT_FREQS_ENABLE (boolean)``
+ If set, then transmit alternate frequencies.
+
+``V4L2_CID_RDS_TX_ALT_FREQS (__u32 array)``
+ The alternate frequencies in kHz units. The RDS standard allows for
+ up to 25 frequencies to be defined. Drivers may support fewer
+ frequencies so check the array size.
+
+``V4L2_CID_AUDIO_LIMITER_ENABLED (boolean)``
+ Enables or disables the audio deviation limiter feature. The limiter
+ is useful when trying to maximize the audio volume, minimize
+ receiver-generated distortion and prevent overmodulation.
+
+``V4L2_CID_AUDIO_LIMITER_RELEASE_TIME (integer)``
+ Sets the audio deviation limiter feature release time. Unit is in
+ useconds. Step and range are driver-specific.
+
+``V4L2_CID_AUDIO_LIMITER_DEVIATION (integer)``
+ Configures audio frequency deviation level in Hz. The range and step
+ are driver-specific.
+
+``V4L2_CID_AUDIO_COMPRESSION_ENABLED (boolean)``
+ Enables or disables the audio compression feature. This feature
+ amplifies signals below the threshold by a fixed gain and compresses
+ audio signals above the threshold by the ratio of Threshold/(Gain +
+ Threshold).
+
+``V4L2_CID_AUDIO_COMPRESSION_GAIN (integer)``
+ Sets the gain for audio compression feature. It is a dB value. The
+ range and step are driver-specific.
+
+``V4L2_CID_AUDIO_COMPRESSION_THRESHOLD (integer)``
+ Sets the threshold level for audio compression freature. It is a dB
+ value. The range and step are driver-specific.
+
+``V4L2_CID_AUDIO_COMPRESSION_ATTACK_TIME (integer)``
+ Sets the attack time for audio compression feature. It is a useconds
+ value. The range and step are driver-specific.
+
+``V4L2_CID_AUDIO_COMPRESSION_RELEASE_TIME (integer)``
+ Sets the release time for audio compression feature. It is a
+ useconds value. The range and step are driver-specific.
+
+``V4L2_CID_PILOT_TONE_ENABLED (boolean)``
+ Enables or disables the pilot tone generation feature.
+
+``V4L2_CID_PILOT_TONE_DEVIATION (integer)``
+ Configures pilot tone frequency deviation level. Unit is in Hz. The
+ range and step are driver-specific.
+
+``V4L2_CID_PILOT_TONE_FREQUENCY (integer)``
+ Configures pilot tone frequency value. Unit is in Hz. The range and
+ step are driver-specific.
+
+``V4L2_CID_TUNE_PREEMPHASIS``
+ (enum)
+
+enum v4l2_preemphasis -
+ Configures the pre-emphasis value for broadcasting. A pre-emphasis
+ filter is applied to the broadcast to accentuate the high audio
+ frequencies. Depending on the region, a time constant of either 50
+ or 75 useconds is used. The enum v4l2_preemphasis defines possible
+ values for pre-emphasis. Here they are:
+
+
+
+.. flat-table::
+ :header-rows: 0
+ :stub-columns: 0
+
+ * - ``V4L2_PREEMPHASIS_DISABLED``
+ - No pre-emphasis is applied.
+ * - ``V4L2_PREEMPHASIS_50_uS``
+ - A pre-emphasis of 50 uS is used.
+ * - ``V4L2_PREEMPHASIS_75_uS``
+ - A pre-emphasis of 75 uS is used.
+
+
+
+``V4L2_CID_TUNE_POWER_LEVEL (integer)``
+ Sets the output power level for signal transmission. Unit is in
+ dBuV. Range and step are driver-specific.
+
+``V4L2_CID_TUNE_ANTENNA_CAPACITOR (integer)``
+ This selects the value of antenna tuning capacitor manually or
+ automatically if set to zero. Unit, range and step are
+ driver-specific.
+
+For more details about RDS specification, refer to :ref:`iec62106`
+document, from CENELEC.