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+ <title>RDS Interface</title>
+
+ <para>The Radio Data System transmits supplementary
+information in binary format, for example the station name or travel
+information, on an inaudible audio subcarrier of a radio program. This
+interface is aimed at devices capable of receiving and/or transmitting RDS
+information.</para>
+
+ <para>For more information see the core RDS standard <xref linkend="iec62106" />
+and the RBDS standard <xref linkend="nrsc4" />.</para>
+
+ <para>Note that the RBDS standard as is used in the USA is almost identical
+to the RDS standard. Any RDS decoder/encoder can also handle RBDS. Only some of the
+fields have slightly different meanings. See the RBDS standard for more
+information.</para>
+
+ <para>The RBDS standard also specifies support for MMBS (Modified Mobile Search).
+This is a proprietary format which seems to be discontinued. The RDS interface does not
+support this format. Should support for MMBS (or the so-called 'E blocks' in general)
+be needed, then please contact the linux-media mailing list: &v4l-ml;.</para>
+
+ <section>
+ <title>Querying Capabilities</title>
+
+ <para>Devices supporting the RDS capturing API set
+the <constant>V4L2_CAP_RDS_CAPTURE</constant> flag in
+the <structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl. Any tuner that supports RDS
+will set the <constant>V4L2_TUNER_CAP_RDS</constant> flag in
+the <structfield>capability</structfield> field of &v4l2-tuner;. If
+the driver only passes RDS blocks without interpreting the data
+the <constant>V4L2_TUNER_CAP_RDS_BLOCK_IO</constant> flag has to be
+set, see <link linkend="reading-rds-data">Reading RDS data</link>.
+For future use the
+flag <constant>V4L2_TUNER_CAP_RDS_CONTROLS</constant> has also been
+defined. However, a driver for a radio tuner with this capability does
+not yet exist, so if you are planning to write such a driver you
+should discuss this on the linux-media mailing list: &v4l-ml;.</para>
+
+ <para> Whether an RDS signal is present can be detected by looking
+at the <structfield>rxsubchans</structfield> field of &v4l2-tuner;:
+the <constant>V4L2_TUNER_SUB_RDS</constant> will be set if RDS data
+was detected.</para>
+
+ <para>Devices supporting the RDS output API
+set the <constant>V4L2_CAP_RDS_OUTPUT</constant> flag in
+the <structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl.
+Any modulator that supports RDS will set the
+<constant>V4L2_TUNER_CAP_RDS</constant> flag in the <structfield>capability</structfield>
+field of &v4l2-modulator;.
+In order to enable the RDS transmission one must set the <constant>V4L2_TUNER_SUB_RDS</constant>
+bit in the <structfield>txsubchans</structfield> field of &v4l2-modulator;.
+If the driver only passes RDS blocks without interpreting the data
+the <constant>V4L2_TUNER_CAP_RDS_BLOCK_IO</constant> flag has to be set. If the
+tuner is capable of handling RDS entities like program identification codes and radio
+text, the flag <constant>V4L2_TUNER_CAP_RDS_CONTROLS</constant> should be set,
+see <link linkend="writing-rds-data">Writing RDS data</link> and
+<link linkend="fm-tx-controls">FM Transmitter Control Reference</link>.</para>
+ </section>
+
+ <section id="reading-rds-data">
+ <title>Reading RDS data</title>
+
+ <para>RDS data can be read from the radio device
+with the &func-read; function. The data is packed in groups of three bytes.</para>
+ </section>
+
+ <section id="writing-rds-data">
+ <title>Writing RDS data</title>
+
+ <para>RDS data can be written to the radio device
+with the &func-write; function. The data is packed in groups of three bytes,
+as follows:</para>
+ </section>
+
+ <section>
+ <title>RDS datastructures</title>
+ <table frame="none" pgwide="1" id="v4l2-rds-data">
+ <title>struct
+<structname>v4l2_rds_data</structname></title>
+ <tgroup cols="3">
+ <colspec colname="c1" colwidth="1*" />
+ <colspec colname="c2" colwidth="1*" />
+ <colspec colname="c3" colwidth="5*" />
+ <tbody valign="top">
+ <row>
+ <entry>__u8</entry>
+ <entry><structfield>lsb</structfield></entry>
+ <entry>Least Significant Byte of RDS Block</entry>
+ </row>
+ <row>
+ <entry>__u8</entry>
+ <entry><structfield>msb</structfield></entry>
+ <entry>Most Significant Byte of RDS Block</entry>
+ </row>
+ <row>
+ <entry>__u8</entry>
+ <entry><structfield>block</structfield></entry>
+ <entry>Block description</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+ <table frame="none" pgwide="1" id="v4l2-rds-block">
+ <title>Block description</title>
+ <tgroup cols="2">
+ <colspec colname="c1" colwidth="1*" />
+ <colspec colname="c2" colwidth="5*" />
+ <tbody valign="top">
+ <row>
+ <entry>Bits 0-2</entry>
+ <entry>Block (aka offset) of the received data.</entry>
+ </row>
+ <row>
+ <entry>Bits 3-5</entry>
+ <entry>Deprecated. Currently identical to bits 0-2. Do not use these bits.</entry>
+ </row>
+ <row>
+ <entry>Bit 6</entry>
+ <entry>Corrected bit. Indicates that an error was corrected for this data block.</entry>
+ </row>
+ <row>
+ <entry>Bit 7</entry>
+ <entry>Error bit. Indicates that an uncorrectable error occurred during reception of this block.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <table frame="none" pgwide="1" id="v4l2-rds-block-codes">
+ <title>Block defines</title>
+ <tgroup cols="4">
+ <colspec colname="c1" colwidth="1*" />
+ <colspec colname="c2" colwidth="1*" />
+ <colspec colname="c3" colwidth="1*" />
+ <colspec colname="c4" colwidth="5*" />
+ <tbody valign="top">
+ <row>
+ <entry>V4L2_RDS_BLOCK_MSK</entry>
+ <entry> </entry>
+ <entry>7</entry>
+ <entry>Mask for bits 0-2 to get the block ID.</entry>
+ </row>
+ <row>
+ <entry>V4L2_RDS_BLOCK_A</entry>
+ <entry> </entry>
+ <entry>0</entry>
+ <entry>Block A.</entry>
+ </row>
+ <row>
+ <entry>V4L2_RDS_BLOCK_B</entry>
+ <entry> </entry>
+ <entry>1</entry>
+ <entry>Block B.</entry>
+ </row>
+ <row>
+ <entry>V4L2_RDS_BLOCK_C</entry>
+ <entry> </entry>
+ <entry>2</entry>
+ <entry>Block C.</entry>
+ </row>
+ <row>
+ <entry>V4L2_RDS_BLOCK_D</entry>
+ <entry> </entry>
+ <entry>3</entry>
+ <entry>Block D.</entry>
+ </row>
+ <row>
+ <entry>V4L2_RDS_BLOCK_C_ALT</entry>
+ <entry> </entry>
+ <entry>4</entry>
+ <entry>Block C'.</entry>
+ </row>
+ <row>
+ <entry>V4L2_RDS_BLOCK_INVALID</entry>
+ <entry>read-only</entry>
+ <entry>7</entry>
+ <entry>An invalid block.</entry>
+ </row>
+ <row>
+ <entry>V4L2_RDS_BLOCK_CORRECTED</entry>
+ <entry>read-only</entry>
+ <entry>0x40</entry>
+ <entry>A bit error was detected but corrected.</entry>
+ </row>
+ <row>
+ <entry>V4L2_RDS_BLOCK_ERROR</entry>
+ <entry>read-only</entry>
+ <entry>0x80</entry>
+ <entry>An uncorrectable error occurred.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+ </section>