     <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>