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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
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interface is aimed at devices capable of receiving and/or transmitting RDS
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information.</para>
<para > For more information see the core RDS standard <xref linkend= "en50067" />
and the RBDS standard <xref linkend= "nrsc4" /> .</para>
<para > Note that the RBDS standard as is used in the USA is almost identical
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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>
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<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>
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<para > Devices supporting the RDS capturing API set
the <constant > V4L2_CAP_RDS_CAPTURE</constant> flag in
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the <structfield > capabilities</structfield> field of &v4l2-capability;
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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_SUB_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_SUB_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>
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<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>
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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_SUB_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_SUB_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>
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</section>
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<section id= "reading-rds-data" >
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<title > Reading RDS data</title>
<para > RDS data can be read from the radio device
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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,
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as follows:</para>
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</section>
<section >
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<title > RDS datastructures</title>
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<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>
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<tgroup cols= "4" >
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<colspec colname= "c1" colwidth= "1*" />
<colspec colname= "c2" colwidth= "1*" />
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<colspec colname= "c3" colwidth= "1*" />
<colspec colname= "c4" colwidth= "5*" />
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<tbody valign= "top" >
<row >
<entry > V4L2_RDS_BLOCK_MSK</entry>
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<entry > </entry>
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<entry > 7</entry>
<entry > Mask for bits 0-2 to get the block ID.</entry>
</row>
<row >
<entry > V4L2_RDS_BLOCK_A</entry>
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<entry > </entry>
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<entry > 0</entry>
<entry > Block A.</entry>
</row>
<row >
<entry > V4L2_RDS_BLOCK_B</entry>
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<entry > </entry>
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<entry > 1</entry>
<entry > Block B.</entry>
</row>
<row >
<entry > V4L2_RDS_BLOCK_C</entry>
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<entry > </entry>
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<entry > 2</entry>
<entry > Block C.</entry>
</row>
<row >
<entry > V4L2_RDS_BLOCK_D</entry>
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<entry > </entry>
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<entry > 3</entry>
<entry > Block D.</entry>
</row>
<row >
<entry > V4L2_RDS_BLOCK_C_ALT</entry>
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<entry > </entry>
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<entry > 4</entry>
<entry > Block C'.</entry>
</row>
<row >
<entry > V4L2_RDS_BLOCK_INVALID</entry>
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<entry > read-only</entry>
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<entry > 7</entry>
<entry > An invalid block.</entry>
</row>
<row >
<entry > V4L2_RDS_BLOCK_CORRECTED</entry>
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<entry > read-only</entry>
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<entry > 0x40</entry>
<entry > A bit error was detected but corrected.</entry>
</row>
<row >
<entry > V4L2_RDS_BLOCK_ERROR</entry>
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<entry > read-only</entry>
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<entry > 0x80</entry>
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<entry > An uncorrectable error occurred.</entry>
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</row>
</tbody>
</tgroup>
</table>
</section>
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