2024-09-09 08:52:07 +00:00
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<title>RDS Interface</title>
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<para>The Radio Data System transmits supplementary
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information in binary format, for example the station name or travel
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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>
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2024-09-09 08:57:42 +00:00
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<para>For more information see the core RDS standard <xref linkend="iec62106" />
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2024-09-09 08:52:07 +00:00
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and the RBDS standard <xref linkend="nrsc4" />.</para>
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<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
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fields have slightly different meanings. See the RBDS standard for more
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information.</para>
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<para>The RBDS standard also specifies support for MMBS (Modified Mobile Search).
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This is a proprietary format which seems to be discontinued. The RDS interface does not
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support this format. Should support for MMBS (or the so-called 'E blocks' in general)
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be needed, then please contact the linux-media mailing list: &v4l-ml;.</para>
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<section>
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<title>Querying Capabilities</title>
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<para>Devices supporting the RDS capturing API set
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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
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will set the <constant>V4L2_TUNER_CAP_RDS</constant> flag in
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the <structfield>capability</structfield> field of &v4l2-tuner;. If
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the driver only passes RDS blocks without interpreting the data
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the <constant>V4L2_TUNER_CAP_RDS_BLOCK_IO</constant> flag has to be
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set, see <link linkend="reading-rds-data">Reading RDS data</link>.
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For future use the
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flag <constant>V4L2_TUNER_CAP_RDS_CONTROLS</constant> has also been
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defined. However, a driver for a radio tuner with this capability does
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not yet exist, so if you are planning to write such a driver you
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should discuss this on the linux-media mailing list: &v4l-ml;.</para>
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<para> Whether an RDS signal is present can be detected by looking
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at the <structfield>rxsubchans</structfield> field of &v4l2-tuner;:
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the <constant>V4L2_TUNER_SUB_RDS</constant> will be set if RDS data
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was detected.</para>
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<para>Devices supporting the RDS output API
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set the <constant>V4L2_CAP_RDS_OUTPUT</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.
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Any modulator that supports RDS will set the
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<constant>V4L2_TUNER_CAP_RDS</constant> flag in the <structfield>capability</structfield>
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field of &v4l2-modulator;.
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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;.
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If the driver only passes RDS blocks without interpreting the data
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the <constant>V4L2_TUNER_CAP_RDS_BLOCK_IO</constant> flag has to be set. If the
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tuner is capable of handling RDS entities like program identification codes and radio
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text, the flag <constant>V4L2_TUNER_CAP_RDS_CONTROLS</constant> should be set,
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see <link linkend="writing-rds-data">Writing RDS data</link> and
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<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>
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<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>
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</section>
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<section id="writing-rds-data">
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<title>Writing RDS data</title>
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<para>RDS data can be written to the radio device
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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>
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<section>
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<title>RDS datastructures</title>
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<table frame="none" pgwide="1" id="v4l2-rds-data">
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<title>struct
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<structname>v4l2_rds_data</structname></title>
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<tgroup cols="3">
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<colspec colname="c1" colwidth="1*" />
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<colspec colname="c2" colwidth="1*" />
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<colspec colname="c3" colwidth="5*" />
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<tbody valign="top">
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<row>
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<entry>__u8</entry>
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<entry><structfield>lsb</structfield></entry>
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<entry>Least Significant Byte of RDS Block</entry>
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</row>
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<row>
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<entry>__u8</entry>
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<entry><structfield>msb</structfield></entry>
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<entry>Most Significant Byte of RDS Block</entry>
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</row>
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<row>
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<entry>__u8</entry>
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<entry><structfield>block</structfield></entry>
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<entry>Block description</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table frame="none" pgwide="1" id="v4l2-rds-block">
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<title>Block description</title>
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<tgroup cols="2">
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<colspec colname="c1" colwidth="1*" />
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<colspec colname="c2" colwidth="5*" />
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<tbody valign="top">
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<row>
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<entry>Bits 0-2</entry>
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<entry>Block (aka offset) of the received data.</entry>
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</row>
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<row>
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<entry>Bits 3-5</entry>
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<entry>Deprecated. Currently identical to bits 0-2. Do not use these bits.</entry>
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</row>
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<row>
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<entry>Bit 6</entry>
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<entry>Corrected bit. Indicates that an error was corrected for this data block.</entry>
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</row>
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<row>
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<entry>Bit 7</entry>
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<entry>Error bit. Indicates that an uncorrectable error occurred during reception of this block.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<table frame="none" pgwide="1" id="v4l2-rds-block-codes">
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<title>Block defines</title>
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<tgroup cols="4">
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<colspec colname="c1" colwidth="1*" />
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<colspec colname="c2" colwidth="1*" />
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<colspec colname="c3" colwidth="1*" />
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<colspec colname="c4" colwidth="5*" />
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<tbody valign="top">
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<row>
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<entry>V4L2_RDS_BLOCK_MSK</entry>
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<entry> </entry>
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<entry>7</entry>
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<entry>Mask for bits 0-2 to get the block ID.</entry>
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</row>
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<row>
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<entry>V4L2_RDS_BLOCK_A</entry>
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<entry> </entry>
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<entry>0</entry>
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<entry>Block A.</entry>
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</row>
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<row>
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<entry>V4L2_RDS_BLOCK_B</entry>
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<entry> </entry>
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<entry>1</entry>
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<entry>Block B.</entry>
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</row>
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<row>
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<entry>V4L2_RDS_BLOCK_C</entry>
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<entry> </entry>
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<entry>2</entry>
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<entry>Block C.</entry>
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</row>
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<row>
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<entry>V4L2_RDS_BLOCK_D</entry>
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<entry> </entry>
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<entry>3</entry>
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<entry>Block D.</entry>
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</row>
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<row>
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<entry>V4L2_RDS_BLOCK_C_ALT</entry>
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<entry> </entry>
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<entry>4</entry>
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<entry>Block C'.</entry>
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</row>
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<row>
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<entry>V4L2_RDS_BLOCK_INVALID</entry>
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<entry>read-only</entry>
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<entry>7</entry>
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<entry>An invalid block.</entry>
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</row>
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<row>
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<entry>V4L2_RDS_BLOCK_CORRECTED</entry>
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<entry>read-only</entry>
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<entry>0x40</entry>
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<entry>A bit error was detected but corrected.</entry>
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</row>
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<row>
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<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>
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</tbody>
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</tgroup>
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</table>
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</section>
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