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<h6><a href="../index.html">Help</a> <span>›</span> <a href="index.html">Extension Tools</a> <span>›</span> Reed-Solomon Decoder</h6>
</div>
<h2>Reed-Solomon Decoder</h2>
<br>
<p><b>Reed-Solomon error correction</b> is the error correcting codes that used widely in two-dimensional barcode, including QR code. It allows to correct reading when some of the data is damaged or unreadable.<br><br>
This is the most essential tool of QRazyBox, since our recovery method is very dependent on Reed-Solomon.<br>This tool implements all type of correction process (also known as <i>Reed-Solomon decoding</i>).</p>
<br>
<h4>Error and Erasure Correction</h4>
<p>There are three types of correction that used by Reed-Solomon codes: <b>Error correction</b>, <b>Erasure correction</b> and both of <b>Error and Erasure correction</b>.<br>
<b>Error correction</b> is type of correction where the errors location are unknown, whereas <b>Erasure correction</b> is type of correction where the errors location are known. <br><br>
Using <b>Error correction</b>, we can correct errors up to half of the number of ECC symbol (<i>Reed-Solomon blocks</i>) and correct errors as many as the number of ECC symbol with <b>Erasure correction</b>. Therefore, Error correction become twice as strong when the error location is known.
<br>
<br>
Thus, according to that fact, when we know position of the errors, the capability to restore the data of each ECC level can be doubled.<br>See the following table to see the difference.</p>
<center><table>
<tr>
<th rowspan="2">Error Correction Level</th>
<th colspan="2">% of data that can be restored</th>
</tr>
<tr>
<th>Without Erasure Correction</th>
<th>With Erasure Correction</th>
</tr>
<tr>
<td>Level L</td>
<td>± 7%</td>
<td>± 14%</td>
</tr>
<tr>
<td>Level M</td>
<td>± 15%</td>
<td>± 30%</td>
</tr>
<tr>
<td>Level Q</td>
<td>± 25%</td>
<td>± 50%</td>
</tr>
<tr>
<td>Level H</td>
<td>± 30%</td>
<td>± 60%</td>
</tr>
</table>
</center>
<br>
<p>However, normal QR code scanner doesn't have <b>erasure correction</b> feature, since it difficult to recognize the error locations of QR code automatically and may resulting in slower scan.</p>
<br>
<h4>Example of Reed-Solomon in General</h4>
<p>Before using this tool, it is better to know how <i>Reed-Solomon</i> works generally. Take a look at the following simple example.<br></p>
<p>Suppose we will encode text <b>"HELLO WORLD"</b> to Reed-Solomon block.</p>
<p>Convert the text to decimal value : <br>
<b>72,69,76,76,79,32,87,79,82,76,68</b></p>
<p>Encode with Reed-Solomon using <b>11</b> ECC symbol : <br>
<b>72,69,76,76,79,32,87,79,82,76,68,<i>123,80,185,183,114,194,10,36,93,160,102</i></b></p>
<br>
<p>Now, that we have the encoded Reed-Solomon block, we will try to calculate the correction possiblity in various damaged states.</p>
<p>According to the ECC symbol that being used to encode (in this case the number of ECC symbol is <b>11</b>), we can correct up to <b>2*e+n <= 11</b>, where <b>e</b> is number of error and <b>n</b> is number of erasure.<br>
Suppose <b>x</b> as error and <b>?</b> as erasure</p>
<p><b>72,69,x,x,x,x,87,79,82,76,68,123,80,185,183,114,194,10,36,93,160,102</b> <i>(4 errors)</i>  <b>=  8 <= 11</b><span>CORRECTABLE</span></p>
<p><b>72,69,x,x,x,x,87,79,82,76,68,123,80,185,183,114,x,x,x,93,160,102</b> <i>(7 errors)</i>  <b>=  14 <= 11</b><span class="red">UNCORRECTABLE</span></p>
<p><b>72,69,?,?,?,?,87,79,82,76,68,123,80,185,183,114,194,10,36,93,160,102</b> <i>(4 erasures)</i>  <b>=  4 <= 11</b><span>CORRECTABLE</span></p>
<p><b>72,69,?,?,?,?,87,79,82,76,68,123,80,185,183,114,?,?,?,93,160,102</b> <i>(7 erasures)</i>  <b>=  7 <= 11</b><span>CORRECTABLE</span></p>
<p><b>?,?,?,?,?,?,87,?,?,76,68,123,80,185,183,114,?,?,?,?,160,102</b> <i>(12 erasures)</i>  <b>=  12 <= 11</b><span class="red">UNCORRECTABLE</span></p>
<p><b>72,69,x,x,x,x,87,79,82,76,68,123,80,185,183,114,?,?,?,93,160,102</b> <i>(4 errors & 3 erasures)</i>  <b>=  11 <= 11</b><span>CORRECTABLE</span></p>
<br>
<h4>Basic Usage</h4>
<p>When you use Reed-Solomon Decoder, you will be prompted by dialog. There will be total 2 page of dialog, for input and output. Although, this tool will input the Reed-Solomon blocks automatically, but you can edit and enter manually if you want to.<br>
You also don't have to input number of ECC symbol and even the error position/location, since this tool will automatically detect from the QR code that you already drawn before (zero values of Reed-Solomon blocks will be treated as error position).</p>
<center>
<img src="../img/rs-decoder-1.png">
</center>
<p>Keep in mind, that Reed-Solomon block can also consist more than one on a larger QR code.<br>The second page of the dialog will display output such as decoded Reed-Solomon blocks and the final message from QR code.</p>
<center>
<img src="../img/rs-decoder-2.png">
</center>
<br>
<p>Jump to <a href="../examples/index.html">examples section</a> to see the further details about using Reed-Solomon Decoder.</p>
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