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14<h1 align="left">axvpb.pb.lpf</h1>
15
16
17<h2>C Specification</h2>
18<BLOCKQUOTE>
19<pre><CODE>typedef struct _AXPBLPF
20{
21u16 on;    // 1=filter is active, 0=filter is off
22u16 yn1;   // history sample. Must be initialized once to zero.
23u16 a0;    // Coefficient
24u16 b0;    // Coefficient
25
26
27} AXPBLPF;</CODE></pre>
28</BLOCKQUOTE>
29<h2>Description</h2>
30<p>The <code>lpf</code> values are used by the DSP to implement a per-voice, low-pass filter.</p>
31<p>The on value indicates whether filtering is active for the given voice. A value of 1 means that the filter is on, and a value of zero means that the filter is off.</p>
32<p>The <code>yn1</code> value is a history value and must be initialized to zero. This value is thereafter maintained by AX and must not be touched.</p>
33<p>The <code>a0</code> and <code>b0</code> values are the coefficients for the filter, and must be provided by the application. These values determine the cut-off frequency of the filter, and may be changed at any time.</p>
34<p>The filter is a simple one-pole function:</p>
35<BLOCKQUOTE>
36<pre><CODE>y(n) = a0*x(n) - b0*y(n-1)</CODE></pre>
37</BLOCKQUOTE>
38<p>Where<code> x(n)</code> is the current <i>input</i> sample, and <code>y(n-1)</code> is the previous <i>output</i> sample. The coefficients <code>a0</code> and <code>b0</code> can be calculated like so:</p>
39<BLOCKQUOTE>
40<pre>c = 2.0 - cos(2.0*pi*freq/32000.0)<br>    b0 = sqrt(c^2 - 1) - c<br>    a0 = 1 + b0</pre>
41</BLOCKQUOTE>
42<p>Where <code>freq</code> is in Hertz (Hz) and must be within the range:</p>
43<BLOCKQUOTE>
44<pre><CODE>0 &lt;= freq &lt;= 16,000</CODE></pre>
45</BLOCKQUOTE>
46<p>The coefficients must be unsigned, 16-bit words in fixed-point format, having 1 bit of integer and 15 bits of fraction. Furthermore, the <code>b0</code> term must be (arithmetically) negated before being sent to the DSP. This is because of an optimization in the DSP filter calculation that is afforded by the absence of a sign bit in the coefficients.</p>
47<p>A sample table of coefficients is provided below:</p>
48<BLOCKQUOTE>
49<pre><CODE>static u16 __lpf_coefs[48] =
50{
51
52//  a0      b0       cut-off frequency
53  //------  ------     -----------------
540x6a09, 0x15f6, // 16000Hz
550x6871, 0x178e, // 12800Hz
560x6463, 0x1b9c, // 10240Hz
570x5db3, 0x224c, // 8000Hz
580x5618, 0x29e7, // 6400Hz
590x4d7a, 0x3285, // 5120Hz
600x4367, 0x3c98, // 4000Hz
610x3a5a, 0x45a5, // 3200Hz
620x31c5, 0x4e3a, // 2560Hz
630x2924, 0x56db, // 2000Hz
640x2244, 0x5dbb, // 1600Hz
650x1c50, 0x63af, // 1280Hz
660x16c0, 0x693f, // 1000Hz
670x1292, 0x6d6d, // 800Hz
680x0f18, 0x70e7, // 640Hz
690x0bf5, 0x740a, // 500Hz
700x09a9, 0x7656, // 400Hz
710x07ca, 0x7835, // 320Hz
720x0646, 0x79b9, // 256Hz
730x04ed, 0x7b12, // 200Hz
740x03f5, 0x7c0a, // 160Hz
750x032d, 0x7cd2, // 128Hz
760x027d, 0x7d82, // 100Hz
770x01fe, 0x7e01  // 80Hz
78};  </CODE></pre>
79</BLOCKQUOTE>
80<p><B>Note:</B> The table above provides cut-off frequencies from 80Hz to 16,000Hz, in (approximately) 1/3 octave steps. This granularity is sufficient to achieve a smooth 'sweep' of the cut-off frequency across the spectrum.</p>
81<p>This filter provides 6dB of attenuation per octave.</p>
82<P>Use <CODE><A href="../Utility/AXGetLpfCoefs.html">AXGetLpfCoefs()</A></CODE>  to get the coefficients of the specified cut-off frequency.</P>
83<P>To reflect changes to <code>a0</code>, or <code>b0</code> in the DSP, the user must assert the <code>AX_SYNC_USER_LPF_COEF</code> bit in the <code>axvpb.sync</code> word.</P>
84<P>To synchronize the entire structure, assert the <code>AX_SYNC_USER_LPF</code> bit.</P>
85<h2>See Also</h2>
86
87<p>
88<font face="Courier New"><a href="../Voice_Parameter_Blocks/sync.html">axvpb.sync</a><br> <a href="../Voice_Parameters/AXSetVoiceLpf.html">AXSetVoiceLpf</a><br> <a href="../Voice_Parameters/AXSetVoiceLpfCoefs.html">AXSetVoiceLpfCoefs</a><br> <a href="../Utility/AXGetLpfCoefs.html">AXGetLpfCoefs</a><br></font>
89</p>
90
91<h2>Revision History</h2>
92<P>03/01/2006 Initial version.</P>
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