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diff --git a/demos/lpc_tutorial.htm b/demos/lpc_tutorial.htm new file mode 100644 index 0000000..a6af640 --- /dev/null +++ b/demos/lpc_tutorial.htm @@ -0,0 +1,186 @@ +<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> +<html> +<head> + <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> + <meta name="GENERATOR" content="Mozilla/4.75 [en] (Windows NT 5.0; U) [Netscape]"> + <title>Nyquist FFT and Inverse FFT Tutorial</title> +</head> +<body> + +<h1> +Nyquist LPC Analysis and Synthesis</h1> +Nyquist provides functions for LPC analysis and synthesis on streams of +audio data. For analysis, the audio stream is broken into successive frames, +each of which is analyzed, resulting in a small set of coefficients. For +synthesis, these frames control a time-varying filter which can be applied +to an audio signal. +<p>As with FFT frames, Nyquist does not have a special data type corresponding +to a sequence of LPC frames. Instead, a sequence of frames is represented +by an XLISP object. Whenever you send the selector <tt>:next</tt> to the +object, you get back either NIL, indicating the end of the sequence, or +you get an LPC frame. (Note that FFT frames and LPC frames are not compatible.) +<p>This tutorial is based on the file <tt>lpcdemo.lsp</tt> in the <tt>demos</tt> +folder fo the Nyquist release. Not every line is covered, but this tutorial +should give you a pretty full explanation of how to use LPC in Nyquist. +<h2> +Preliminaries</h2> +Before using LPC functions, you must load the LPC library: +<pre>(load "lpc")</pre> +At the top of <tt>lpcdemo.lsp</tt>, you see the following lines: +<pre>(setf *lpc-path* (current-path)) +(setf *lpc-data* (strcat *lpc-path* "lpc-exmpl.dat"))</pre> +The <tt>(current-path)</tt> function call returns the full file system +path to the file being loaded (<tt>lpcdemo.lsp</tt>). We save this into +<tt>*lpc-path*</tt> +so we can find other related files later, even if the current directory +is changed. <tt>*lpc-data*</tt> is one of the file names we will use later. +<h2> +LPC Analysis</h2> +Another file we want is <tt>a-snd-file.snd</tt>, a very short vocal sound +that is part of the Nyquist release (it is also used by <tt>examples.lsp)</tt>. +To get this file, we concatenate it with the path we obtained above. The +resulting full path can be passed to <tt>s-read </tt>to read the sound. +Find the expressions in the following function that build a file name and +read the file as a sound: +<pre>(defun do-lpc-analysis () + (let ((myfile (strcat *lpc-path* "a-snd-file.snd"))) + (save-lpc-file (make-lpanal-iterator (s-read *myfile*) 0.08 0.04 50) + "temp.dat")))</pre> +You can analyze a sound using <tt>make-lpanal-iterator</tt>. This function +takes a sound, a frame size, a step size, and the number of poles. In <tt>do-lpc-analysis</tt> +(above), we used 0.08 seconds, 0.04 seconds, and 50 poles for these parameters. +The result of <tt>make-lpanal-iterator</tt> is an object that will deliver +LPC frames on demand. In this function, we will grab all of the frames +and write them to a file using <tt>save-lpc-file</tt>. The data is written +to <tt>"temp.dat"</tt>. You should run this function and look at the file, +which contains ASCII data. Later, we will use this data to filter a sound. +<p>Since <tt>a-snd-file.snd</tt> is a very short and uninteresting sound, +we will use a different set of LPC frames in the following synthesis examples. +The data is in <tt>lpc-exmpl.dat</tt>, and the full path for this file +was computed earlier and stored in <tt>*lpc-data*</tt>. +<p>The first example will just show how to read the LPC data file using +<tt>make-lpc-file-iterator</tt>, +which takes the file name as a parameter. You can print some frame data +using <tt>show-lpc-data</tt> as shown in the following: +<pre>(defun ex-1 () + (show-lpc-data (make-lpc-file-iterator *lpc-data*) 100 120 NIL))</pre> +The additional parameters to <tt>show-lpc-data</tt> are the starting frame +(100), the ending frame (120), and a flag (<tt>NIL</tt>) indicating +whether to print filter coefficients. If you want to look a little closer +at the inner workings of this code, you can send a <tt>:next</tt> message +to an LPC iterator object as follows: +<pre>(setf iterator (make-lpc-file-iterator *lpc-data*)) +(send iterator :next)</pre> +This will return the first frame of the LPC analysis. Send <tt>:next</tt> +again to get the next frame. +<h2> +LPC Synthesis With Fixed All-Pole Filter</h2> +The next example creates a sequence of vowel sounds. The vowel filters +are based on the +<br>30th, 60th, and 100th frames taken from the file <tt>lpc-exmpl.dat</tt>. +We use <tt>make-lpc-file-iterator</tt> as before to read the LPC frames +from the file, but we use <tt>nth-frame</tt> to get the desired frames. +<p>The function <tt>allpoles-from-lpc</tt> constructs a filter from the +frame and applies it to a sound. In this case, the source sound is created +by <tt>buzz</tt> with a little vibrato provided by <tt>lfo</tt>: +<br> +<pre>(defun ex-4 () + (play (seq + (allpoles-from-lpc (buzz 12 f2 (lfo 5.0 4.0)) + (nth-frame (make-lpc-file-iterator *lpc-data*) + 30)) + (allpoles-from-lpc (buzz 12 f2 (lfo 5.1 4.0)) + (nth-frame (make-lpc-file-iterator *lpc-data*) + 60)) + (allpoles-from-lpc (buzz 12 g2 (lfo 5.3 4.0)) + (nth-frame (make-lpc-file-iterator *lpc-data*) + 100)) )))</pre> +Rather than iterate through a file to find the desired frame, you can also +just store the desired frames as in the following: +<pre>(setf a-lpcdata + '(63.2144 0.674387 0.103287 + #(-0.0381026 0.00804115 0.0109905 0.0145117 0.00199174 + -0.00129314 0.0171826 0.0181176 0.00179391 -0.0114089 + -0.0120949 -0.000410595 -0.0122539 -0.0209354 -0.00804976 + -0.00345041 -0.00409532 -0.00227011 0.014224 0.0135451 + 0.0056023 -0.00651142 -0.00564953 -0.0168921 -0.0377939 + -0.0449506 -0.0355592 -0.0339316 -0.0454434 1.19336)))</pre> + +<pre>(setf e-lpcdata + '(40.7157 0.149753 0.0606467 + #(0.0244574 -0.0225545 -0.0172724 -0.0122709 -0.0042946 + 0.00886974 0.0121516 0.0120936 0.00197545 -0.00582163 + -0.018367 -0.0201546 -0.00440599 0.00638936 0.0166275 + 0.0185066 0.00890464 -0.00158013 -0.00494974 -0.00479037 + 0.0130814 0.0138648 -0.0022018 -0.021368 -0.0343532 + -0.0312712 -0.0574975 -0.0918824 -0.112016 1.31398)))</pre> + +<pre>(setf i-lpcdata + '(5.5391 0.0321825 0.0762238 + #(-0.0341124 -0.0149688 -0.00585657 -0.0111572 0.00769712 + 0.0190367 0.00885366 0.0112762 0.0118286 -0.00059044 + -0.0140864 -0.0123688 -0.0151128 0.00214354 -0.00810219 + -0.00538188 0.00631382 0.020771 0.0356498 0.0295531 + 0.0242797 0.0124296 0.00445127 -0.013062 -0.0387178 + -0.0527783 -0.0685511 -0.076575 -0.0846335 1.24521)))</pre> +The following function applies a filter to noise: +<pre>(defun noise-vocal (lpcdata dur) + (allpoles-from-lpc (noise dur) lpcdata))</pre> +Combining this with our definitions of different frames, we can write a +little sequence +<br>of vowel sounds: +<pre>(defun ex-5 () + (play + (seq (noise-vocal a-lpcdata 1) + (noise-vocal e-lpcdata 1) + (noise-vocal i-lpcdata 1))))</pre> +We can do the same thing using a buzz sound rather than noise: +<pre>(defun buzz-vocal (lpcdata dur) + (allpoles-from-lpc (buzz 16 e2 (const 0.0 dur)) + lpcdata))</pre> + +<pre>(defun ex-6 () + (play + (seq (buzz-vocal a-lpcdata 1) + (buzz-vocal e-lpcdata 1) + (buzz-vocal i-lpcdata 1))))</pre> + +<h2> +Time-Varying LPC Filter</h2> +The most interesting LPC effect is to use a sequence of frames to drive +a time-varying all-pole filter. If the frames were analyzed from a vocal +source, then the resulting filter will transfer the speech articulation +(or at least some of it) to another sound. The time-varying LPC filter +is called <tt>lpreson</tt>. +<p>Here, the LPC data from <tt>*lpc-data*</tt> is used to modulate noise: +<pre>(defun ex-7a () + ;; parameters are sound, lpc-iterator, skiptime + (lpreson (noise 6.5) (make-lpc-file-iterator *lpc-data*) 0.04))</pre> + +<pre>(defun ex-7 () + (play (ex-7a)))</pre> +The same thing can be done to filter a buzz sound. This example generates +some vocal-like sounds in two-part harmony: +<br>(defun ex-8a (p dur)<br> + (lpreson (buzz 16 p (scale 1.5 (lfo 3.0 dur))) <br> + (make-lpc-file-iterator +*lpc-data*)<br> + 0.04))<br> +<br> +(defun ex-8 ()<br> + (play<br> + (sim (seq (ex-8a c4 1) (ex-8a g3 1) (ex-8a a3 1) (ex-8a +b3 1) (ex-8a c4 1))<br> + (seq (ex-8a c2 2) (ex-8a +f2 1) (ex-8a g2 1) (ex-8a e2 1)))))<br> +<br> +Note that you can change the <i>skiptime</i> parameter to <tt>lpreson</tt> +to change the rate at which the filter moves through the frames. The result +is to caues the speech articulations to go faster or slower. +<p>More examples can be found in <tt>lpcdemo.lsp</tt>. +<h2> +Acknowledgments</h2> +Pedro J. Morales created the LPC functions for Nyquist and wrote <tt>lpcdemo.lsp</tt> +on which this tutorial is based. +</body> +</html> |