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226 lines
7.7 KiB
HTML
226 lines
7.7 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML>
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<HEAD>
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<TITLE>
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Secret Rabbit Code (aka libsamplerate)
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</TITLE>
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<META NAME="Author" CONTENT="Erik de Castro Lopo (erikd AT mega-nerd DOT com)">
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<META NAME="Version" CONTENT="libsamplerate-0.1.8">
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<META NAME="Description" CONTENT="The Secret Rabbit Code Home Page">
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<META NAME="Keywords" CONTENT="libsamplerate sound resample audio dsp Linux">
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<LINK REL=StyleSheet HREF="SRC.css" TYPE="text/css" MEDIA="all">
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</HEAD>
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<BODY TEXT="#FFFFFF" BGCOLOR="#000000" LINK="#FB1465" VLINK="#FB1465" ALINK="#FB1465">
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<!-- pepper -->
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<CENTER>
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<IMG SRC="SRC.png" HEIGHT=100 WIDTH=760 ALT="SRC.png">
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</CENTER>
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<BR>
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<TABLE ALIGN="center" WIDTH="98%">
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<TR>
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<TD VALIGN="top">
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<BR>
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<DIV CLASS="nav">
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<BR>
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<A HREF="index.html">Home</A><BR>
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<BR>
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<A HREF="api_simple.html">Simple API</A><BR>
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<A HREF="api_full.html">Full API</A><BR>
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<A HREF="api_misc.html#ErrorReporting">Error Handling</A><BR>
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<A HREF="api_misc.html">Miscellaneous</A><BR>
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<BR>
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<DIV CLASS="block">
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Author :<BR>Erik de Castro Lopo
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<BR><BR>
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</DIV>
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<IMG SRC=
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"/cgi-bin/Count.cgi?ft=6|frgb=55;55;55|tr=0|md=6|dd=B|st=1|sh=1|df=src_api.dat"
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HEIGHT=30 WIDTH=100 ALT="counter.gif">
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</DIV>
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</TD>
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<!-- ######################################################################## -->
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<TD VALIGN="top">
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<DIV CLASS="block">
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<H1><B>Miscellaneous API Documentation</B></H1>
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<A NAME="ErrorReporting"></A>
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<H3><BR>Error Reporting</H3>
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<P>
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Most of the API functions either return an integer error (ie <B>src_simple</B>
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and <B>src_process</B>) or return an integer error value via an int pointer
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parameter (<B>src_new</B>).
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These integer error values can be converted into a human readable text strings by
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calling the function:
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</P>
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<PRE>
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const char* src_strerror (int error) ;
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</PRE>
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<P>
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which will return an error string for valid error numbers, the string "No Error"
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for an error value of zero or a NULL pointer if no error message has been defined
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for that error value.
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</P>
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<A NAME="Converters"></A>
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<H3><BR>Converters</H3>
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<P>
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Secret Rabbit Code has a number of different converters which can be selected
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using the <B>converter_type</B> parameter when calling <B>src_simple</B> or
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<b>src_new</B>.
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Currently, the five converters available are:
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</P>
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<PRE>
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enum
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{
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SRC_SINC_BEST_QUALITY = 0,
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SRC_SINC_MEDIUM_QUALITY = 1,
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SRC_SINC_FASTEST = 2,
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SRC_ZERO_ORDER_HOLD = 3,
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SRC_LINEAR = 4
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} ;
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</PRE>
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<P>
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As new converters are added, they will given a number corresponding to the
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next inetger.
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</P>
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<P>
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The details of these converters are as follows:
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</P>
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<UL>
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<LI> <B>SRC_SINC_BEST_QUALITY</B> - This is a bandlimited interpolator derived
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from the mathematical <B>sinc</B> function and this is the highest
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quality sinc based converter, providing a worst case Signal-to-Noise
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Ratio (SNR) of 97 decibels (dB) at a bandwidth of 97%.
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All three SRC_SINC_* converters are based on the techniques of
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<A HREF="http://ccrma-www.stanford.edu/~jos/resample/">Julius O. Smith</A>
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although this code was developed independantly.
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<LI> <B>SRC_SINC_MEDIUM_QUALITY</B> - This is another bandlimited interpolator
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much like the previous one. It has an SNR of 97dB and a bandwidth of 90%.
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The speed of the conversion is much faster than the previous one.
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<LI> <B>SRC_SINC_FASTEST</B> - This is the fastest bandlimited interpolator and
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has an SNR of 97dB and a bandwidth of 80%.
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<LI><B>SRC_ZERO_ORDER_HOLD</B> - A Zero Order Hold converter (interpolated value
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is equal to the last value). The quality is poor but the conversion speed is
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blindlingly fast.
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<li><b>SRC_LINEAR</b> - A linear converter. Again the quality is poor, but the
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conversion speed is blindingly fast.
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</UL>
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<P>
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There are two functions that give either a (text string) name or description
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for each converter:
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</P>
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<PRE>
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const char *src_get_name (int converter_type) ;
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const char *src_get_description (int converter_type) ;
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</PRE>
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<P>
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The name will typically be a short string for use in a dialog box, while the
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description string is longer.
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</P>
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<P>
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Both of these functions return a NULL pointer if there is no converter for the
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given <B>converter_type</B> value.
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Since the converters have consecutive <B>converter_type</B> values, the caller
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is easily able to figure out the number of converters at run time.
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This enables a binary dynamically linked against an old version of the library
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to know about converters from later versions of the library as they become
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available.
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</P>
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<A NAME="SRC_DATA"></A>
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<H3><BR>SRC_DATA</H3>
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<P>
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Both the simple and the full featured versions of the API use the <B>SRC_DATA</B>
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struct to pass audio and control data into the sample rate converter.
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This struct is defined as:
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</P>
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<PRE>
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typedef struct
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{ float *data_in, *data_out ;
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long input_frames, output_frames ;
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long input_frames_used, output_frames_gen ;
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int end_of_input ;
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double src_ratio ;
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} SRC_DATA ;
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</PRE>
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<P>
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The <B>data_in</B> pointer is used to pass audio data into the converter while the
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<B>data_out</B> pointer supplies the converter with an array to hold the converter's
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output.
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For a converter which has been configured for mulitchannel operation, these pointers
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need to point to a single array of interleaved data.
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</P>
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<P>
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The <B>input_frames</B> and <B>output_frames</B> fields supply the converter with
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the lengths of the arrays (in frames) pointed to by the <B>data_in</B> and
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<b>data_out</B> pointers respectively.
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For monophinc data, these values would indicate the length of the arrays while
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for multi channel data these values would be equal to the the length of the array
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divided by the number of channels.
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</P>
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<P>
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The <B>end_of_input</B> field is only used when the sample rate converter is used
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by calling the <B>src_process</B> function.
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In this case it should be set to zero if more buffers are to be passed to the
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converter and 1 if the current buffer is the last.
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</P>
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<P>
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Finally, the <B>src_ratio</B> field specifies the conversion ratio defined as
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the input sample rate divided by the output sample rate.
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For a connected set of buffers, this value can be varies on each call to
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<B>src_process</B> resulting in a time varying sample rate conversion
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process.
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For time varying sample rate conversions, the ratio will be linearly
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interpolated between the <B>src_ratio</B> value of the previous call
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to <B>src_process</B> and the value for the current call.
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</P>
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<P>
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The <B>input_frames_used</B> and <B>output_frames_gen</B> fields are set by the
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converter to inform the caller of the number of frames consumed from the
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<B>data_in</B> array and the number of frames generated in the <B>data_out</B>
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array respectively.
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These values are for the current call to <B>src_process</B> only.
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</P>
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<A NAME="Aux"></A>
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<H3><BR>Auxillary Functions</H3>
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<P>
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There are four auxillary functions for converting arrays of float data
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to and from short or int data.
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These functions are defined as:
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</P>
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<PRE>
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void src_short_to_float_array (const short *in, float *out, int len) ;
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void src_float_to_short_array (const float *in, short *out, int len) ;
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void src_int_to_float_array (const int *in, float *out, int len) ;
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void src_float_to_int_array (const float *in, int *out, int len) ;
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</PRE>
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<P>
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The float data is assumed to be in the range [-1.0, 1.0] and it is
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automatically scaled on the conversion to and from float.
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On the float to short/int conversion path, any data values which would overflow
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the range of short/int data are clipped.
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</P>
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</DIV>
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</TD></TR>
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</TABLE>
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</BODY>
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