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Chris@41: Secret Rabbit Code (aka libsamplerate)
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Chris@41: <META NAME="Author" CONTENT="Erik de Castro Lopo (erikd AT mega-nerd DOT com)">
Chris@41: <META NAME="Version" CONTENT="libsamplerate-0.1.8">
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Chris@41: <A HREF="index.html">Home</A><BR>
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Chris@41: <A HREF="api_simple.html">Simple API</A><BR>
Chris@41: <A HREF="api_full.html">Full API</A><BR>
Chris@41: <A HREF="api_misc.html#ErrorReporting">Error Handling</A><BR>
Chris@41: <A HREF="api_misc.html">Miscellaneous</A><BR>
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Chris@41: <DIV CLASS="block">
Chris@41: Author :<BR>Erik de Castro Lopo
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Chris@41:
Chris@41: <H1><B>Miscellaneous API Documentation</B></H1>
Chris@41: <A NAME="ErrorReporting"></A>
Chris@41: <H3><BR>Error Reporting</H3>
Chris@41: <P>
Chris@41: Most of the API functions either return an integer error (ie <B>src_simple</B>
Chris@41: and <B>src_process</B>) or return an integer error value via an int pointer
Chris@41: parameter (<B>src_new</B>).
Chris@41: These integer error values can be converted into a human readable text strings by
Chris@41: calling the function:
Chris@41: </P>
Chris@41: <PRE>
Chris@41: const char* src_strerror (int error) ;
Chris@41: </PRE>
Chris@41: <P>
Chris@41: which will return an error string for valid error numbers, the string "No Error"
Chris@41: for an error value of zero or a NULL pointer if no error message has been defined
Chris@41: for that error value.
Chris@41: </P>
Chris@41:
Chris@41: <A NAME="Converters"></A>
Chris@41: <H3><BR>Converters</H3>
Chris@41: <P>
Chris@41: Secret Rabbit Code has a number of different converters which can be selected
Chris@41: using the <B>converter_type</B> parameter when calling <B>src_simple</B> or
Chris@41: <b>src_new</B>.
Chris@41: Currently, the five converters available are:
Chris@41: </P>
Chris@41: <PRE>
Chris@41: enum
Chris@41: {
Chris@41: SRC_SINC_BEST_QUALITY = 0,
Chris@41: SRC_SINC_MEDIUM_QUALITY = 1,
Chris@41: SRC_SINC_FASTEST = 2,
Chris@41: SRC_ZERO_ORDER_HOLD = 3,
Chris@41: SRC_LINEAR = 4
Chris@41: } ;
Chris@41: </PRE>
Chris@41: <P>
Chris@41: As new converters are added, they will given a number corresponding to the
Chris@41: next inetger.
Chris@41: </P>
Chris@41:
Chris@41: <P>
Chris@41: The details of these converters are as follows:
Chris@41: </P>
Chris@41: <UL>
Chris@41: <LI> <B>SRC_SINC_BEST_QUALITY</B> - This is a bandlimited interpolator derived
Chris@41: from the mathematical <B>sinc</B> function and this is the highest
Chris@41: quality sinc based converter, providing a worst case Signal-to-Noise
Chris@41: Ratio (SNR) of 97 decibels (dB) at a bandwidth of 97%.
Chris@41: All three SRC_SINC_* converters are based on the techniques of
Chris@41: <A HREF="http://ccrma-www.stanford.edu/~jos/resample/">Julius O. Smith</A>
Chris@41: although this code was developed independantly.
Chris@41: <LI> <B>SRC_SINC_MEDIUM_QUALITY</B> - This is another bandlimited interpolator
Chris@41: much like the previous one. It has an SNR of 97dB and a bandwidth of 90%.
Chris@41: The speed of the conversion is much faster than the previous one.
Chris@41: <LI> <B>SRC_SINC_FASTEST</B> - This is the fastest bandlimited interpolator and
Chris@41: has an SNR of 97dB and a bandwidth of 80%.
Chris@41: <LI><B>SRC_ZERO_ORDER_HOLD</B> - A Zero Order Hold converter (interpolated value
Chris@41: is equal to the last value). The quality is poor but the conversion speed is
Chris@41: blindlingly fast.
Chris@41: <li><b>SRC_LINEAR</b> - A linear converter. Again the quality is poor, but the
Chris@41: conversion speed is blindingly fast.
Chris@41: </UL>
Chris@41: <P>
Chris@41: There are two functions that give either a (text string) name or description
Chris@41: for each converter:
Chris@41: </P>
Chris@41: <PRE>
Chris@41: const char *src_get_name (int converter_type) ;
Chris@41: const char *src_get_description (int converter_type) ;
Chris@41: </PRE>
Chris@41: <P>
Chris@41: The name will typically be a short string for use in a dialog box, while the
Chris@41: description string is longer.
Chris@41: </P>
Chris@41: <P>
Chris@41: Both of these functions return a NULL pointer if there is no converter for the
Chris@41: given <B>converter_type</B> value.
Chris@41: Since the converters have consecutive <B>converter_type</B> values, the caller
Chris@41: is easily able to figure out the number of converters at run time.
Chris@41: This enables a binary dynamically linked against an old version of the library
Chris@41: to know about converters from later versions of the library as they become
Chris@41: available.
Chris@41: </P>
Chris@41:
Chris@41: <A NAME="SRC_DATA"></A>
Chris@41: <H3><BR>SRC_DATA</H3>
Chris@41: <P>
Chris@41: Both the simple and the full featured versions of the API use the <B>SRC_DATA</B>
Chris@41: struct to pass audio and control data into the sample rate converter.
Chris@41: This struct is defined as:
Chris@41: </P>
Chris@41: <PRE>
Chris@41: typedef struct
Chris@41: { float *data_in, *data_out ;
Chris@41:
Chris@41: long input_frames, output_frames ;
Chris@41: long input_frames_used, output_frames_gen ;
Chris@41:
Chris@41: int end_of_input ;
Chris@41:
Chris@41: double src_ratio ;
Chris@41: } SRC_DATA ;
Chris@41: </PRE>
Chris@41: <P>
Chris@41: The <B>data_in</B> pointer is used to pass audio data into the converter while the
Chris@41: <B>data_out</B> pointer supplies the converter with an array to hold the converter's
Chris@41: output.
Chris@41: For a converter which has been configured for mulitchannel operation, these pointers
Chris@41: need to point to a single array of interleaved data.
Chris@41: </P>
Chris@41: <P>
Chris@41: The <B>input_frames</B> and <B>output_frames</B> fields supply the converter with
Chris@41: the lengths of the arrays (in frames) pointed to by the <B>data_in</B> and
Chris@41: <b>data_out</B> pointers respectively.
Chris@41: For monophinc data, these values would indicate the length of the arrays while
Chris@41: for multi channel data these values would be equal to the the length of the array
Chris@41: divided by the number of channels.
Chris@41: </P>
Chris@41:
Chris@41: <P>
Chris@41: The <B>end_of_input</B> field is only used when the sample rate converter is used
Chris@41: by calling the <B>src_process</B> function.
Chris@41: In this case it should be set to zero if more buffers are to be passed to the
Chris@41: converter and 1 if the current buffer is the last.
Chris@41: </P>
Chris@41: <P>
Chris@41: Finally, the <B>src_ratio</B> field specifies the conversion ratio defined as
Chris@41: the input sample rate divided by the output sample rate.
Chris@41: For a connected set of buffers, this value can be varies on each call to
Chris@41: <B>src_process</B> resulting in a time varying sample rate conversion
Chris@41: process.
Chris@41: For time varying sample rate conversions, the ratio will be linearly
Chris@41: interpolated between the <B>src_ratio</B> value of the previous call
Chris@41: to <B>src_process</B> and the value for the current call.
Chris@41: </P>
Chris@41: <P>
Chris@41: The <B>input_frames_used</B> and <B>output_frames_gen</B> fields are set by the
Chris@41: converter to inform the caller of the number of frames consumed from the
Chris@41: <B>data_in</B> array and the number of frames generated in the <B>data_out</B>
Chris@41: array respectively.
Chris@41: These values are for the current call to <B>src_process</B> only.
Chris@41: </P>
Chris@41:
Chris@41: <A NAME="Aux"></A>
Chris@41: <H3><BR>Auxillary Functions</H3>
Chris@41: <P>
Chris@41: There are four auxillary functions for converting arrays of float data
Chris@41: to and from short or int data.
Chris@41: These functions are defined as:
Chris@41: </P>
Chris@41: <PRE>
Chris@41: void src_short_to_float_array (const short *in, float *out, int len) ;
Chris@41: void src_float_to_short_array (const float *in, short *out, int len) ;
Chris@41: void src_int_to_float_array (const int *in, float *out, int len) ;
Chris@41: void src_float_to_int_array (const float *in, int *out, int len) ;
Chris@41: </PRE>
Chris@41: <P>
Chris@41: The float data is assumed to be in the range [-1.0, 1.0] and it is
Chris@41: automatically scaled on the conversion to and from float.
Chris@41: On the float to short/int conversion path, any data values which would overflow
Chris@41: the range of short/int data are clipped.
Chris@41: </P>
Chris@41:
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