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	Secret Rabbit Code (aka libsamplerate)
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Author : Erik de Castro Lopo

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<H1>Miscellaneous API Documentation</H1>
<A NAME="ErrorReporting"></A>
<H3> Error Reporting</H3>

Most of the API functions either return an integer error (ie src_simple 
and src_process) or return an integer error value via an int pointer 
parameter (src_new).
These integer error values can be converted into a human readable text strings by 
calling the function:

<PRE>
 const char* src_strerror (int error) ;
</PRE>

which will return an error string for valid error numbers, the string "No Error" 
for an error value of zero or a NULL pointer if no error message has been defined 
for that error value.

<A NAME="Converters"></A>
<H3> Converters</H3>

Secret Rabbit Code has a number of different converters which can be selected
using the converter_type parameter when calling src_simple or
src_new.
Currently, the five converters available are:

<PRE>
 enum
 { 
 SRC_SINC_BEST_QUALITY = 0,
 SRC_SINC_MEDIUM_QUALITY = 1,
 SRC_SINC_FASTEST = 2,
 SRC_ZERO_ORDER_HOLD = 3,
 SRC_LINEAR = 4
 } ;
</PRE>

As new converters are added, they will given a number corresponding to the 
next inetger.

The details of these converters are as follows:

<UL>
	<LI> SRC_SINC_BEST_QUALITY - This is a bandlimited interpolator derived 
		from the mathematical sinc function and this is the highest
		quality sinc based converter, providing a worst case Signal-to-Noise
		Ratio (SNR) of 97 decibels (dB) at a bandwidth of 97&#37;.
		All three SRC_SINC_* converters are based on the techniques of 
		<A HREF="http://ccrma-www.stanford.edu/~jos/resample/">Julius O. Smith</A>
		although this code was developed independantly.
	<LI> SRC_SINC_MEDIUM_QUALITY - This is another bandlimited interpolator 
		much like the previous one. It has an SNR of 97dB and a bandwidth of 90&#37;.
		The speed of the conversion is much faster than the previous one.
	<LI> SRC_SINC_FASTEST - This is the fastest bandlimited interpolator and
		has an SNR of 97dB and a bandwidth of 80&#37;.
	<LI>SRC_ZERO_ORDER_HOLD - A Zero Order Hold converter (interpolated value
		is equal to the last value). The quality is poor but the conversion speed is
		blindlingly fast.
	<li>SRC_LINEAR - A linear converter. Again the quality is poor, but the 
		conversion speed is blindingly fast.
</UL>

There are two functions that give either a (text string) name or description
for each converter:

<PRE>
 const char *src_get_name (int converter_type) ;
 const char *src_get_description (int converter_type) ;
</PRE>

The name will typically be a short string for use in a dialog box, while the 
description string is longer.


Both of these functions return a NULL pointer if there is no converter for the 
given converter_type value.
Since the converters have consecutive converter_type values, the caller
is easily able to figure out the number of converters at run time. 
This enables a binary dynamically linked against an old version of the library 
to know about converters from later versions of the library as they become 
available.

<A NAME="SRC_DATA"></A>
<H3> SRC_DATA</H3>

Both the simple and the full featured versions of the API use the SRC_DATA
struct to pass audio and control data into the sample rate converter.
This struct is defined as:

<PRE>
 typedef struct
 { float *data_in, *data_out ;

long   input_frames, output_frames ;
          long   input_frames_used, output_frames_gen ;

int    end_of_input ;

double src_ratio ;
 } SRC_DATA ;
</PRE>

The data_in pointer is used to pass audio data into the converter while the
data_out pointer supplies the converter with an array to hold the converter's
output.
For a converter which has been configured for mulitchannel operation, these pointers
need to point to a single array of interleaved data.


The input_frames and output_frames fields supply the converter with 
the lengths of the arrays (in frames) pointed to by the data_in and 
data_out pointers respectively.
For monophinc data, these values would indicate the length of the arrays while
for multi channel data these values would be equal to the the length of the array
divided by the number of channels.

The end_of_input field is only used when the sample rate converter is used
by calling the src_process function.
In this case it should be set to zero if more buffers are to be passed to the 
converter and 1 if the current buffer is the last.


Finally, the src_ratio field specifies the conversion ratio defined as
the input sample rate divided by the output sample rate.
For a connected set of buffers, this value can be varies on each call to 
src_process resulting in a time varying sample rate conversion 
process.
For time varying sample rate conversions, the ratio will be linearly
interpolated between the src_ratio value of the previous call
to src_process and the value for the current call.


The input_frames_used and output_frames_gen fields are set by the
converter to inform the caller of the number of frames consumed from the
data_in array and the number of frames generated in the data_out
array respectively.
These values are for the current call to src_process only.

<A NAME="Aux"></A>
<H3> Auxillary Functions</H3>

There are four auxillary functions for converting arrays of float data
to and from short or int data.
These functions are defined as:

<PRE>
 void src_short_to_float_array (const short *in, float *out, int len) ;
 void src_float_to_short_array (const float *in, short *out, int len) ;
 void src_int_to_float_array (const int *in, float *out, int len) ;
 void src_float_to_int_array (const float *in, int *out, int len) ;
</PRE>

The float data is assumed to be in the range [-1.0, 1.0] and it is
automatically scaled on the conversion to and from float.
On the float to short/int conversion path, any data values which would overflow
the range of short/int data are clipped.

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