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Secret Rabbit Code (aka libsamplerate)
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Author :<BR>Erik de Castro Lopo
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<H1><B>Miscellaneous API Documentation</B></H1>
<A NAME="ErrorReporting"></A>
<H3><BR>Error Reporting</H3>
<P>
Most of the API functions either return an integer error (ie <B>src_simple</B>
and <B>src_process</B>) or return an integer error value via an int pointer
parameter (<B>src_new</B>).
These integer error values can be converted into a human readable text strings by
calling the function:
</P>
<PRE>
const char* src_strerror (int error) ;
</PRE>
<P>
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.
</P>
<A NAME="Converters"></A>
<H3><BR>Converters</H3>
<P>
Secret Rabbit Code has a number of different converters which can be selected
using the <B>converter_type</B> parameter when calling <B>src_simple</B> or
<b>src_new</B>.
Currently, the five converters available are:
</P>
<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>
<P>
As new converters are added, they will given a number corresponding to the
next inetger.
</P>
<P>
The details of these converters are as follows:
</P>
<UL>
<LI> <B>SRC_SINC_BEST_QUALITY</B> - This is a bandlimited interpolator derived
from the mathematical <B>sinc</B> 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%.
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> <B>SRC_SINC_MEDIUM_QUALITY</B> - This is another bandlimited interpolator
much like the previous one. It has an SNR of 97dB and a bandwidth of 90%.
The speed of the conversion is much faster than the previous one.
<LI> <B>SRC_SINC_FASTEST</B> - This is the fastest bandlimited interpolator and
has an SNR of 97dB and a bandwidth of 80%.
<LI><B>SRC_ZERO_ORDER_HOLD</B> - 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><b>SRC_LINEAR</b> - A linear converter. Again the quality is poor, but the
conversion speed is blindingly fast.
</UL>
<P>
There are two functions that give either a (text string) name or description
for each converter:
</P>
<PRE>
const char *src_get_name (int converter_type) ;
const char *src_get_description (int converter_type) ;
</PRE>
<P>
The name will typically be a short string for use in a dialog box, while the
description string is longer.
</P>
<P>
Both of these functions return a NULL pointer if there is no converter for the
given <B>converter_type</B> value.
Since the converters have consecutive <B>converter_type</B> 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.
</P>
<A NAME="SRC_DATA"></A>
<H3><BR>SRC_DATA</H3>
<P>
Both the simple and the full featured versions of the API use the <B>SRC_DATA</B>
struct to pass audio and control data into the sample rate converter.
This struct is defined as:
</P>
<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>
<P>
The <B>data_in</B> pointer is used to pass audio data into the converter while the
<B>data_out</B> pointer supplies the converter with an array to hold the converter's
output.
</P>
<P>
The <B>input_frames</B> and <B>output_frames</B> fields supply the converter with
the lengths of the arrays pointed to by the <B>data_in</B> and <b>data_out</B>
pointers respectively.
For monophinc data, these values would indicate the length of the arrays while
for stereo data these values would be equal to the half the length of the array
and so on.
</P>
<P>
The <B>end_of_input</B> field is only used when the sample rate converter is used
by calling the <B>src_process</B> 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.
</P>
<P>
Finally, the <B>src_ratio</B> 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
<B>src_process</B> resulting in a time varying sample rate conversion
process.
For time varying sample rate conversions, the ratio will be linearly
interpolated between the <B>src_ratio</B> value of the previous call
to <B>src_process</B> and the value for the current call.
</P>
<P>
The <B>input_frames_used</B> and <B>output_frames_gen</B> fields are set by the
converter to inform the caller of the number of frames consumed from the
<B>data_in</B> array and the number of frames generated in the <B>data_out</B>
array respectively.
These values are for the current call to <B>src_process</B> only.
</P>
<A NAME="Aux"></A>
<H3><BR>Auxillary Functions</H3>
<P>
There are two auxillary functions for converting arrays of float data
to and from short data.
These functions are defined as:
</P>
<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) ;
</PRE>
<P>
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 conversion path, any data values which would overflow
the range of short data are clipped.
</P>
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