Add libsamplerate for resampling decoder output

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+---
+layout: default
+---
+
+# Callback API
+
+The callback API consists of the following functions :
+
+```c
+typedef long (*src_callback_t) (void *cb_data, float **data) ;
+
+SRC_STATE* src_callback_new (src_callback_t func,
+                    int converter_type, int channels,
+                    int *error, void* cb_data) ;
+
+SRC_STATE* src_delete (SRC_STATE *state) ;
+
+long src_callback_read (SRC_STATE *state, double src_ratio,
+                    long frames, float *data) ;
+
+int src_reset (SRC_STATE *state) ;
+int src_set_ratio (SRC_STATE *state, double new_ratio) ;
+```
+
+Like the [simple API](api_simple.md) and the [full API](api_full.md), the
+callback based API is able to operate on interleaved multi channel data.
+
+An example of the use of the callback based API can be found in the
+**varispeed-play** program in the **examples/** directory of the source code
+tarball.
+
+## Initialisation
+
+```c
+SRC_STATE* src_callback_new (src_callback_t func,
+                    int converter_type, int channels,
+                    int *error, void* cb_data) ;
+```
+
+The **src_callback_new** function returns an anonymous pointer to a sample rate
+converter callback object, src_state. This is the same type of object as that
+returned by [src\_new](api_full.md#initialisation), but with different internal
+state. Although these are the same object type, they cannot be used
+interchangeably. If an error occurs the function returns a NULL pointer and
+fills in the error value pointed to by the **error** pointer supplied by the
+caller.
+
+The caller then passes the SRC_STATE object to the **src_callback_read**
+function to pull data out of the converter. When the caller is finished with the
+converter they should pass it to the clean up function [src_delete](api_full.md#cleanup).
+
+The **func** parameter is a user supplied function which must match the
+**src_callback_t** type signature while **cb_data** is a pointer to data which
+be passed as the first parameter to the user supplied callback function. This
+function is called by the converter whenever it needs input data as a result of
+being calls to **src_callback_read**.
+
+If the converter was initialised to work with more than one channel, the
+callback function must work with mutiple channels of interleaved data. The
+callback function should return the number of frames it supplying to the
+converter. For multi channel files, this return value should be the number of
+floats divided by the number of channels.
+
+The converter must be one of the supplied converter types documented [here](api_misc.md#converters).
+
+The caller then passes the SRC_STATE pointer to the **src_callback_read**
+function to pull data out of the converter.
+
+## Callback Read
+
+```c
+long src_callback_read (SRC_STATE *state, double src_ratio,
+                    long frames, float *data) ;
+```
+
+The **src_callback_read** function is passed the [**SRC_STATE**](api_misc.md#src_data)
+pointer returned by **src_callback_new**, the coversion ratio
+(output_sample_rate / input_sample_rate), the maximum number of output frames to
+generate and a pointer to a buffer in which to place the output data. For multi
+channel files, the data int the output buffer is stored in interleaved format.
+
+The **src_callback_read** function returns the number of frames generated or
+zero if an error occurs or it runs out of input (ie the user supplied callback
+function returns zero and there is no more data buffered internally). If an
+error has occurred, the function [src_error](api_misc.md#error-reporting)
+will return non-zero.
+
+See also : [**src_set_ratio**](api_full.md#set-ratio)