libsamplerate/doc/api_misc.html

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   6         Secret Rabbit Code (aka libsamplerate)
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  37 Author :<BR>Erik de Castro Lopo
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  55 <H1><B>Miscellaneous API Documentation</B></H1>
  56 <A NAME="ErrorReporting"></A>
  57 <H3><BR>Error Reporting</H3>
  58 <P>
  59 Most of the API functions either return an integer error (ie <B>src_simple</B>
  60 and  <B>src_process</B>) or return an integer error value via an int pointer
  61 parameter (<B>src_new</B>).
  62 These integer error values can be converted into a human readable text strings by
  63 calling the function:
  64 </P>
  65 <PRE>
  66       const char* src_strerror (int error) ;
  67 </PRE>
  68 <P>
  69 which  will return an error string for valid error numbers, the string "No Error"
  70 for an error value of zero or a NULL pointer if no error message has been defined
  71 for that error value.
  72 </P>
  73
  74 <A NAME="Converters"></A>
  75 <H3><BR>Converters</H3>
  76 <P>
  77 Secret Rabbit Code has a number of different converters which can be selected
  78 using the <B>converter_type</B> parameter when calling <B>src_simple</B> or
  79 <b>src_new</B>.
  80 Currently, the five converters available are:
  81 </P>
  82 <PRE>
  83       enum
  84       {
  85           SRC_SINC_BEST_QUALITY       = 0,
  86           SRC_SINC_MEDIUM_QUALITY     = 1,
  87           SRC_SINC_FASTEST            = 2,
  88           SRC_ZERO_ORDER_HOLD         = 3,
  89           SRC_LINEAR                  = 4
  90       } ;
  91 </PRE>
  92 <P>
  93 As new converters are added, they will given a number corresponding to the
  94 next inetger.
  95 </P>
  96
  97 <P>
  98 The details of these converters are as follows:
  99 </P>
 100 <UL>
 101         <LI> <B>SRC_SINC_BEST_QUALITY</B> - This is a bandlimited interpolator derived
 102                 from the mathematical <B>sinc</B> function and this is the highest
 103                 quality sinc based converter, providing a worst case Signal-to-Noise
 104                 Ratio (SNR) of 97 decibels (dB) at a bandwidth of 97&#37;.
 105                 All three SRC_SINC_* converters are based on the techniques of
 106                 <A HREF="http://ccrma-www.stanford.edu/~jos/resample/">Julius O. Smith</A>
 107                 although this code was developed independantly.
 108         <LI> <B>SRC_SINC_MEDIUM_QUALITY</B> - This is another bandlimited interpolator
 109                 much like the previous one. It has an SNR of 97dB and a bandwidth of 90&#37;.
 110                 The speed of the conversion is much faster than the previous one.
 111         <LI> <B>SRC_SINC_FASTEST</B> - This is the fastest bandlimited interpolator and
 112                 has an SNR of 97dB and a bandwidth of 80&#37;.
 113         <LI><B>SRC_ZERO_ORDER_HOLD</B> - A Zero Order Hold converter (interpolated value
 114                 is equal to the last value). The quality is poor but the conversion speed is
 115                 blindlingly fast.
 116         <li><b>SRC_LINEAR</b> - A linear converter. Again the quality is poor, but the
 117                 conversion speed is blindingly fast.
 118 </UL>
 119 <P>
 120 There are two functions that give either a (text string) name or description
 121 for each converter:
 122 </P>
 123 <PRE>
 124       const char *src_get_name (int converter_type) ;
 125       const char *src_get_description (int converter_type) ;
 126 </PRE>
 127 <P>
 128 The name will typically be a short string for use in a dialog box, while the
 129 description string is longer.
 130 </P>
 131 <P>
 132 Both of these functions return a NULL pointer if there is no converter for the
 133 given <B>converter_type</B> value.
 134 Since the converters have consecutive <B>converter_type</B> values, the caller
 135 is easily able to figure out the number of converters at run time.
 136 This enables a binary dynamically linked against an old version of the library
 137 to know about converters from later versions of the library as they become
 138 available.
 139 </P>
 140
 141 <A NAME="SRC_DATA"></A>
 142 <H3><BR>SRC_DATA</H3>
 143 <P>
 144 Both the simple and the full featured versions of the API use the <B>SRC_DATA</B>
 145 struct to pass audio and control data into the sample rate converter.
 146 This struct is defined as:
 147 </P>
 148 <PRE>
 149       typedef struct
 150       {   float  *data_in, *data_out ;
 151
 152           long   input_frames, output_frames ;
 153           long   input_frames_used, output_frames_gen ;
 154
 155           int    end_of_input ;
 156
 157           double src_ratio ;
 158       } SRC_DATA ;
 159 </PRE>
 160 <P>
 161 The <B>data_in</B> pointer is used to pass audio data into the converter while the
 162 <B>data_out</B> pointer supplies the converter with an array to hold the converter's
 163 output.
 164 For a converter which has been configured for mulitchannel operation, these pointers
 165 need to point to a single array of interleaved data.
 166 </P>
 167 <P>
 168 The <B>input_frames</B> and <B>output_frames</B> fields supply the converter with
 169 the lengths of the arrays (in frames) pointed to by the <B>data_in</B> and
 170 <b>data_out</B> pointers respectively.
 171 For monophinc data, these values would indicate the length of the arrays while
 172 for multi channel data these values would be equal to the the length of the array
 173 divided by the number of channels.
 174 </P>
 175
 176 <P>
 177 The <B>end_of_input</B> field is only used when the sample rate converter is used
 178 by calling the <B>src_process</B> function.
 179 In this case it should be set to zero if more buffers are to be passed to the
 180 converter and 1 if the current buffer is the last.
 181 </P>
 182 <P>
 183 Finally, the <B>src_ratio</B> field specifies the conversion ratio defined as
 184 the input sample rate divided by the output sample rate.
 185 For a connected set of buffers, this value can be varies on each call to
 186 <B>src_process</B> resulting in a time varying sample rate conversion
 187 process.
 188 For time varying sample rate conversions, the ratio will be linearly
 189 interpolated between the <B>src_ratio</B> value of the previous call
 190 to <B>src_process</B> and the value for the current call.
 191 </P>
 192 <P>
 193 The <B>input_frames_used</B> and <B>output_frames_gen</B> fields are set by the
 194 converter to inform the caller of the number of frames consumed from the
 195 <B>data_in</B> array and the number of frames generated in the <B>data_out</B>
 196 array respectively.
 197 These values are for the current call to <B>src_process</B> only.
 198 </P>
 199
 200 <A NAME="Aux"></A>
 201 <H3><BR>Auxillary Functions</H3>
 202 <P>
 203 There are four auxillary functions for converting arrays of float data
 204 to and from short or int data.
 205 These functions are defined as:
 206 </P>
 207 <PRE>
 208     void src_short_to_float_array (const short *in, float *out, int len) ;
 209     void src_float_to_short_array (const float *in, short *out, int len) ;
 210     void src_int_to_float_array (const int *in, float *out, int len) ;
 211     void src_float_to_int_array (const float *in, int *out, int len) ;
 212 </PRE>
 213 <P>
 214 The float data is assumed to be in the range [-1.0, 1.0] and it is
 215 automatically scaled on the conversion to and from float.
 216 On the float to short/int conversion path, any data values which would overflow
 217 the range of short/int data are clipped.
 218 </P>
 219
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