Secret Rabbit Code (aka libsamplerate)
Author: Erik de Castro Lopo (erikd AT mega-nerd DOT com)
Description: The Secret Rabbit Code Home Page
Keywords: libsamplerate sound resample audio dsp Linux

| # Frequently Asked Questions Q1 : Is it normal for the output of libsamplerate to be louder than its input? Q2 : On Unix/Linux/MacOSX, what is the best way of detecting the presence and location of libsamplerate and its header file using autoconf? Q3 : If I upsample and downsample to the original rate, for example 44.1->96->44.1, do I get an identical signal as the one before the up/down resampling? ## Q1 : Is it normal for the output of libsamplerate to be louder than its input? The output of libsamplerate will be roughly the same volume as the input. However, even if the input is strictly in the range (-1.0, 1.0), it is still possible for the output to contain peak values outside this range. Consider four consecutive samples of [0.5 0.999 0.999 0.5]. If we are up sampling by a factor of two we need to insert samples between each of the existing samples. Its pretty obvious then, that the sample between the two 0.999 values should and will be bigger than 0.999. This means that anyone using libsamplerate should normalise its output before doing things like saving the audio to a 16 bit WAV file. ## Q2 : On Unix/Linux/MacOSX, what is the best way of detecting the presence and location of libsamplerate and its header file using autoconf? libsamplrate uses the pkg-config (man pkg-config) method of registering itself with the host system. The best way of detecting its presence is using something like this in configure.ac (or configure.in):
        PKG_CHECK_MODULES(SAMPLERATE, samplerate >= 0.0.15,
                ac_cv_samplerate=1, ac_cv_samplerate=0)

        AC_DEFINE_UNQUOTED([HAVE_SAMPLERATE],${ac_cv_samplerate},
                [Set to 1 if you have libsamplerate.])

        AC_SUBST(SAMPLERATE_CFLAGS)
        AC_SUBST(SAMPLERATE_LIBS)
This will automatically set the SAMPLERATE_CFLAGS and SAMPLERATE_LIBS variables which can be used in Makefile.am or Makefile.in like this:
            SAMPLERATE_CFLAGS = @SAMPLERATE_CFLAGS@
            SAMPLERATE_LIBS = @SAMPLERATE_LIBS@
If you install libsamplerate from source, you will probably need to set the PKG_CONFIG_PATH environment variable's suggested at the end of the libsamplerate configure process. For instance on my system I get this:
        -=-=-=-=-=-=-=-=-=-= Configuration Complete =-=-=-=-=-=-=-=-=-=-=-

          Configuration summary :

            Version : ..................... 0.1.0
            Enable debugging : ............ no

          Tools :

            Compiler is GCC : ............. yes
            GCC major version : ........... 3

          Extra tools required for testing and examples :

            Have FFTW : ................... yes
            Have libsndfile : ............. yes
            Have libefence : .............. no

          Installation directories :

            Library directory : ........... /usr/local/lib
            Program directory : ........... /usr/local/bin
            Pkgconfig directory : ......... /usr/local/lib/pkgconfig
## Q3 : If I upsample and downsample to the original rate, for example 44.1->96->44.1, do I get an identical signal as the one before the up/down resampling? The short answer is that for the general case, no, you don't. The long answer is that for some signals, with some converters, you will get very, very close. In order to resample correctly (ie using the SRC_SINC_* converters), filtering needs to be applied, regardless of whether its upsampling or downsampling. This filter needs to attenuate all frequencis above 0.5 times the minimum of the source and desination sample rate (call this fshmin). Since the filter needed to achieve full attemuation at this point, it has to start rolling off a some frequency below this point. It is this rolloff of the very highest frequencies which causes some of the loss. The other factor is that the filter itself can introduce transient artifacts which causes the output to be different to the input.