Частота выходного звука, с bass.dll

Retrieves the immediate sample data (or an FFT representation of it) of a stream or MOD music channel. Can also be used with a recording channel.

DWORD BASS_ChannelGetData(
DWORD handle,
void *buffer,
DWORD length
);


Parameters
handle The channel handle... a HMUSIC, HSTREAM, or HRECORD.
buffer Pointer to a buffer to receive the data... can be NULL when handle is a recording channel (HRECORD), to discard the requested amount of data from the recording buffer.
length Number of bytes wanted, and/or the following flags.
BASS_DATA_FLOAT Return floating-point sample data.
BASS_DATA_FFT256 256 sample FFT (returns 128 floating-point values).
BASS_DATA_FFT512 512 sample FFT (returns 256 floating-point values).
BASS_DATA_FFT1024 1024 sample FFT (returns 512 floating-point values).
BASS_DATA_FFT2048 2048 sample FFT (returns 1024 floating-point values).
BASS_DATA_FFT4096 4096 sample FFT (returns 2048 floating-point values).
BASS_DATA_FFT8192 8192 sample FFT (returns 4096 floating-point values).
BASS_DATA_FFT16384 16384 sample FFT (returns 8192 floating-point values).
BASS_DATA_FFT_INDIVIDUAL Perform a separate FFT for each channel, rather than a single combined FFT. The size of the data returned (as listed above) is multiplied by the number of channels.

BASS_DATA_FFT_NOWINDOW Prevent a Hann window being applied to the sample data when performing an FFT.
BASS_DATA_FFT_REMOVEDC Remove any DC bias from the sample data when performing an FFT.
BASS_DATA_AVAILABLE Query the amount of data the channel has buffered for playback, or from recording. This flag cannot be used with decoding channels as they do not have playback buffers. buffer is ignored when using this flag.



Return value
If an error occurs, -1 is returned, use BASS_ErrorGetCode to get the error code. When requesting FFT data, the number of bytes read from the channel (to perform the FFT) is returned. When requesting sample data, the number of bytes written to buffer will be returned (not necessarily the same as the number of bytes read when using the BASS_DATA_FLOAT flag). When using the BASS_DATA_AVAILABLE flag, the number of bytes in the channel's buffer is returned.

Error codes
BASS_ERROR_HANDLE handle is not a valid channel.
BASS_ERROR_ENDED The channel has reached the end.
BASS_ERROR_NOTAVAIL The BASS_DATA_AVAILABLE flag was used with a decoding channel.
BASS_ERROR_BUFLOST Should not happen... check that a valid window handle was used with BASS_Init.


Remarks
This function can only return as much data as has been written to the channel's buffer, so it may not always be possible to get the amount of data requested, especially if you request large amounts. If you really do need large amounts, then increase the buffer lengths (BASS_CONFIG_BUFFER). The BASS_DATA_AVAILABLE flag can be used to check how much data a channel's buffer contains at any time, including when stopped or stalled.

When requesting data from a "decoding channel" (BASS_STREAM_DECODE or BASS_MUSIC_DECODE was used at creation), there are no intermediate buffers involved, so as much data as is available can be decoded in one go.

When retrieving sample data, 8-bit samples are unsigned (0 to 255) , 16-bit samples are signed (-32768 to 32767), 32-bit floating-point samples range from -1 to +1 (not clipped, so can actually be outside this range). That is unless the BASS_DATA_FLOAT flag is used, in which case, the sample data will be converted to 32-bit floating-point if it is not already.

When requesting FFT data, only the first half of the FFT result is returned. For example, with a 2048 sample FFT, there will be 1024 floating-point values returned. Each value, or "bin", ranges from 0 to 1 (can actually go higher if the sample data is floating-point and not clipped). The 1st bin contains the DC component, the 2nd contains the amplitude at 1/2048 of the channel's sample rate, then the amplitude at 2/2048, 3/2048, etc. A Hann window is applied to the sample data to reduce leakage, unless the BASS_DATA_FFT_NOWINDOW flag is used. When a window is applied, it causes the DC component to leak into the next bin, but that can be removed (reduced to 0) by using the BASS_DATA_FFT_REMOVEDC flag. Doing so slightly increases the processing required though, so it should only be done when needed, which is when a window is applied and the 2nd bin value is important.

Channels that have 2 or more sample channels (ie. stereo or above) may have FFT performed on each individual channel, using the BASS_DATA_FFT_INDIVIDUAL flag. Without this flag, all of the channels are combined, and a single mono FFT is performed. Performing the extra individual FFTs of course increases the amount of processing required. The return values are interleaved in the same order as the channel's sample data, eg. stereo = left,right,left,etc.

This function is most useful if you wish to visualize (eg. spectrum analyze) the sound.

Example
Get an 1024 sample FFT representation of a playing channel.

никак...bass.dll не предоставляет такой возможности, жаль =(
(вы можете загрузить музыку и просто промотать её при воспроизведении, но не уверен правильности результата.)