Delphi: BASS.dll - how to copy part of MP3 stream to another file

Question

Im using BASS.dll library and all I want to do is to "redirect" part of MP3 Im playing using for example BASS_StreamCreateFile to another file (may be MP3 or WAVe). I dont know how to start? Im trying to use help to find an answer, but still nothing. I can play this stream. Read some data I need. Now I need to copy ile for example from 2:00 to 2:10 (or by position). Any ideas how should I start?

Regards, J.K.

Answer 1

Well, I don't know BASS specifically, but I know a little about music playing and compressed data formats in general, and copying the data around properly involves an intermediate decoding step. Here's what you'll need to do:

1. Open the file and find the correct position.
2. Decode the audio into an in-memory buffer. The size of your buffer should be (LengthInSeconds * SamplesPerSecond * Channels * BytesPerSample) bytes. So if it's 10 seconds of CD quality audio, that's 10 * 44100 * 2 (stereo) * 2 (16-bit audio) = 1764000 bytes.
3. Take this buffer of decoded data and feed it into an MP3 encoding function, and save the resulting MP3 to a file.

If BASS has functions for decoding to an external buffer and for encoding a buffer to MP3, you're good; all you have to do is figure out which ones to use. If not, you'll have to find another library for MP3 encoding and decoding.

Also, watch out for generational loss. MP3 uses lossy compression, so if you decompress and recompress the data multiple times, it'll hurt the sound quality.