Variable Bitrate
I’ve been having a go at improving the accuracy and format of my DreamyAudio podcast. The accuracy being the part where I read information embedded within the MP3 files.
I decided to challenge myself and display the playtime for a mp3 file encoded with a variable bitrate (VBR). It was tough just trying to detect that the file had a VBR. It’s actually simple now that I figured it out. You need to detect if an Xing header is present in the audio data a few bytes after the MPEG header. The tricky part is figuring out what position those bytes are at in the file.
After figuring out if the file has a VBR, I noticed that the estimated playtime is out of wack. It’s based on a constant bitrate (CBR). So now I’m plagued with a problem.
I assume that I could go ahead and read through the individual audio frames and tally the bitrates from each one and divide it by the number of frames found. The thing is, there are thousands of frames. Perhaps I can find a way to read the first few hundred and give an estimate based on the number of bytes remaining.
Bah. No time for it today.