Why can I convert from MP3 to M4A or M4B at a fixed bit rate?
The M4A and M4B file is encoded with MPEG4AAC codec. The codec doesn’t fix bit rate like MP3. The real bit rate used in target M4A and M4B depends on input sound. If MPEG4AAC encoder thinks it’s possible to get the best quality by a lower bit rate than number user set, it use the lower bit rate to save space. What you care about is if sound quality loses. If the encoder think no needs to use 192 kbps, it just choose a best bit rate, which will get same quality and use less space. The input format doesn’t affect the bit rate; only real input sound affects. The bit rate probably aren’t same with different files.
