How is bitrate calculated in mp3?

Question

Programming

I read an article on Habré about the structure of mp3 files. I opened the music in a hex editor to try the whole thing in practice.
It turned out that my file was compressed in MPEG-1 Layer II format. Here's the mp3 frame header just in case: 11111111111110111101001000000100
bits [16-19] is the bitrate index. If you look at the table, it turns out that almost all frames in the file have a bitrate of 384 Kbps. However, if you access the mp3 properties through Windows Media Player, the bitrate is 256 kbps. How so? Tell me, maybe I'm doing something wrong?

Answer 1

It's simple, you have Layer 3 and not 2, so 256 kbps

[0-10]	1111 1111 111
[11-12]	11 - MPEG 1
[13-14]	01 - Layer 3
[15]		1
[16-19]  	1101 - 256 kbit/s
[20-21]	00 - 44100Hz
[22]		1
[23]		0
[24-25]	00 - stereo
[26-27]	00
[28]		0
[29]		1
[30-31]	00