I have a set of bare MP3 files. Bare as in I removed all tags (no ID3, no Xing, no Info) from those files.
Just before sending one of these files to the client, I want to add an Info tag. All of my files are CBR so we will use an Info tag (no Xing).
Right now I get the first 4 bytes of the existing MP3 to get the Version (MPEG-1, Layer III), Bitrate, Frequency, Stereo Mode, etc. and thus determine the size of one frame. I create the tag that way, reusing these 4 bytes for the Info tag and determining the size of the frame.
For those wondering, these 4 bytes may look like this:
FF FB 78 04
To me it felt like you are expected to use the exact same first 4 bytes in the Info tag as found in the other audio frames of the MP3, but when using ffmpeg, they stick an Info tag with a hard coded header (wrong bitrate, wrong frequency, etc.)
My question is: Is ffmpeg really doing it right? (LAME doesn't do that) Could I do the same, skipping the load of the first 4 bytes and still have the greater majority of the players out there play my files as expected?
Note: since I read these 4 bytes over the network, it would definitely save a lot of time and some bandwidth to not have to load these 4 bytes on a HEAD request. Resources I could use for the GET requests instead...
The reason for the difference is that with certain configurations, the size of a frame is less than 192 bytes. In that case, the full Info/Xing tag will not fit (and from what I can see, the four optional fields are always included, so an Info/Xing tag is always full even if not required to be).
So, for example, if you have a single channel with 44.1kHz data at 32kbps, the MP3 frame is 117 or 118 bytes. This is less than what is necessary to save the Info/Xing tag.
What LAME does in that situation is forfeit the Info/Xing tag. It's not going to be seen anywhere in the file.
On the other hand, what FFMPEG does is create a frame with a higher bitrate. So instead of 32kbps, it will try with 48kbps and then 64kbps. Once it finds a configuration which offers a frame large enough to support the Info/Xing tag, it stops. (I have not looked at the code, so how FFMPEG really finds a large enough frame, I do not know, but on my end I just incremented the bitrate index field by one until frame size >= 192 and it works).
You can replicate the feat by first creating (or converting) a WAVE file at 44.1kHz using a 32kbps bitrate then try to convert it to MP3 using ffmpeg and see that the Info/Xing tag has a different bitrate.