Skip to main content

Notice

Please note that most of the software linked on this forum is likely to be safe to use. If you are unsure, feel free to ask in the relevant topics, or send a private message to an administrator or moderator. To help curb the problems of false positives, or in the event that you do find actual malware, you can contribute through the article linked here.
Topic: FAAD2 decoding aac to 32-bit float, can someone explain the bit-magic? (Read 3531 times) previous topic - next topic
0 Members and 1 Guest are viewing this topic.

FAAD2 decoding aac to 32-bit float, can someone explain the bit-magic?

I recently wrote a little AAC decoder using libfaad2. Just to test my work, I compared my decoded files (decoded to 32 bit float RAW) with the output generated by the faad frontend and found some unexpected differences. By comparing the output in a hex editor, I found the faad frontend will actually turn all floats with value -0.0 into 0.0. (It is done here: https://github.com/gypified/libfaad/blob/master/frontend/audio.c#L464, in lines 469-474, where -0.0 == 0.0 is true).

Can someone explain to me why this is done? I've tried finding something about it in WAV specifications, but as far as I can tell, it requires IEEE floats (which allow -0.0). Do media players or sound servers not like -0.0 floats?

Also, can someone explain the bit magic done to the floats in the lines that follow (473-492)? I can read code, but I can't figure out what (mathematically) those bit-hacks do to the floats. And, if it's not obvious from the answer, why is this done? When I turn all -0.0 floats in my own output into 0.0, the files my program produces (without the extra bit-hacks) are identical to the faad frontend output, so it seems (at least on the files I tested) that the other stuff has no effect.

Thanks!

Re: FAAD2 decoding aac to 32-bit float, can someone explain the bit-magic?

Reply #1
Can someone explain to me why this is done? I've tried finding something about it in WAV specifications, but as far as I can tell, it requires IEEE floats (which allow -0.0). Do media players or sound servers not like -0.0 floats?

Also, can someone explain the bit magic done to the floats in the lines that follow (473-492)? I can read code, but I can't figure out what (mathematically) those bit-hacks do to the floats. And, if it's not obvious from the answer, why is this done?
As far as I can tell, it's just a way to get the floats written with a known endianness (byte order). The change from -0.0 to 0.0 is just a side effect.

 

Re: FAAD2 decoding aac to 32-bit float, can someone explain the bit-magic?

Reply #2
Of course! That makes sense, I should have thought of that. Thanks for the quick reply!

 
SimplePortal 1.0.0 RC1 © 2008-2021