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Topic: why do m4a files change bit rate when I edit tags? (Read 12720 times) previous topic - next topic
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why do m4a files change bit rate when I edit tags?

I have numerous m4a files bought from Itunes and When I edit the date in the ID tag which apple has as 2009:29837272287 And I change it to just 2009

it will change my kbps from 256 to sometimes 258 or sometimes 252, why is that?



P.s.  Not sure if m4a is considered AAC so I apologize if this is in wrong area.

why do m4a files change bit rate when I edit tags?

Reply #1 –
You're in the right area.  Songs from iTMS store that are encoded with the iTunes Plus setting are actually encoded with a variable bit rate setting.  Play a song with foobar and you'll see they are not 256kbps constant.

why do m4a files change bit rate when I edit tags?

Reply #2 –
Quote from: antman on
You're in the right area.  Songs from iTMS store that are encoded with the iTunes Plus setting are actually encoded with a variable bit rate setting.  Play a song with foobar and you'll see they are not 256kbps constant.




Well winamp displays a song to be 256 kbps the whole time  but if  i edit the date tag it will change the kbps which will stay constant the whole time

why do m4a files change bit rate when I edit tags?

Reply #3 –
Download and play it in foobar.  The bitrate that is displayed in Winamp's main window will just round the bitrate to either 128, 192, 224, 256, 320, etc.  So 252 & 258 will still be 256.

why do m4a files change bit rate when I edit tags?

Reply #4 –
Quote from: antman on
Download and play it in foobar.  The bitrate that is displayed in Winamp's main window will just round the bitrate to either 128, 192, 224, 256, 320, etc.  So 252 & 258 will still be 256.

but why is changing the date changing that number, is a frame being removed or added or something?

why do m4a files change bit rate when I edit tags?

Reply #5 –
Quote from: airtas on
Quote from: antman on
Download and play it in foobar.  The bitrate that is displayed in Winamp's main window will just round the bitrate to either 128, 192, 224, 256, 320, etc.  So 252 & 258 will still be 256.

but why is changing the date changing that number, is a frame being removed or added or something?



try changing the year guys from an itunes store purchase

why do m4a files change bit rate when I edit tags?

Reply #6 –
bump

why do m4a files change bit rate when I edit tags?

Reply #7 –
Quote from: airtas on
I have numerous m4a files bought from Itunes and When I edit the date in the ID tag which apple has as 2009:29837272287 And I change it to just 2009

it will change my kbps from 256 to sometimes 258 or sometimes 252, why is that?



P.s.  Not sure if m4a is considered AAC so I apologize if this is in wrong area.


Have you compared the file size before and after the change? Change of m4a tags is a tricky business and it might involve the change of the file size (in order to do the change faster, there might be some empty "holes" left in the file). And if bitrate is calculated using file size, this might explain it. Some tools offer to optimize m4a/mp4 files - this will remove those unused spaces in the file.

why do m4a files change bit rate when I edit tags?

Reply #8 –
Quote from: Xire on
Quote from: airtas on
I have numerous m4a files bought from Itunes and When I edit the date in the ID tag which apple has as 2009:29837272287 And I change it to just 2009

it will change my kbps from 256 to sometimes 258 or sometimes 252, why is that?



P.s.  Not sure if m4a is considered AAC so I apologize if this is in wrong area.


Have you compared the file size before and after the change? Change of m4a tags is a tricky business and it might involve the change of the file size (in order to do the change faster, there might be some empty "holes" left in the file). And if bitrate is calculated using file size, this might explain it. Some tools offer to optimize m4a/mp4 files - this will remove those unused spaces in the file.



I will take  a look at that!  is it safe to edit m4a meta data then?



and for everyone posting about vbr I know the file is vbr I am talking about the summary bitrate that is always 256

normally I do not buy from itunes as I prefer mp3 but some songs only itunes has and when I was labeling my collection I had notcied the album year had extra characters thinking it was a mistake

I went ahead and just made it 2009 but when I made it 2009 the vbr display also changed

Now I know it is vbr, but still why the change all itunes songs show up as 256 in the summary filed until that date change

Why would meta data affect that?  is it an m4a thing?  this doesnt happen with mp3

and why have all those extra characters to begin with?

why do m4a files change bit rate when I edit tags?

Reply #9 –
Quote from: airtas on
and why have all those extra characters to begin with?


I think you shouldn't worry too much about this  Bitrate in this case is only informational (no audio quality is affected by this change).

why do m4a files change bit rate when I edit tags?

Reply #11 –
I can give you an answer:
iTunes saves that 256 bitrate display in a tag field (Encoding Params). When you change the tags with some other software, it might corrupt that field (since it has an unusual structure).
So when that tag is corrupted/deleted iTunes shows you the real(?) average bitrate because it can't read it from that tag.

why do m4a files change bit rate when I edit tags?

Reply #12 –
Quote from: dano on
I can give you an answer:
iTunes saves that 256 bitrate display in a tag field (Encoding Params). When you change the tags with some other software, it might corrupt that field (since it has an unusual structure).
So when that tag is corrupted/deleted iTunes shows you the real(?) average bitrate because it can't read it from that tag.



gotcha, so whats the extra data for? why not just have it say 2010?

why do m4a files change bit rate when I edit tags?

Reply #13 –
You mean in the date tag?
I don't know, I've only seen this kind of structure in it:
2009-12-08T08:00:00Z

why do m4a files change bit rate when I edit tags?

Reply #14 –
The bitrate changes because 256kbps isn't the real bitrate.  There is a place in the m4a file for the nominal average bitrate, but it's only used for informational/display purposes.  The iTunes encoder is likely writing "256" here even though the exact bitrate is different.  When modifying the file, Winamp calculates the true bitrate and replaces it.

why do m4a files change bit rate when I edit tags?

Reply #15 –
Quote from: airtas on
I will take  a look at that!  is it safe to edit m4a meta data then?


Don't worry. I always kick out all the rubbish tags which iTunes files incorporate. No problem at all.