Hi Nic,
The problem of the maximum time for working with an audio file is not
specific to iTunes. Basically none of the music programs, including
QuickTime, can correctly handle sound files where the number of
samples exceeds 2 billion (or, to be precise, the maximum number that
can be represented with a 32-bit unsigned integer, or 2 raised to the
exponent of 31, which is about 2.1478 billion samples). One of the
numbers in the file header for the audio file is a counter that turns
over when you exceed this maximum. This means that the actual maximum
file length (in time) that can be correctly read from these audio
files depends on the quality of the file encoding. CD quality music
files sample the music at 44.1 kHz (44.1 thousand samples per
second). Voice memo files might sample at 8 kHz (8 thousand samples
per second) -- a rate that is more than 5 times smaller. The total
number of samples is the encoding sample rate (e.g. 44.1 kHz for a CD)
multiplied by the time of your audio file in seconds. This number
hits the 2 billion maximum for a file length of 13.5 hours, assuming
this is stereo music. This is an absolute maximum that the file
structure can correctly represent -- you can still run into problems
before this. When music programs like QuickTime or any comparable
programs on any platform (Windows, Linux, Mac, etc.) read these files,
they all compute the time from the number of samples, and they all get
incorrect answers when the counter is exceeded. That's why you're
able to play the files with QuickLook, which just starts streaming
without trying to read the time. The exact wrong number depends on
the rollover value of the counter.
Moreover, if you think back to recent posts by James looking for the
intro and other music files that the Mac plays on startup, you'll
notice that the file extensions are .caf instead of .aiff (Audio
Interchange File Format). The new format is "Core Audio File Format",
and one of the reasons for the new file format is that these files can
correctly represent samples that exceed the 2 billion counter maximum.
All of this comes up in discussions of the maximum length you can make
a single audiobook file and play it correctly.
HTH. You can get longer files to play, and get correct times if you
reduce the audio quality.
Cheers,
Esther
Nicolai Svendsen wrote:
Hi guys,
I sometimes get really huge audio files, sometimes files that last
more than eight hours in length. The problem is this.
While iTunes can actually measure the time properly, it won't play
it all. I have a file which is nine hours long, but it will only
play two hours of it. The LCD just stops counting, even though it
shows that seven hours are left of the total time. If I use Quick
Look, I can go to 100 percent of the file, where iTunes will usually
cut it off. However, if I leave it to continue in Quick Look, what
will actually happen is that it will go beyond 100 percent because
the file is longer than it thinks, even though it actually measures
the time properly. I played a similar file earlier today, and it hit
405 percent before it finished, however if I stopped playback or
attempted to go backwards, it'd put me back at where it cut off.
If anyone can help, I'd appreciate it. And please, don't come with
useless comments like "Your iTunes is broken". I know for a fact
that it is not, because I just reinstalled out of interest. When I
figured out that wasn't the problem, I just reverted the changes.
Thanks in advance. :)
Regards,
Nic
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