This is one of the most interesting post I have ever read on a mailing list. Thank you for this Esther. On Mar 12, 2010, at 4:14 PM, Esther wrote:
> 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 >> Skype: Kvalme >> MSN Messenger: [email protected] >> AIM: cincinster >> yahoo Messenger: cin368 >> Facebook Profile >> My Twitter >> >> > > > -- > You received this message because you are subscribed to the Google Groups > "MacVisionaries" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/macvisionaries?hl=en. -- You received this message because you are subscribed to the Google Groups "MacVisionaries" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/macvisionaries?hl=en.
