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MB COMAND: large AAC file playback problem, large AAC files cannot be played
Pete7874
post Sep 17 2012, 18:04
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The wife's car is a 2008 Mercedes C300 with the COMAND multimedia/navigation system. According to the owner's manual, it'll play MP3 and AAC/M4A files.

However, when trying to play AAC files, I noticed that larger files will not play. For example, a 10 MB AAC file will play, but a 25 MB file will not. Such limitation does not exist for MP3 files.

I've got 1-2 hour long mixes that I often convert to AAC for portable players since even down at about 128 kbps AAC files sound very good but take up significantly less space. Alas, the COMAND will not read these.

When I contacted MB about it, they stated that the reason why these larger AAC files will not play is because of a small internal cache. While this does sound like a believable explanation, I wonder why this "small internal cache" is still big enough to play very large MP3 files, but not big enough to play large AAC files? I'm guessing it has to do with the way the COMAND reads AAC files... maybe it tries to load the entire file into cache before playing it as opposed to loading it a few frames at a time like may be the case with MP3 files? Even the cheapest smartphones out there will properly play large AAC files these days, so this is a bit weird.

FYI, I am using the Nero AAC encoder, but I also tried FhG with the same result.

Anyway, this is just an FYI more than anything else, but if someone can provide some insight into how an AAC file is typically read/processed by a player that might explain the above issue, I'd be thankful.

This post has been edited by Pete7874: Sep 17 2012, 18:05
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saratoga
post Sep 18 2012, 03:32
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QUOTE (Pete7874 @ Sep 17 2012, 13:04) *
When I contacted MB about it, they stated that the reason why these larger AAC files will not play is because of a small internal cache. While this does sound like a believable explanation, I wonder why this "small internal cache" is still big enough to play very large MP3 files, but not big enough to play large AAC files? I'm guessing it has to do with the way the COMAND reads AAC files... maybe it tries to load the entire file into cache before playing it as opposed to loading it a few frames at a time like may be the case with MP3 files?


MP4 seektables are complicated and can be very large for long files. If a device has limited memory it may not be able to play long AAC files.

MP3 does not use the same type of seek tables, so there is no limit on how long MP3 files can be.
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Pete7874
post Sep 18 2012, 04:16
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QUOTE (saratoga @ Sep 17 2012, 20:32) *
MP4 seektables are complicated and can be very large for long files. If a device has limited memory it may not be able to play long AAC files.

Do MP4 files with higher bit rates have larger seek tables?

The reason I ask is that if I start with the same 27-minute long source file and then create two MP4 files out of it:
1) 48 kbps, about 10 MB in size
2) 128 kbps, about 25 MB in size

Then COMAND will play file (1), but not file (2).

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saratoga
post Sep 18 2012, 05:13
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QUOTE (Pete7874 @ Sep 17 2012, 23:16) *
QUOTE (saratoga @ Sep 17 2012, 20:32) *
MP4 seektables are complicated and can be very large for long files. If a device has limited memory it may not be able to play long AAC files.

Do MP4 files with higher bit rates have larger seek tables?

The reason I ask is that if I start with the same 27-minute long source file and then create two MP4 files out of it:
1) 48 kbps, about 10 MB in size
2) 128 kbps, about 25 MB in size

Then COMAND will play file (1), but not file (2).


Not sure how its handled in practice. I suppose you'd have to dig into the spec to see how the tables are likely to allocated.
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