Seeking in AAC files - Clip+

[benshepherd]

I just bought and immediately Rockboxed my Sansa Clip+. I'm really really impressed with it - the detail and responsiveness of the interface, the playback, the extras, the customisability - I could go on. I'm just astonished at the incredible work that's gone into this project, so thanks very much.

I mostly use my player to listen to BBC radio programmes downloaded using get_iplayer. I've been transcoding them to MP3 since my old player wouldn't play AACs. Rockbox does play the AAC files, but the seeking doesn't seem very accurate. Specifically, if I seek anywhere in the first minute or so, it restarts from the beginning (although the reported time is still where I seek to). If I seek between 1:00 and 2:00, it goes to approximately 1:10 every time, and so on. I've put a file for testing purposes here.

Is this a known bug, or is there a patch for it or something? I've searched in the forums and the documentation, but there doesn't seem to be anything obvious, other than the codecs page, which suggests that support for HE-AAC (which I believe this is, although I'm a bit confused about AAC in general) doesn't include 'realtime', but I'm not sure what that means.

Obviously this isn't a major problem, but it would be nice to not have to transcode to MP3 if I didn't have to. Please bear with me, I'm a newcomer to Rockbox.

Thanks very much!

[Julian67]

If you remux the get_iplayer m4a files they will play better and seek OK, even on low powered devices.  If you're on OS X or a Linux system you can do:

Code: [Select]

MP4Box -ipod INFILE -out OUTFILE

and that will produce an ipod compatible file that your Clip+ will (almost always) play without a problem.  Metadata is retained.  I used to use MP4Box -hint but found some files still didn't seek right or could even hang the player.  Using MP4Box -ipod gives files that even play and seek on my ancient iRiver players (they max out the processor though) so you should have no problems on a Clip+.  Be aware that HE-AAC get_iplayer files are usually play back in Rockbox at half speed, see  http://www.rockbox.org/tracker/task/12856 but the usual LC-AAC get_iplayer stuff works great when remuxed.

MP4Box is part of gpac which you may already have installed if you use get_iplayer.  It is hosted at http://sourceforge.net/projects/gpac/ and is usually available through OS package managers.  There is also a windows build available at the sourceforge page.

[benshepherd]

Terrific - that seems to work, at least on the Clip+ Windows simulator. Thanks very much! What exactly is the -ipod switch doing?

Will the battery life be reduced if I'm playing AAC files rather than MP3s?

I'm still not clear on the difference between HE- and LC-AAC.

[saratoga]

Battery life should be pretty similar with aac.

Lc is a normal aac file. He is a low bitrate optimized file that is much slower to decode. They are somewhat less common for music.

[Julian67]

...What exactly is the -ipod switch doing?
...

The -ipod switch changes the location of the moov atom (a description/index of the file and media content).  It can be in a different place of the file depending on the intended usage i.e. streaming vs.playback from disk.  I don't know much more than that but you can get plenty of info at http://atomicparsley.sourceforge.net/mpeg-4files.html

I *think* that when you first create an m4a/mp4 file with ffmpeg the moov atom is always at the finish (because content and size are not predictable but space must be preallocated) so if you need it at the beginning you have to remux it.  There doesn't seem to be a convenient way around this in get_iplayer, at least according to `get_iplayer --long-help` but I know that often useful features are present in git that aren't in stable releases or docs so it's probably worth getting onto the get_iplayer mailing list and asking.

[benshepherd]

OK - thanks for that. If I find out that I can do the remuxing in get_iplayer natively, that would be preferable; I'll post it here too.

[benshepherd]

Got it - the option to add to get_iplayer is

Code: [Select]

--ffmpeg-radio-opts="-movflags rtphint"

M4A files from get_iplayer now seek to the correct place each time.

Thanks to dinkypumpkin on the get_iplayer mailing lists for this.

[Julian67]

Thanks for that.  It doesn't work with older versions of get_iplayer such as those found in distributions' stable or long term support version repositories but it works fine with git master get_iplayer and recent ffmpeg.