Major audio weirdness in Mac OS X

Posted by Pierre Igot in: iTunes, Music
October 13th, 2003 • 5:51 pm

I’ve just experience a baffling series of problems when trying to edit audio tracks before burning them to disk.

The tracks are a concert recorded live, so there shouldn’t be any gaps/pauses between tracks. In both iTunes and Toast, I’ve set the pause to “0 seconds”, thereby indicating that I want to record the disk using the “Disk-At-Once” approach.

Yet when playing the tracks back in both iTunes (4) and Toast (6), there is a tiny pause between each track.

The tracks are MP3 files. So I open them in Amadeus II and zoom in on the beginnings and ends. I can see tiny pauses (flat sound wave) at the beginning and end of each track. So I carefully edit them out in Amadeus II, and save the tracks as MP3 files again.

I recreate a new playlist in iTunes and a new audio CD in Toast, and drag and drop the tracks. I make sure the pause between tracks is set to zero seconds. I try to play the transitions between tracks. The pauses are still there (in both iTunes and Toast).

I reopen the MP3 tracks in Amadeus II and… the tiny pauses are back! I edit them out again, close the file, open the file again, and the pauses are back! I don’t understand.

So I choose to save as AIFF instead of MP3. AIFF is an uncompressed format and Amadeus II is much faster saving and opening AIFF files anyway. I edit the tiny pauses out, save the files as AIFF, close them, reopen them, and the pauses are still gone. Good.

I go back to iTunes and recreate a new playlist with the AIFF files this time. I try to play back the transitions and… the pauses are still there! What the hell? I try in Toast, same thing! There is definitely a tiny pause between each track, even though in both programs the pause is set to 0 seconds.

Doubting myself, I reopen the AIFF files in Amadeus II and zoom in on the beginnings and ends. No pause. Why on earth do both iTunes and Toast play a pause when there isn’t one?

Out of desperation, I fiddle around with the settings in iTunes. I uncheck “Sound Check” (which brings all tracks to the same volume level, shouldn’t have any impact on pauses) and “Sound Enhancer” (shouldn’t have any impact either). The pauses are still there.

Then check this: I check the “Crossfade” option (which was unchecked) and set the length of the crossfade to “0”. I go back to my playlist and try playing the transitions. And it works! No pauses between the tracks!

In a weird sense, it sort of makes sense, of course. (A crossfade whose duration is 0 equals no crossfade, hence no pause between the tracks.) But why does it only work right if the “Crossfade” option is checked? And why on earth does Toast 6 itself also have pauses between the tracks even though the pauses are set to 0 seconds?

I must admit I’m completely miffed. For now, I’m going to record my audio CD with “Crossfade” on and set to 0 seconds, but this is not normal!

4 Responses to “Major audio weirdness in Mac OS X”

  1. Pierre Igot says:

    Thanks for the clarification. As you said, this doesn’t really explain why both iTunes and Toast are still playing things incorrectly after I’ve converted to AIFF.

    The author of Amadeus II also did confirm that this was an inherent problem with MP3 files.

    iTunes CAN be used to convert from MP3 to AIFF. You just need to change the “Importing” settings in the application’s prefs and then use the “Convert to AIFF” command. Not very convenient, though.

  2. Will says:

    Oh, yeah!

    What I meant to say was that iTunes _automatically_ converted to AIFF before burning to prevent the gaps.

    Although the more I think it about it, the less certain I am.

    What I find irritating about the whole situation is that Minidiscs can handle the whole thing perfectly, and the encoding they use (Sony’s ATRAC) is not that dissimilar to MP3 (AFAIK).

  3. Pierre Igot says:

    Well, iTunes *must* convert the MP3 files into a non-compressed format before burning them as an audio CD — so the process takes place somewhere, at some point, even if we don’t see it. Anyway, if the gaps are an intrinsic limitation of the MP3 format, then there’s nothing iTunes can do to prevent them. Converting to AIFF doesn’t remove them. You still have to edit them out manually, and I doubt iTunes was ever able to edit them out automatically.

    I guess we just can’t blame Apple for the limitations of the MP3 format. I’d have to check with VBR MP3s, though. (However, converting a fixed bit rate MP3 to AIFF and then to VBR MP3 might not be a very good idea quality-wise.)

  4. Will says:

    The reason that MP3s do this is slightly technical (which I will try to explain), but I don’t know why iTunes is playing your AIFFs incorrectly.

    Very basically, as I understand it, the reason that MP3s have those annoying small gaps is because it compresses frequency domain information. Frequency domain information is gained by chopping the time signal into small chunks (the size of which is one of the factors in the size of the resultant MP3). If the song isn’t made up of an exact integer number of these chunks, the encoder pads the end of the track with zeros in order to make the length of the song an integer multiple of the chuck length.

    Why then the encoder can’t add the number of padded zeros as a tag in the track to tell the audio player not to play those added zeros, I don’t know.

    Variable bit rate MP3s, I have heard, do not have this problem, but I haven’t had any experience with them. Until iTunes and my iPod can remove the gaps, I’ll just have to live with it.

    The moral of the story is to convert to AIFF before burning a CD, as you have found. iTunes used to do this, if I recall correctly, but no longer does. Which is irritating.

    Hope I made sense!


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