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TomAckroyd

95 posts

Master Geek


#21340 23-Apr-2008 14:48
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[This was a Geekzone blog entry that didn't get much response, so I'm re-posting here. Might also have some relevance to this HD/.ac3 thread: http://www.geekzone.co.nz/forums.asp?ForumId=83&topicid=21288 ]

For years now on analogue and cable free-to-air TV I've been hearing a dodgy audio artifact on broadcast US TV shows - the likes of Six Feet Under or The Sopranos. It's most noticeable on music featuring sustained solo notes on violins or flutes and is best described as a regular (1 or 2 Hz-ish) momentary jump, bump or gap in the audio. To my old-skool ears it sounds as if someone stopped a 1/4" takeup spool and then let go, bouncing the tape as it comes to rest.

I think I've figured out what it is, from watching Battlestar Galactica Season 3 on PAL DVD. There's an option to choose a 5.1 or stereo soundtrack on these discs. The stereo is sped-up from the original 24fps to 25fps, but pitch-corrected back down. The 5.1 is sped up *and* pitched up from 24 to 25 - just like speeding up an analogue tape.

The artifact I described is exactly what I hear on the stereo DVD soundtrack, and is unbearably bad, especially on the BSG theme tune - that plaintive solo wind instrument and female voice.

Obviously it's a rough-and-ready realtime effect of pitch-correction and speed-adjustment of the soundtrack. It drives me nuts BECAUSE IT'S SOOOOOO BAD AND NO-ONE COMPLAINS.

Well, until now, obviously.

Does anyone know what I mean and/or can shed light?

Cheers,
Tom


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digitalepsilon
41 posts

Geek


  #126207 24-Apr-2008 19:47
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You're probably right. PAL encodes of 24fps material run at an artificially faster framerate which raises the pitch of the audio and increases the tempo of the music, among other things. You can apply pitch correction to fix one aspect of it, but that won't fix the speedup.

In your case, it sounds like they applied pitch correction to the DVD DD2.0 track but not the DD5.1 track. Who knows why they might do that. I do know that 2.0 and 5.1 tracks are usually prepared and mixed separately so perhaps they came from different sources for the BSG DVD authors to use.

I remember reading a while back that pitch correction was applied to some of the LOTR DVDs (there's been so many releases of that I can't remember which exactly) much to the widespread disgust of those with a keen ear who could hear the artifacts in the music (particularly the long vocals). You would think that with the budget that movie would have had for DVD release that they would have carefully supervised the pitch correction during the authoring. Despite that the artifacts are still there.

I don't know how it's done in the TV broadcast world -- ie. whether it's done in realtime during transmission, prepared in advance, or not done at all.

Seems to me that there's no easy answer since uncorrected audio has a noticably higher pitch, but ptich 'corrected' audio has artifacts that can be quite severe if done with bad software or by someone who doesn't really know what they're doing. If I were to choose I'd probably go with uncorrected pitch since unless I've heard/seen the broadcast or movie in its original timing I'm probably not likely to even notice. If all we hear and see in NZ is PAL TV and buy only PAL DVDs, most of us probably never hear or see the shows at their 'correct' speed and have no frame of reference to see the difference.

From a purist point of view though, it's good to know most 24p blu-ray discs shouldn't have this problem and with the lossless audio tracks included on many releases, what you're getting is not far off the original master mixes used by the studios -- no speedup, no pitch correction, and no loss of fidelity because of compression.

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