Timeline quantization effect perhaps?

Gammaburst wrote on 6/24/2002, 7:12 AM
In VV3, if I create a new default NTSC DV project, enable Quantize To Frames, import a DV clip that has incorrect frame rate 29.970000 (the Adobe After Effects default), and then preview or render, I see that frame 4 becomes duplicated, and an ugly frame (with only one field) appears at the end of the clip. If I force the VV3 project frame rate to 29.970000, the problem disappears.

That almost makes sense - I figure it's a timeline quantization effect.
In general, what's the best way to deal with this situation?
How about adding a new VV3 preference, "tolerate slight frame misalignment"?

Comments

HPV wrote on 6/24/2002, 9:08 PM
You could turn off Quantize to Frames. Use of the Alt/Arrow keys will move your cursor at perfect single frame spacing on the timeline, then snap your media to the cursor. That might do it.
Craig H.
SonyDennis wrote on 6/25/2002, 12:01 PM
Gammaburst:

If applied in the timeline-to-media conversion, such a slop factor would only delay the inevitable frame repeat. The real solution would be to "correct" media that says it's 29.970000 to the actual correct NTSC rate. However, that bring up issues with the audio, it would have to be resampled, with possible degradation. So, I think the present solution works fine. If you prefer to simulate the first method, just "slip" the media a little to the left by 90% of a frame time, that will move the place where the frame gets dropped somewhere far into the timeline.

///d@
Chienworks wrote on 6/25/2002, 5:15 PM
I'm staring at the screen really really hard, and i still can't see a difference between 29.970000 and 29.970000. What am i missing here?
SonyDennis wrote on 6/25/2002, 8:38 PM
Chienworks:

That's because it's all in the part after the third decimal that you can't see.

NTSC video runs at a frame rate of 30,000 / 1,001, or 29.97002997002997... (repeating decimal). Some programs incorrectly use a frame rate of 29.97 (exactly) instead, for whatever reason. If such a clip is placed on the timeline, the two rates slip over time, and a frame gets repeated. The actual frame varies with clip placement and other factors, but apparently with Quantize to Frames on, it's often frame 4 according to Gammaburst. If you override QtF and slip the media or move the clip a little (like 30mS to the left), the duplicated frame gets moved out past the end of the media and everything is fine.

///d@
BillyBoy wrote on 6/25/2002, 11:45 PM
OK, OK... you say 29.97002997002997 for the frame rate. Someone's got to ask the dumb question so I will. Why, like how come the standard was set to such a figure? If its anything like that 755x480 DV pixel thing Dennis and other SoFo guys explained countless times, just say yes and I'll understand. Don't want to start another long thread on that kind of thing, gives me a headache.
SonyDennis wrote on 6/25/2002, 11:57 PM
It was done back when color television was invented. In order to make it compatible with existing black & white sets, the color (chroma) information was modulated on top of the luma (brightness) information. In order to not interfere with the audio subcarrier (in some way that I don't fully understand), either the audio or video frequencies had to be "tweaked" slightly, and they picked the video, which has caused endless headaches for anyone who's ever had to implement 29.97-related stuff, and anyone who's ever had to work with "drop frame" timecode (no frames are dropped, just frame labels, so that timecode stays in sync with wall clock time). The world would be a better place if they would have tweaked the audio instead, at least for us video guys <g>. If I ever meet the guy who did it, I'm going to have a word with him.
///d@
Gammaburst wrote on 6/26/2002, 12:05 AM
Thank you, I agree. The best solution is to fix the erroneous imported clips. One culprit is After Effects 5.5. If I have access to the entire AE project, I can override its default 29.970000 project settings and clip interpretations. However, if that's gone and all I have is the final rendered AVI ... well ... I wonder what would happen if I hacked the AVI header from 2997/100 to 30000/1001. ;-)

I too discovered that I can nudge the bogus 29.970000 clips very slightly to the left in the VV3 timeline to relocate the problem far away, exactly as you describe. The difference between 29.970000 and 4.5MHz/286/525 (thank you SF for getting this number right!) causes only one frame slip every 9 hours or so. Too bad the slip always occurs (for me) at frame 4.

I wonder if anyone else has noticed this stuff.
Gammaburst wrote on 6/26/2002, 12:48 AM
That weird number exactly equals 4.5 MHz / 286 / 525. The 4.5 MHz is the nominal audio subcarrier frequency of a TV broadcast transmitter. 525 is the number of lines per frame. 286 is a clever division factor that tends to hide beat-frequency interference caused by the audio subcarrier slightly leaking into the video.

For a similar reason, the color subcarrier frequency is 4.5MHz / 286 * 227.5 = 3579545.4545...

Yes, those numbers don't divide out pretty. Unfortunately, most people don't know the precise equations, so they use various rounded-off values, and that causes all sorts of subtle problems.

Most people round-off aspect ratios too. :-(