jerky MPEG clips

[Maukka]

Question concerning MSP7.3:

A couple of clips which have until now rendered ok have become jerky in MPEG files created by MSP. Above the timeline (General Timeline view)there is a cointinuous green line. However, there is abreak on this green line just above these two clips. What does the green line stand for?

Any ideas what might be causing this?

The speed of the clips is their original 100 per cent, and they do not contain fast movements. And like I said they used to render ok until now...

MPEG settings are:
24 Bits, 720 x 576, 25 fps
Lower Field First
(DVD-PAL), 4:3
Video data rate: Variable (Max. 8000 kbps)
LPCM Audio, 48 KHz, Stereo

Thanks,

Maukka

[Maukka]

I just did some testing and discovered that if I use one pass encoding the clips render ok, if I use two pass encoding, they become jerky. They are normal DV-1 avi files. The strange thing is that they have rendered ok for so many times until now. They are on a 41 minute timeline, but the tests I made were just 10 seconds long.

Why would 2 pass encoding cause this on clips which earlier rendered ok?

Maukka

[Devil]

Reduce your settings to 6000 kbit/s CBR and AC3 (Dolby) stereo 2.0 at 192 kbit/s. You are probably simply sending too much data to whatever device you are using for playback. Many players cannot cope with much data from DVD.R disks.

The red and green lines indicate where the timeline is in the project settings format and do not require rendering. I can't remember for sure as far back as 7.3, but I think green is for video and red for audio (but it may be the other way round).

[Maukka]

Thanks for the quick reply, Devil.

I have rendered and watched this project many many times, and watched it on computer as well as on 5 different DVD players. Never any problems until now.

The clips which now have the problem were recently edited. One I slowed down by changing its speed setting in MSP and the other one was reversed. Again I have done similar things to other clips on the same timeline before, and never had problems. And the slowed down clip is now back to 100 per cent speed, but it still did not solve the problem.

I am aware that some DVD players might have problems with 8000 kbit/s, but like I said, it certainbly has not been a problem with the devices I use.

I think that for some reason the two pass encoding does not handle these clips correctly. The project is quite complex and I recently added several complicated transitions. Could it be that the two pass encoding somehow dedicates so much data to these complex transitions that it then tries to keep the data rate low elsewhere - and hence produces a jerky image on some more simple bits?

Anyway, in terms of space available on the DVD I am ok with using one pass encoding. But will I lose anything in terms of picture quality if I do so?

Thanks again,
Maukka

[Devil]

You have discovered something that I've been saying for a long time: variable bitrates speed up the transitions and not so much the fast motion.

It stands to reason: MPEG mathematically deduces the differences of each frame, compared with the key I frames. Imagine a complex transition: what moves faster than that? and you probably have a couple of I frames in it and a dozen or so interpolated B frames between them. The difference per frame is probably much more than if you were shooting Schumi in his Ferrari. So that's where your bitrate in VBR is going to shoot through the ceiling, for something which, if you have done your job correctly, the viewer should not even consciously notice.

On single pass, it will adjust the bitrates cautiously. On 2-pass, it knows where the fastest cghanges take place and push the bitrate of those up to the limit (actually often passing the limit, as max, min and average rates are never closely adhered to). In addition to these high video bitrates, you have chosen the worst possible audio bitrates, with LPCM, so you are really pushing things to the limit.

And don't forget that there are limits to burning bitrates, into the bargain. It is always recommended to burn at 1/2 max speed or lower (I always burn at 4x, even if the blank is rated at 12 or 16x. I wouldn't mind betting that a DVD+/-R burnt at 4 x with the settings I gave in my previous post and with the quality set at 90 will work fine and with a quality as good as your dragster settings.

[GeorgeW]

Anyway, in terms of space available on the DVD I am ok with using one pass encoding. But will I lose anything in terms of picture quality if I do so?

Just to note -- using 2-pass vs. 1-pass won't save you much in terms of space.

2-pass can improve quality when you are forced to use lower bitrates (like in the 4mbps neighbborhood).

How long are your transitions, and how long are your GOP's?

Regards,
George

[Maukka]

Thanks again Devil and GeorgeW,

Sorry if this discussion spread over to two separate threads...

I tried rendering a ten second preview bit from the timeline; this bit contains the clips in question and not much else. So no complicated transitions rendered this time. The result was the same: jerky after 2 pass encoding, and ok after one pass.

And this happens even after I returned the clip to its original speed. I am quite puzzled.
This is a project which used to be 52 minutes long, and which I have re-edited to 41 min. I rendered it many times with 2 pass encoding without problems when it was still 52min. Back then I used exactly the same settings as now. And I also had several reversed clips and clips with changed speed already then. So why this problem now?

I will try to render the entire project with lower data rate and Dolby Digital sound. Just so time consuming to try to figure out these things...

In the worst case I will stick to one pass encoding which sems to work for me. The whole rendered video is less than 3 gigabytes, so I guess the advantage of 2 pass encoding would be limited anyway. Right?

Thanks,
Maukka

[Maukka]

Devil,

I gotta grant it to you, it seems to have worked. I reduced data rate to 6500 and quality to 90 per cent, and set sound to Dolby Digital and then rendered with VBR two pass encoding. Et voilà! No jerky video!

There are a few artifacts though. But nothing too dramatic (much less visible than the jerky video). They have actually always been there, but perhaps not quite as visible as now. I get this "lightning" shaped artifact that quickly appears qnd disappears in two places, both of which involve panning movement in the picture. I think this is more visible on computer screen, possibly because the video is interlaced (lower field first). I have not yet had the time to see this latest result on TV screen.

Any further hints as to how to get rid of such artifacts?

Somebody recommended to me Sorenson Squeeze. It has been praised alot in the press. I see that they have a trial version available... If I made an avi file of my 41 minute prject than then tried making the MPEG-2 file using Sorenson, would I have a chance to get rid of the artifacts? Does anyone have any experience to share?

Thanks again for all the help to sort out the jerky video problem!

Regards,