Quality loss when rendering mini-DVD

[Johan Lundqvist]

By the way, I just noticed that the only thing I really had to change in my previous settingss to get the “No Rec…”-sign constant (on parts with no cuts) during rendering was to change the maximum bps to 9 100 000.

[Johan Lundqvist]

I have found one way to improve the picture at the cuts – setting the bitrate to constant 9 100 000. When you do this, the picture in itself looks ok (and the “No Rec…”-sign i shown during rendering. But one problem remains – and this problem was there with the variable bitrate as well: There are som “time-shifts” at the cuts. The picture is not “flowing” and it looks like there are several cuts sometimes. This sensation is best seen when the cut is made at a place where there is motion in the picture (so it could be hard to detect it in the 4 sec clip).

[John Rofrano]

What you are seeing is caused by the way MPEG-2 is encoded. Remember the GOP I talked about? That stands for Group Of Pictures. In your case, a group of 12 pictures where only 1 contains all of the information to build a complete frame. This is called an I-Frame. Then there are 2 B-Frames which are deltas from the I-Frame and one P-Frame which is a predictive frame following the format IBBPBBPBBPBB then it repeats with another I-Frame and so on. When you cut at any location except an I-Frame, the data change is so dramatic that the delta and predictive frames fall apart until the next I-Frame is encountered. Unfortunately, there is no way in Vegas to know where the I-Frames are.

There is a tool called Womble (I used it before to determine the data-rate of your video) that does a great job and cuts-only MPEG editing. You might want to try the trial and see if it gives you better results.

Of course… buying a camera that shoots in a format made for editing is the ultimate solution. I know I keep repeating that but you are doing a lot of workarounds and limiting your potential because of the format you are using.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Johan Lundqvist]

Before I start with Womble – don’t you think that there’s any setting(s) in Vegas that would make the cuts look ok? The picture looks so fine now in between the cuts, and it feels like I´m very close to what I want to achieve.

I mean – the cuts were never a problem before when the overall quality was weakened. Now the quality is great, but the cuts are horrible. Right now, I would have to go with my previos settings if I wanted to make a DVD. The bad cuts are much worse than the quality issue I had before.

Well, I might start looking for a new camera. But the thing is I have 2,5 years of footage stored on mini-DVD:s that I’ve edited. Even if I buy a new camera, I still want to make the best of the content on my mini-DVD:s.

[Johan Lundqvist]

I´ve been looking at the Womble-program. Perhaps it´s ok, but it doesn´t seem very user-friendly.

I still want to solve this thing in Vegas. It seems absurd that now – when I finally know the right bitrate – that same bitrate should totally mess up parts that were ok when the bitrate-setting was wrong! Any suggestions?

[John Rofrano]

> It seems absurd that now – when I finally know the right bitrate – that same bitrate should totally mess up parts that were ok when the bitrate-setting was wrong! Any suggestions?

Perhaps you didn’t understand my explanation. When the bitrate was wrong, the file was being re-encoded so that the delta and predictive frames were similar to the I-frames (because they ALL got rebuilt). When you splice a long GOP format together, and you don’t cut at an I-frame, you throw too much data at the encoder than it can represent as just a delta from the previous frame so it breaks apart for a second. I don’t know how else to explain it.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Johan Lundqvist]

Ok, I understand. But I still find it strange that Vegas can’t compensate this in any way – or that Vegas can’t find I-frames automatically if I’d like the program to. The way I understand it, it’s all about luck if I cut the movie in Vegas and render it in the right bitrate. On every cut I have less than 10 % chance of cutting at a right spot. If I don’t cut at a right spot, there’s a big chance that the cut will look very strange if rendered in the right bitrate. This means that Vegas forces me to use a bitrate I know is wrong, in order to recompress the movie from the beginning.

The “No recompression”-thing seems totally pointless – even for people with other cameras. If the cutting is made in Vegas that is. Would you agree?

[John Rofrano]

> The “No recompression”-thing seems totally pointless – even for people with other cameras. If the cutting is made in Vegas that is. Would you agree?

“No Recompression” isn’t only a feature of MPEG-2. It also works for DV AVI’s. Since DV AVI has a full frame every frame it is not affected by this and works extremely well. So it’s not totally pointless, it’s just that editing DVD MPEG-2 doesn’t lend itself as well to this concept.

You might try changing your cuts to crossfades. This will allow the encoder to gradually change from one scene to the next and it should look similar to what you had before.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Johan Lundqvist]

I’ve tried some crossfading but I’m not too happy about it. Well, I think I give up now on the “No-Compression…”-thing. Instead, I’ll try to look for the optimal “wrong” bitrate when rendering in Vegas.

I´m thinking about setting the bitrate to constant on 9 200 000. Or perhaps variable with a maximum of 9 200 000, average of 6 000 000, and a minimum of 4 000 000. The default minimum, 192 000, sounds very low. Do you have any ballpark suggestion?

One last question: Right now, I’m downloading 26 GB of film, split onto two files .mov-files, filmed with a normal DV-camera (a friend of mine filmed my wedding with his camera and I’ll edit myself in Vegas). Is it possible to cut .mov-files in Vegas and use the right bitrate during rendering without getting the problem I just had?

Well I guess these were my last questions to you for now. You’ve been extremely helpful and I have learned a lot! Two weeks ago, I had never edited movies at all, so forgive me for my noobie questions… Thank you very much, John!

[Douglas Spotted eagle]

Bit rate is determined by resolution x length. You get to choose quality based on length, because resolution is essentially determined for you. There is no “right” bitrate. Ultimate S gives you a bitrate calculator to help figure out the “opimized” bitrate for the length of your project. Anything lower than about 4.5 is challenged, anything higher than 8.5 runs a risk of not being playable on some systems.

What codec is contained in your .mov file? DV? MJPEG? MPEG? something else?

Douglas Spotted Eagle
VASST

Certified Sony Vegas Trainer
Aerial Camera/Instructor