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alwayslearning
VERY Pixelated MPEG-2
on Jul 17, 2002 at 1:27:14 pm

Good morning.
Ok, as some of you read yesterday, I was finally successful at burning a DVD. I viewed the DVD on a Toshiba DVD player and the image and audio were identical to the original on the hard drive. I know that there are theoretical differences, but there were no artifacts that I could detect. This is good.

Then, last night, I tried another. After I exported from the Premiere timeline to convert the .avi to .mp2 and viewed it, it was extremely pixelated. A second try did the same thing.
What is "I frame", "b frame" and "p frame"? Should I be doing anything with them when necessary?

Hopefully someone's reply will help me formulate a decent question beyond "help?". :)

AlwaysLearning


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alwayslearning
Re: VERY Pixelated MPEG-2
on Jul 17, 2002 at 1:33:47 pm

Just in case this matters......

The first .avi file I converted to MPEG-2 was just that... .avi, as generated by Premiere. When I converted it, it made the .mp2 file as well as a .wav file.

The second project I am trying to convert to .MP2 already has the audio converted to .wav. I did that prior to "project trim".

When I tried playing the .mp2 in Impression SE, the audio was fine. Just the image is grossly pixelated.

Thanks, AlwaysLearning


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WTS
Re: VERY Pixelated MPEG-2
by
on Jul 19, 2002 at 1:40:42 am

First off--congratulations on getting everything to work.

Second: What settings are you using for your mpeg2 compression? If you are seeing pixelation in the preview window of Impression, then I would wonder if there has been an inadvertant change in the compression setting you are using, or change in field order, or something to that effect. I frames are like single jpeg images that contain the most detailed information of the scene you are encoding. The B and P frames are predictive frames that contain partial information based on the frames next to themselves. The standard for NTSC is a GOP (group of pictures) setting of no more than 18 frames(15 is typical, and is the max for PAL), with a repeating pattern that looks like this: IBBPBBPBBPBBPBB...and so on. I think its best to just use the preset that most encoders have for this, rather than to monkey around with it. If the GOP setting is less (ie 10), then you would have perhaps better looking compression, but it would come at the expense of a greater file size. In a 60 second clip, there would be '6' I frames (which contain the most detail) instead of '4' like you would with a GOP of 15. However, playback on a set top might suffer if the GOP setting is too low as well. Its hard to explain, and I don't understand all of the details, but I frames take more to process than the predictive frames, so a lower GOP setting (and thus more I frames per given time segment) might result in some stuttering in playback. For that reason, I think you will find that the default is a GOP of 15 for most encoders.


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alwayslearning
Re: VERY Pixelated MPEG-2
on Jul 19, 2002 at 1:36:57 pm

Thanks WTS for the info.
Do you know of a good book or web site(s) that would explain all this I, B and P frame stuff? I understand what you said in your post (thank you) but would like to study it more.

As far as GOP at 15. On the Pinnacle discussion board, I thought I saw someone say that it should always be 3. Whichever it should be though, I'm not sure I understand how to set that.

When I (in Premiere) go: File --> Export --> Movie, (there might be something in there about timeline too, but I'm at work now and can't look), but anyway, I get to a box with a settings button. I click on that and that is where I see the GOP stuff. What I see is in the lower left, GOP... to the right of that, positioned vertically, is a M and a N. M can be set at 1, 2 or 3. N can be set from 1 to 15? Under the GOP is a box where the letters show up, I, B or P or combinations of them. When I change the M and/or N, is that changing the GOP? Both are defaulted to 1 and under GOP is IB. If I change the N to anything more than 3 (I think), then when I go back to it test another setting (I was building a table of different combinations and trying to keep track of the results), it was always back at 3.

Thoughts?

Thanks for your time which I assume is at least as valuable as mine, AlwaysLearning.

P.S. I just got out of a meeting (engineering group where I've been for 20 years) where I found out that I may be going into video production full time. If you have any thoughts along those lines, tips, etc., please let me know, if you want and as is convenient for you.


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WTS
Re: VERY Pixelated MPEG-2
by
on Jul 19, 2002 at 3:37:34 pm

I don't have a Pinnacle based system, so someone out there who does might be better at answering your question. My guess is the 'N' represents the GOP setting, and I would pick 15. I think 3 is way too low, and for mpeg2 for video would give you a file size that is unnecessarily large (and I think would have troubles with playback). The 'M' I'm not as sure of, but my bet would be that it represents the number of repeating 'B' frames. If that's the case, I would select '2', since that would fit the standard repeating IBP frames that I posted earlier.

Also, make sure you have the proper field order. Most NTSC stuff in lower field, and I would make sure my encoder was set the same way to match.

As far as resources for dvd creation are concerned, DVD De-mystified by I believe J. Taylor would be a good start. I've generally picked up things here and there, and don't remember a particular source. I do have that book, and although there is a lot in there that I don't use, it is quite helpful in explaining a lot of this stuff. There is a Stanford prof., Ben Waggoner, who has a course on video compression who is going to have a book come out in August, and I think that would be a great one to get. A smaller version of it came included with Procoder (a Canopus encoding software) and has a lot of good little tid bits.

As far as help I could provide for video production--I'm afraid I probably wouldn't be of as much help. I do this stuff for fun on the side (although it's gotten to be almost an addiction). I have been tempted to open a side business however--perhaps to at least cover the costs of all the software/hardware toys I buy :)



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AD3
Re: VERY Pixelated MPEG-2
by
on Jul 20, 2002 at 6:05:37 am

Hey guys,

Been experimenting with the DV500DVD, Impressions Pro and my DVR-A04 myself. If you check the DV500 DVD manual, there's a brief blurb in there about export settings. It suggests setting the GOP fields to n=15 with m=3. It also suggests setting the bit rate to 6 and constant. Depending on your Authoring software, you might be better going for a higher, variable bit rate (Impressions Pro 2.0.2 is kosher with a VBR, I haven't played with the other progs yet). But having played with various GOP settings at the same bit rate...the image quality changes ever so slightly...but, strangely, the file size does not.

Let me know if this helps...or if you have any luck trying to compress audio...as my current .wav files are close to a gig for the 90 minute wedding videos I've been doing.


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WTS
Re: VERY Pixelated MPEG-2
by
on Jul 20, 2002 at 12:03:00 pm

I'm curious as to what the 'm' stands for. Without the manual I can only guess. I guessed right on the 'n' being the number of frames in a GOP, and Pinnacle, like most other encoders, suggests using the typical 15. It's a safe number for NTSC and PAL footage. I generally would stick with what a company rec's would be for settings. How an mpeg file plays on a computer will be vastly different for a set top player. Computers will tolerate wide variations in encoding parameters, frame size, etc--where as your set top player won't. So, files with different GOP settings may play just fine on your computer, may not play at all on a set top player, or even be accepted as an asset in a dvd authoring program.
There are two ways to compress your audio, one would be to mpeg1 layer2 and the other would be Dolbly Digital AC3. Mpeg1 layer2 can be created by a number of shareware programs out there (a good one is TMPGEnc), however that type of audio may not play back on NTSC set top players--although there are more and more players that will. AC3 audio is your best bet for set top acceptibility, but requires an AC3 encoder--which aren't cheap. Some dvd authoring apps (like ReelDVD and DVDitPE) include the encoder, otherwise you would need a stand alone like SurCode from Minnetonka audio (~$600-$700). PCM wave (which is the audio file created with the DV500) is uncompressed, and the bitrate is ~1500, that's why it takes up so much room. With an AC3 encoder, you can get that compressed down to about an 1/6th, and still maintain excellent quality.


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AD3
Re: VERY Pixelated MPEG-2
by
on Jul 22, 2002 at 12:47:56 am

I did a little more reading. The "M" stands for how many how B-frames are required before a reference (P-frame) is to appear. The default is 3, which corresponds to a GOP length of 15 (N in this case) and with a data rate of 6 Mbit/s...is, according to the manual "a common value within the DVD standard." Personally, I'd love to know the "range" of the DVD standard and whether this affects playback compatibility. Any thoughts? Or have I completely lost my mind? For instance, does using VBR (now supported by Impressions) mess anything up compatibility wise?

AC3...I did manage to find an AC3 encoder called SoftEncode (an old, apparently discontinued product by Sonic Foundry) which converts .wav files to .AC3. It's slow (realtime compression), but pretty funky especially when you get to play with the LFE (nothing like adding strange sounds to the subwoofer channel). It was able to compress the .wav file to 1/5th the original size...allowing me to have a little more bit-rate headroom. It can do both 2.0 and 5.1 Dolby Digital. And, thus far, has imported into Impressions Pro and seems to sync up OK (though the track length in the media assets is listed incorrectly...even though the audio plays for the entire duration of the video clip). Like you said, the real test will be when I build and burn and try it out on my set-top players. Has anyone had any thoughts/experience doing AC3 encoding to know what all the fun variables are (there are a lot of checkboxes and bit-rate options)? I don't normally condone "liberating software", but if Sonic Foundry is no longer selling a product...


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WTS
Re: VERY Pixelated MPEG-2
by
on Jul 22, 2002 at 3:35:19 am

I suspected the 'm' stood for the B frames, and if that is the case, I would change the default to 2 instead of 3.
For the most part, a GOP of 15, and a GOP structure of IBBPBBPBBPBBPBB (that repeats) is a commonly used setting by a variety of encoders. I don't think much is to be gained changing this too much(at least in my mind) when those much more familiar with the details have decided on this "sweet spot".
CBR and VBR are different issues. Both encoding parameters are accepted parameters of the universal dvd spec, so whether you use CBR or VBR to encode shouldn't in theory affect the compatibility of playback. However, there are other things you need to control when setting up VBR so that ultimately the file is compatible with your dvd authoring application. I use VBR all the time, and it works without a hitch in Impression Pro.
I have Softencode, but if you don't have the parameters set exactly right, you might have problems with playback. I have ReelDVD which includes a 2 channel AC3 encoder, and I let it do all the work and not worry about all the settings. Because of that, and the fact that ReelDVD will include a timecode stamp when encoding, the timecodes display properly in Impression.


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