Understanding a bitrate

[alex1234]

I have a 600MB AVI that I want to convert to a DVD MPG. Since the size of the source is 600MB, (correct me if I'm wrong) there is no point of increasing the size of the resulting MPG to over 600MB because there will be no quality improvemet anyway. So, I choose a minimal bitrate (about 1500) to keep the size smaller.

Is this a right approach or I'm doing things wrong and I should choose a bitrate 2000,3000,4000 and make the resulting MPG as big as 3 or 4GB? If I should, why? Please explain

[DiablosuX102]

if the .avi file has a codec in it, when u remove that codec and keep the same bitrate its gonna look terrible, open it up in gspot n see if there's a video codec

josh

[alex1234]

OK, I'll re-phrase my question - will my resulting MPG loose any quality if I, manipulating the bitrate (this is the only thing that seems to affect the size), make the encoded file the same size as the encoding one (original)?

[DiablosuX102]

u might already know but let me explain anyways,
video codecs like xvid, divx, 3ivx, help with compressing the movie in a way, so its not just bitrate that effects the size. DVD players(as far as i know of) can't play videos with these codecs inside them so u hafta remove the codecs, and to keep the same quality the file size has to go up.

hope this helps, josh

[ZippyP.]

Bitrate in an avi and bitrate in an mpeg is not equal because they use completely different methods of compression. In the case of your file we have no idea what codec was used to compress it but it doesn't really matter. Different codecs compress differently so you can't even compare directly between different avis. There are bitrate thresholds that experienced encoders will try to stick to when creating their mpegs but it depends on the type of source, and most importantly the resolution being used. If you do not increase the bitrate when going from avi to mpeg the results will be pretty bad, try it and see.

[ZippyP.]

. DVD players(as far as i know of) can't play videos with these codecs inside them so u hafta remove the codecs, and to keep the same quality the file size has to go up.

Not exactly. There are divx DVD players, they're getting pretty common.

Uncompressed audio and video is very large, that means that the bitrate is very large as well. Mpeg is a type of codec (encoder-decoder). You do not have to uncompress to convert, you're going from one type of compression (divx, xvid or ?) to another (mpeg!). Divx and xvid are more efficient than mpeg at reducing file sizes and maintaining quality, that's why file sizes need to increase to keep it looking good.

[alex1234]

I still do not understand why you are so concern about codecs used in my source AVI? I don't care - my encoder understands it and I'm OK with it. This is the resulting bitrate that I do not understand.

Once again, I have 600MB AVI and if I use bitrate 2000 when I'm encoding it to MPG, the MPG will be 3GB! Should I go for it? Why can't I use bitrate 1000? and make my file 1 GB? My understanding is that there is no point of increasing the size of the destination file over the size of the source. Am I right or not?

Here is an other example: I have a 600MB AVI file with certain quality. I can:

1. convert it to 4 GB DVD MPG with bitrate 6000
2. convert it to 1 GB DVD MPG with bitrate 1000

I think that 2) is smarter because the quality of 1) and 2) are the same - the space just wasted. Am I right?

[DiablosuX102]

just try encoding to 3 gig's, if u have enough time do a smaller size if u want, it won't look near as good though man.

josh

[DiablosuX102]

hey zippy, when they say compatible with divx, does that mean other video codecs also?

[alex1234]

just try encoding to 3 gig's, if u have enough time do a smaller size if u want, it won't look near as good though man.

josh

Can you explain why? what if I encode to a 9 GB? Will the quality be even better? Would it be even better than the original?? See where I'm leading to? Where is the boundary that makes the destination to have the same quality as the source and beyond which there is no point of going?

[bugster]

if the .avi file has a codec in it, josh

Avi file do not have codecs in them. An avi file just contains the video and audio that has in some way been compressed using a codec of some sort. The avi file does include some info that inform the player application what codec is required to decompress the video and audio (note: Video and audio can, and often do, use different codecs).

Once again, I have 600MB AVI and if I use bitrate 2000 when I'm encoding it to MPG, the MPG will be 3GB!

The size of an mpeg file is dependant on two things. These are the bitrate that was used to encode it (if it was Variable bitrate (VBR) then its the average bitrate that matters) and the playing time. Nothing else. (The same is true of divx, which is mpeg-4).

In terms of getting the best quality from a divx to Mpeg-2 conversion, a lot depends on the quality of the source. Assuming a 90 minute movie in 600Mb, it won't be that great. I would probably encode that 1/2 D1 resolution (352*480/576 (NTSC/PAL)) with an average bitrate of around 3Mbps.

[DiablosuX102]

i think 9 gigs is pushing it, and without doing some hardcore editing i don't think u can make the video look "better" but....if that 600MB film looks good, it most likely has really good compression(video codecs)dvd video's compression isn't near as good as the compression already in that video, so to go from really good compression(the avi) to a lot less compression(mpeg for the dvd)u hafta increase the bitrate(size) to maintain quality.

josh

[bugster]

just try encoding to 3 gig's, if u have enough time do a smaller size if u want, it won't look near as good though man.

josh

Can you explain why? what if I encode to a 9 GB? Will the quality be even better? Would it be even better than the original?? See where I'm leading to? Where is the boundary that makes the destination to have the same quality as the source and beyond which there is no point of going?

The boundary is the quality of the source material and if it is really good then the boundary becomes the Max bitrate available for the destination format, which for DVD is 9.8Mbps. However, from experience I can tell you that if your movie is much over an hour long, in a 600Mb divx, the point at which you will not get anything better is noweher near the 9.8Mps. As I said, try around 3Mbps at 1/2 D1, you probably won't improve much on that.

[alex1234]

[quote="bugster"]

Assuming a 90 minute movie in 600Mb, it won't be that great. I would probably encode that 1/2 D1 resolution (352*480/576 (NTSC/PAL)) with an average bitrate of around 3Mbps.

Why? Please explain! Why 2Mbps won't have the same quality? Again, where is the boundary? How do you know that, let's say 3Mbps will give better quality for my source than 2Mbps and 4Mbps will be a waste of space?

[alex1234]

The boundary is the quality of the source material...

Exactly! Now back to my initial question - can't the size of the source be that boundary since I can not compare the bitrate of AVI with bitrate to use for MPG encoding?

In other words if the source is 600MB AVI and I choose the bitrate to produce 600MB MPG won't this be the boundary bitrate beyond which there is no point of going?

[ZippyP.]

Where is the boundary that makes the destination to have the same quality as the source and beyond which there is no point of going?

There is always a point beyond which you will not see (to your eyes) any increase in quality. Like I said, it depends on the source and the resolutions (of both the source and the output) what your ideal settings should be. Bitrate settings are really a matter of personal choice as what looks good to one person looks like crap to another. You will need to do some experimenting and decide what your settings will be. If you give some details about your source and destination then some people (like bugster) can give some recommendations. If you're putting it on one DVD then just fill it up. If you take bugsters advice (1/2 D1) then you can probably get 2 on a DVD.

[ZippyP.]

- can't the size of the source be that boundary since I can not compare the bitrate of AVI with bitrate to use for MPG encoding?

In other words if the source is 600MB AVI and I choose the bitrate to produce 600MB MPG won't this be the boundary bitrate beyond which there is no point of going?

No. You would be using the exact same bitrate to produce a file of the same size. Already said, you better not do that.

[alex1234]

Thanks to all who were explaining the situation. Please read my question below and please let me know if I'm right or wrong:


can't the size of the source be that boundary since I can not compare the bitrate of AVI with bitrate to use for MPG encoding?

In other words if the source is 600MB AVI and I choose the bitrate to produce 600MB MPG won't this be the boundary bitrate beyond which there is no point of going?

[alex1234]

It looks like we are out of sync - I'm too fast typing my questions - I'm gonna wait for a while...

[bugster]

In other words if the source is 600MB AVI and I choose the bitrate to produce 600MB MPG won't this be the boundary bitrate beyond which there is no point of going?[/b]

No. The avi is compressed using divx (or something similiar), which compresses the file much more than mpeg-2. To get the same quality in an mpeg-2 file, you need to have a much bigger file. Exactly how much bigger it is impossible to tell, you can only go on experience and your own eyes.

[duhmez]

Mpeg2 does not compress as well as divx and the like, which is probably what your movie is. Being the same size does nto mean same quality. You MUST increase the bitrate if you want tomaintain quality. the opposite is true going from mpeg 2 to divx. A 5 gig mpeg 2 file can compress to1.5gig orless and look almost identical.
If the size of the video data was the same for every codec, there would only be one codec. Depending on the length of your 600 mb movie, and trhe resolution, you may need to double, or more,. the bitrate for mpeg2.

[Gazorgan]

Very simply:

The size of the source AVI has NO/Zero/None affect of the size of the resulting MPEG1/2.

The Resolution of the source AVI has no bearing on the final MPEG resolution.

The length of the source AVI does have an affect on the final size of the MPEG. This is because VCD is a fixed bitrate, SVCD has a range you want to stay in or it looks like crap, same for MPEG2.

The size of the source avi DOES affect the quality of the final MPEG. Garbage In = Garbage Out . Don't expect better than VCD quality from a 1 disk AVI. DVD quality takes 2-3 disk AVI, depending on resolution and length.

The resolution of the source avi affects mpeg quality. You can't take a 320x240 AVI and expect to get DVD quality 720x480 DVD, aint' going to ever happen. Generally speaking you can't increase the resoltuion and expect to get any improvement.

All AVI's have a source aspect of 1:1, never 4:3/16:9/Anamorphic .

Never confuse resolution ratio with Aspect Ratio. An AVI that is 320x240 still has a DAR (Display Aspect Ratio) of 1:1, but its resoltuion ratio is 4:3. When converting to MPEG, you have to make sure you get this right or you get skinny/fat people.

Lastly, it's much much easier to work from teh original DVD instead of download over-compressed crap AVI's from soem P2P.

[alex1234]

Depending on the length of your 600 mb movie, and trhe resolution, you may need to double, or more,. the bitrate for mpeg2.

I'm trying to avoid referencing to bitrates and operate the sizes instead. I see that you are saying that 600MB AVI can hold a much better quality than 600MB MPG. I see that I was wrong trying to maintain sizes 1:1. Would 1:2 proportion be enough not to waste space? In other words, will converting 600MB AVI to 1.2GB MPG produce the same quality for destination (approximatelly)?

[bugster]

I'm trying to avoid referencing to bitrates and operate the sizes instead.

Assuming you are trying to convert to one of the standard formats ( VCD, SVCD or DVD) then that is a mistake, bitrate is VERY important.

In other words, will converting 600MB AVI to 1.2GB MPG produce the same quality for destination (approximatelly)?

In the end only you can tell. Quality is very very subjective. Try encoding some short clips at various bitrates and see what looks good enough to YOU.

[alex1234]

In the end only you can tell. Quality is very very subjective.

But there must be some general rules that will work for all AVI-MPG convertions. If AVI used the same compression rate as MPG I would say that 1:1 (sizewise) convertion would be a wise decision not to waste space. But since AVI is about twice more compressed than MPG, 1:2 convertion would be the recommendation (600MB AVI to 1.2GB MPG), isn't it?

[bugster]

In the end only you can tell. Quality is very very subjective.

But there must be some general rules that will work for all AVI-MPG convertions.

No, there isn't, thats the whole point.

[trevlac]

In the end only you can tell. Quality is very very subjective.

But there must be some general rules that will work for all AVI-MPG convertions. If AVI used the same compression rate as MPG I would say that 1:1 (sizewise) convertion would be a wise decision not to waste space. But since AVI is about twice more compressed than MPG, 1:2 convertion would be the recommendation (600MB AVI to 1.2GB MPG), isn't it?

AVI is like saying 'car'. MPEG2 is like saying 'Mercedes Benz ML320'.

Which one gets the better gas milage?

Probably the car

[Gazorgan]

It's an apples to oranges comparison.

How about looking at it this way: A 5 GB DVD MPEG2 stream will look pretty much exactly like a 2 GB properly done DivX in the same resolution. A 1 GB DivX in the same resolution will have artifacts. The question is where between the 1GB and 2GB mark is 'good enough'? Changing to a lower resolution means a smaller file. A 640x480 DivX is pretty much the same as a 720x480 MPEG2.

Nearly all decisions for AVI's are based on 1 or 2 disk for final product. This completely ignores any factors you would normally consider (audio/resolution/quality). Going the other way is the same thing. If you are shooting for SVCD, then typically it's 1 SVCD disk for every 40-50 minutes of video. This is based on tried and tested SVCD bitrates that work. For DVD you have a 4.2 GB limit (allow for overhead) to shoot for, which ultimately determines your bitrate for you. You do have the option of 1/2D1 resolution (among others), to increase your bitrate and make the MPEG2 just as good or bad as the source AVI.

[alex1234]

Thanks, but I still don't understand whey can't we predict the optimal output size knowing the input size (I'm talking here of AVItoMPG conversions). Gazorgan said that "5 GB DVD MPEG2 stream will look pretty much exactly like a 2 GB properly done DivX". That means that if I convert this 2GB AVI into a 5 GB MPG I will not loose any quality. In other words, if I use a 2.5 size rate converting AVIs to MPGs I will not loose the quality.

Doesn't this sound like a general recommendation - when converting AVI to MPG use a bitrate that produces MPG 2.5 size of the AVI. You may use a higher bitrate but there will be no quality improvement, just a waste of space?

If the AVI was not "properly done", the conversion rate would be even less than 2.5. So 2.5 would suite all AVItoMPG conversions.

[jimmalenko]

alex,

No one here is a qaulified expert on the subject - we have got our knowledge from TRYING different things to see what works best. You will never increase the quality of the source without running filters. If anything, whenever you re-encode you are losing quality. This is fact !There is no formula you can plug in numbers to that will give the optimum bitrate in MPEG2 for any given AVI.

Work it out based on your required output size. If you want this AVI to take up 1 DVDR, use a bitrate calculator to get the required bitrate. Test it, see what it's like...If its good enough for you then happy daze.....