Blu-ray movie quality - Blu-ray Forum

DealsR4theDevil · 01-08-2008, 08:00 AM

Can anyone verify if this guy knows what he is talking about? I realize this mentions HD DVDs, but the point is to try and understand what it is that makes Blu-ray movies superior.

http://forums.highdefdigest.com/show...4&postcount=82

robgmun · 01-08-2008, 11:31 AM

Quote:

Originally Posted by DealsR4theDevil

Can anyone verify if this guy knows what he is talking about? I realize this mentions HD DVDs, but the point is to try and understand what it is that makes Blu-ray movies superior.

http://forums.highdefdigest.com/show...4&postcount=82

He missed the point entierly in my opinion, it's not about resolution. it's about compression. with 20Gb more space to play with, and with both formats using the same method of compression. The Blu-ray has the potential to look less compressed on the screen with higher bit-rates.

Of course there is the law of deminising returns and it comes to a point where the difference in picture quaility is hard to see.

Take the Prestige for example a direct port from HD-DVD, the bit-rate is about 15-20mps and rarely goes above 20. yet the PQ is absolutely stunning, and i was quite shocked that such a good picture can come from such a low bit-rate. and if the bit rate goes up to 25-30 then i think it would be very hard for me to see the any improvement.

Grisle · 01-08-2008, 11:42 AM

What examples can you give us of movies that are over-compressed?

robgmun · 01-08-2008, 11:44 AM

Quote:

Originally Posted by Grisle

What examples can you give us of movies that are over-compressed?

You've mis-read my post, i never said any movies are over-compressed, in fact i gave you an example where a low-bit-rate movied still looked amazing! i'm saying blu-ray has a POTENTAIL to look even less compressed then HD-DVD.

Anthony P · 01-09-2008, 01:34 AM

the guy has no idea what he is talking about. The problem is not capacity (well it can play a role, but let's face it you can always split something on two disks if you need to) but bandwidth.

The issue is that you need 24+ bits per pixel (to describe the colours of each pixel) you need 1080x1920 pixels per frame and you need 24 frames per second this meeans that in order to describe each pixel in the movie you need hundreds (and thousands) of mbps. Obviously no consumer has that and so we can't get that. hwat happens is that we use compression algorithms (MPEG2, AVC, VC-1) to make the mbps needed much lower. Now CODECS are pretty complex (and work in time as well as image), but to simplify if the image is the lesft 1/2 black and the right half white then you don't need to discribe each pixel all you need to do it correctly is convey that it is 1/2 black and 1/2 white. so the codecs job is to block "similar" pixels together to shorten how much information is needed. In real life in many scenes the information cannot be blocked together that nicely and the blocks tend to be of restricted sizes (i.e. MPEG 2 has two block sizes). So what happens is the encoder will look at the pick and say "I need to encoded in less then Xmbps" and sooner or later it will say "well those pixel's aren't the same, but they are close enough and I will pretend they are the same and they are colour Y". The lower the BW and the more complex the scene the more this will happen and you lose the accuracy of the image (i.e. image quality goes down).

The CODEC's are what is called variable bitrate (VBR) so the encoder will have a max bitrate (MBR) which is how high it can get in the most complex scenes and average bitrate (ABR- but sometimes called VBR as well). so the encoder looks through the whole movie to decide where it needs more BW (while being less then the max) or less BW in order to meet the ABR. If you think of traveling the MBR is like the speed limit while ABR would be like how fast you must travel to get to where you are going on time (i.e. if you drive at 30km/h the first 1/2 of the trip and 100km/h the second then you won't be 80km away in 1h).

The sums of MBR for all things that are together<= the what the media can handle and the ABR tells you how much disk space it will take. The thing that happens is that the disk will have several languages, PiP, subtitles, menus.... and all of them will take up BW and capacity.

For example HD DVD has 30mbps max, if it has a lossless track that needs a max of 3mbps, three lossy of 1.5mbps each (you tend to need a lossy and a lossless for most), a PiP (with PiP audio) at 4mbps and 1mbps of headroom, then the guy incharge of the videofor an HD DVD must use a max of 30-3-4.5-4-1=17.5 mbps while on the BD the max would be 48-12.5=35.5 mbps.

so let's say you are at a complex scene and the people used the above spec, the HD DVD would have a max of 17.5 mbps and so would need more blocks then the BD that can go all the way to 35.5mbps and so more information will be lost on the HD DVD.

Now the truth is that studios could do anything they want, for example a studio could decide to keep the 19mbps for the BD video but still get to 48mbps by changing the main audio to 9mbps and add 6mbps lossless for the other two languages to get up there, or they could decide to add more tracks or add more features (like a PiP of the director and a different PiP of the main actor) or even not go up to 48mbps (in which case we are all a bit sadder because we are stuck with an HD DVD level of the movie)

01-08-2008, 08:00 AM	#1
DealsR4theDevil Power Member Member since:Sep 2007 Trading Score:76	Blu-ray movie quality Can anyone verify if this guy knows what he is talking about? I realize this mentions HD DVDs, but the point is to try and understand what it is that makes Blu-ray movies superior. http://forums.highdefdigest.com/show...4&postcount=82

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01-08-2008, 11:42 AM	#3
Grisle Power Member Member since:Sep 2007 Location:Northern California 4K collection:18 Blu-ray collection:18 Trading Score:12	What examples can you give us of movies that are over-compressed?

01-09-2008, 01:34 AM	#5
Anthony P Blu-ray Count Member since:Jul 2007 Location:Montreal, Canada	the guy has no idea what he is talking about. The problem is not capacity (well it can play a role, but let's face it you can always split something on two disks if you need to) but bandwidth. The issue is that you need 24+ bits per pixel (to describe the colours of each pixel) you need 1080x1920 pixels per frame and you need 24 frames per second this meeans that in order to describe each pixel in the movie you need hundreds (and thousands) of mbps. Obviously no consumer has that and so we can't get that. hwat happens is that we use compression algorithms (MPEG2, AVC, VC-1) to make the mbps needed much lower. Now CODECS are pretty complex (and work in time as well as image), but to simplify if the image is the lesft 1/2 black and the right half white then you don't need to discribe each pixel all you need to do it correctly is convey that it is 1/2 black and 1/2 white. so the codecs job is to block "similar" pixels together to shorten how much information is needed. In real life in many scenes the information cannot be blocked together that nicely and the blocks tend to be of restricted sizes (i.e. MPEG 2 has two block sizes). So what happens is the encoder will look at the pick and say "I need to encoded in less then Xmbps" and sooner or later it will say "well those pixel's aren't the same, but they are close enough and I will pretend they are the same and they are colour Y". The lower the BW and the more complex the scene the more this will happen and you lose the accuracy of the image (i.e. image quality goes down). The CODEC's are what is called variable bitrate (VBR) so the encoder will have a max bitrate (MBR) which is how high it can get in the most complex scenes and average bitrate (ABR- but sometimes called VBR as well). so the encoder looks through the whole movie to decide where it needs more BW (while being less then the max) or less BW in order to meet the ABR. If you think of traveling the MBR is like the speed limit while ABR would be like how fast you must travel to get to where you are going on time (i.e. if you drive at 30km/h the first 1/2 of the trip and 100km/h the second then you won't be 80km away in 1h). The sums of MBR for all things that are together<= the what the media can handle and the ABR tells you how much disk space it will take. The thing that happens is that the disk will have several languages, PiP, subtitles, menus.... and all of them will take up BW and capacity. For example HD DVD has 30mbps max, if it has a lossless track that needs a max of 3mbps, three lossy of 1.5mbps each (you tend to need a lossy and a lossless for most), a PiP (with PiP audio) at 4mbps and 1mbps of headroom, then the guy incharge of the videofor an HD DVD must use a max of 30-3-4.5-4-1=17.5 mbps while on the BD the max would be 48-12.5=35.5 mbps. so let's say you are at a complex scene and the people used the above spec, the HD DVD would have a max of 17.5 mbps and so would need more blocks then the BD that can go all the way to 35.5mbps and so more information will be lost on the HD DVD. Now the truth is that studios could do anything they want, for example a studio could decide to keep the 19mbps for the BD video but still get to 48mbps by changing the main audio to 9mbps and add 6mbps lossless for the other two languages to get up there, or they could decide to add more tracks or add more features (like a PiP of the director and a different PiP of the main actor) or even not go up to 48mbps (in which case we are all a bit sadder because we are stuck with an HD DVD level of the movie)

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