Does letterbox or B&W compress better?

vbimport

#1

This might be too technical for this forum and its mostly just a personal curiosity thing. It has to so with MPEG encoding in either hardware or software. I’ll use a hardware example:

If I burn a letterbox movie off-air to DVD with a set-top box, will the picture quality be better than for a “full” screen movie? Since a good portion of each frame is black, that part should easily compress nearly 100% allowing the remaining picture to be compressed less to hit the target for the space available for each frame.

And the same question for a black and white movie. Since there is less than 1/3 the information in a black and white picture of equal quality/resolution, will a black and white movie compress “better” i.e better picture quality?


#2

This question can only be answered if you really know the specs from the source of content. Bitrate, res, etc.


#3

I can see how the chip/algorithym can effect the anser but I don’t think the inout specs matter much. Let me try to explain the question better.

Say I am using a set-top DVD recorder to record a letterbox movie from a 4x3 source on any over-the-air TV channel. I hope there is a standard bitrate/resolution, if not pick any one commonly used.

The DVD recorder is producing 720x480 frames. With a full screen 4x3 movie all the frame will have “picture” in it. With a 16x9 letterbox movie, only 3/4 of each frame has “picture” in it. I realize the DVD recorder doesn’t know its letterbox. As far as it knows, its a full screen picture that just happens to contain a black band at the top and bottom of each frame.

The set-top recorder clearly uses x bytes to store each frame on a DVD. I’m basing that on the observed fact that no matter what I record, it will run out of space on a DVD after a precise number of seconds of video.

Since 1/4 of each input frame is large contiguous black bands, that portion should compress like crazy, leaving more space for the 3/4 of the frame that contains the actual “picture”. So the algorithym doesn’t have to compress the “picture” part of each input frame as much to meet the output frame size target. Less compression loss should mean a better picture.

I am positive that a letterbox movie COULD have a better picture if the algorithym was smart enough. Are they? I don’t know if the MPEG2 specs dictate that or if different chipsets may do it differently.

And I see the same situation with B&W input. I am assuming that after decoding the input signal, a B&W frame is essentially just all 3 color values being identical. As such, it would seem that it would also compress much easier and result in less loss due to compression to meet the output frame byte size target. That would result in a better picture compared to a video where the 3 color values are different for each pixel. Again the question is: Is this true in real life or just theoretical?


#4

any over-the-air TV channel.

How ever would you know that they all come with the same bitrate, resolution etc.???
I think that only counts in this example, and most likely they all have different res & bitrate…


#5

I never said they all have the same bitrate and resolution. As I said before, I suspect it doesn’t matter but if it does use any bitrate and resolution you want.

Since even vacuum tube TV sets made in the 50s can still play signals broadcast today, I really doubt there is a lot of variation. I don’t think 60 year old circuits were sophisticated enough to handle a miriad of resolutions. And since I’m not even talking about a digital signal, I don’t know how “bitrate” even applies.

For the purpose of my question, chose any bitrate and resolution you want. I personally thing the answer will be the same no matter what they are, but an answer for any one bitrate/resolution will suffice for me. If you can’t answer the question anyway for any particular combination of bitrate.resolution, no response is necessary.


#6

I can understand what you are trying to say, and the answer to your question is[B] NO[/B], When you compress each frame it compresses the total size of the frame by a X amount of % that includes the black band as well as the picture. You can’t just compress ¼ of each frame by example 80% and ¾ by 20%, the only way to do it is by manually frame by frame cut the black band off and compress it separately to the picture and rejoin it.


#7

Before i get into bit rates or anything else to answer your question (s). No a widescreen movie will not compress better than a full screen movie. The reason being that its only widescreen on a full screen tv not on a widescreen tv. If you take a widescreen version dvd or movie copied to dvdr from a tv channel and put it on widescreen tv there are no black areas. the black areas are a by product of no signal in that portion of the television. This comes not from the amount of info being sent but where it is sent to. The television or dvd player makes that distinction for you. The information is still the same as before. Now with a black and white movie the compression should be a little bit better, because you dont have all of the colors to try and compress down. The fun thing is that unless its an extremely long black and white movie you shouldnt need to compress it anyway because most of the time they are less than 4.7 gigs or less once transcoded because of the lack of colors. The Bit rate delima is a little heavy for a beginners forum. but i think i can answer it for you because ive been working with dvds and mpegs for a while. Bit rates themselves are what matters most when you compress a movie. No two movies have the same average bit rate. The higher the bit rate the more you can compress a movie and it still look good. Say your bitrates are at 8800, you at least need to keep around 3000 bit rate to keep a decent looking movie. So if you take 8800 and compress it 50% you still have 4400 bit rate wich is acceptable and unless you have a huge tv it will look really decent. However if you start out with 4400 bit you will start getting heavy pixelation around 80 percent or so, maybe higher.
To answer your last question. The algs. and transcoders of today cannot compress half of any one frame and leave the other half alone. All that the most common transcoders can do is compress each title indivdually and cut entire frames off.
If you need further info on this just ask.


#8

Thanks cwrightthruya,

So a B&W movie will have a better picture than a color movie when processed by the same MPEG2 algorithym.

And a letterbox won’t. I will take your word, but as someone who has programmed compex algorithyms (not compression) it seems a shame that an algorithym can’t compress an image with large bands of identical (or nearly so) colored pixels (such as the “black” bands above and below a letterbox movie) better and/or with less loss than an image with widely varying pixels throughout. That tells me that there isn’t enough power/speed with realtime compression to try different strategies on each frame and pick the best one. That also tells me that there must be wasted space on the DVD for every frame that happens to compress very well with the algorithym used. A monochome frame may compress down to almost nothing, but takes up the same space on the DVD as any other frame. :Z

I follow what you say about bitrates. Am I right that with a set-top box and analog input, I have no control over the bitrates going in. And the bitrates on the DVD are dictated by the algorithym used for the recording mode I select.


#9

Gotcha. I think it would be possible for it to work like I hoped, but it doesn’t work that way. I suspect its a matter of not having enough power/speed to tailor the compression differently for each frame and what’s in it.

Still beats analog tapes all to heck though :wink:


#10

HoosierDaddy
You are correct. You have no control over the origional bitrates of a movie that is produced or copied off of a telivision or origional Hollywood dvd for that matter. The only thing you can do is test the origional bitrates and then use a compression engine (of your choice) and try and compress the movie as much as possible without going below the recommended bit rates.
If you are really worried about disk space you also have a choice to reauthor the movie or reencode the movie. This can be a tedius and somewhat complicated process and well beyond the scope of this forum. But if you are working with base algorythms you should be able to handle it with no problem. If you are interested in a cheap reauthor program you might want to try Dvd Remake Pro. If you are ready to go to a more expensive program that can do almost anything you might want it to You could go with a program like CCE.