Resolution & Bitrate a question about quality XVCD XSVCD

[Reinder]

I have a question about the relation between bitrate and quality and size of the file. When I make a XVCD the only thing that changes the outputfile size is the bitrate, changing resolution does not affect the size but does affect the quality. I want to make the best quality with the smallest filesize (as anyone would). But the normal VCD resolution is gonna be streched by my PalTV (or is it the DVD?), this would also affect the quality I guess. So is it better to blow up the resolution while encoding with Tmpgenc or is keeping it small and have the TV/DVD do the zooming. And are there good bitrate/resolution combinations or does that not matter. A third question is about aspect ratio. When I have a widescreen movie you can make a XVCD/XSVCD with the same ratio in stead of having Tmpgenc making extra black bars, it works on my DVD but it's hard to notice any difference in quality. Does this make a difference or aren't the black bars using up any quality or space in the file?

So in short:
1. Does enlarging the resolution in Tmpgenc while having the same bitrate give you a better quality than having small resolution and having the TV/DVD doing the enlarging?
2. Is there a clear relation between bitrates and resolution?
3. Does changing the resolution to the same aspect ratio as the original have an effect on quality?

Thanks for taking the time to read or answer.

[furball6969]

You can think of bitrate and resolution in terms of bread and butter. The butter is the bitrate and the size of the bread is the resolution. The larger the slice of bread the thinner the butter is going to be (not good if yoiu like butter).

answers to your questions?

1: No, enlarging resolution while keeping the same bitrate will give your poorer quality; as you have already experienced.

2: yes, the relation is obvious in the output; as you have experienced.

3: Any time you re-encode an already-compressed file you will have an affeft on quality. Perception is arbitrary.

[Reinder]

Like the metaphore I know the quality gets poorer but my TV/DVD does the same, it enlarges the picture, and which one should you choose. Maybe the last question is not exactly clear. When you have a widescreen movie with a resolution of 576x304 and you want to make it a XVCD, you can leave the resolution at 352x288 with black bars or you can make it 352x192 without the black bars. Resolution in the second case is smaller and would make a better quality (?) Is that the case or not?

[chewyshaun@hotmail.com]

Sorry, just had to comment on the bread and butter, superb, that should go in the newbie guide!

[EddyH]

If u have a widescreen downloaded movie and it doesn't have extra black bars, u just have to resize it to fit at the right shape inside of the vcd frame. You're always going to be encoding a 352x288 image for a PAL TV, the average hardware player is unable to play a file that doesn't conform to the standard sizes (*most* of the time!). But, the black bar area is not going to take much quality away from the image as it both compresses very well on keyframes, and doesn't change very much in the delta frames (pure black for a whole picture seems to take about 1500 (bits? bytes?) for key and 32 or 33 for each delta, at PAL rez - so the bars will need even less

What the size you have to make it as depends on the shape of the movie and the shape of your TV, and maybe a little personal preference or tailoring to your equipment. Personally I find, for a film that would be full frame on a widescreen TV (as a good number of downloads are), 352x224 inside of 352x288 works well on my PAL 4:3 set. Making it about 212 high would be even better, but getting it to crop exactly on the 16x16 macroblock boundaries is apparently a lot better for compression and image quality It appears only slightly stretched vertically, and is unnoticable unless you've just been watching the original from DVD or on the PC.
192 high also works well, slightly squashed instead... it may depend on your actual TV as to which works better, of course the 192 high picture will compress better, less distortion for the same bitrate, but won't be quite as sharp. Swings and roundabouts!
Some TVs even overscan enough to allow horizontal shrinkage, ie to 336 or 320 within the 352 frame and small 'sideboxing'... for just a little better compression (and avoiding loss of subtitles off the side for the less skillfully subbed films). I'm hoping for this in my movie-on-an-8cm-disc experiment

576x304... let me have a little think here ... that should work pretty well at 352x192, only the very very slightest of vertical squash. (95% apparent height compared to the original).

Quick way of working it out, for a 4:3 letterboxed movie on PAL, if you always keep the horizontal size at 352:

Take the original height
Divide by the original width
Multiply by 384 (as VCD pixels aren't square - for NTSC, use 320)
'Snap' it to the nearest multiple of 16 or 32 if you like.
Voila, 352 x your result is the size you need

For use on a widescreen 16:9 TV... well you really should be buying the DVDs you rich showoff or making SVCDs, but still.. the horizontal sizes to use in the maths increase from 384 to 512, and 320 to 426.

And of course, for SVCD/CVDs, just double the number before 'snapping' and make the final horizontal 480 for an SVCD.

Complicated enough for ya? Good luck!


If it comes ready supplied with black bars, you can either resize to fit including them (similar maths, though fitting a 4:3 film to 16:9 will need some of it to be cut out), or crop them out before resizing and work from that figure. Especially if it's been captured from VHS, as they will be a bit noisy and so eat up their fair share of bits. Only problem with that, lies in that any film that's been encoded with the borders intact usually has a good reason - eg subtitles that drop off the bottom of the letterboxed image. These can be a real pain the backside to encode well, especially trying to get them centred if e.g. you're able to precisely crop the top border but have to leave the lower one untouched because of subs - so best wishes!!


(Bitrate is a little more simple - use whatever figure you think looks good. Maybe start with enough of an average or constant to fit it within a single CD, and if that looks too nasty for you, double or triple it and use two or even three discs I think I touched on how much the totals are in another thread, but simple enough to math - audio and video combined is 1374k/s to just squeeze 80 minutes of film on an 80 minute VCD, if you turn off CDi support and all menus etc. Divide 80 (79?) by the run time of your movie, and multiply by 1374, to see how much you have to divide and 'spend' on your image and sound streams! Always leave 30-60s spare on the imaginary disc for multiplexing overheads and all though..)