DAR, PAR, framesizes, analog captures

[migf1]

Ok guys, I've really had it with all this mess! No matter how much and what I read I always end up being confused

What I'm really looking for is the IDEAL framesize settings for capturing a PAL_BG signal from a standard 4:3 TV (connected to a GeForce4 Ti ViVo, via a composite RCA -yellow marked- video cable), in order to encode it

• to a VCD mpeg1
• to an SVCD mpeg2
• to a 352x576 DVD mpeg2
• to a 720x576 DVD mpeg2

In all cases, the objective is to retain the best possible picture quality and the exact aspect-ratio (both DAR and PAR) of the original broadcast, for the viewable part of the output (meaning, cropping or padding if we have to).

Although a quick answer of the form "for VCD capture in that fs, then crop/pad so many pixels, encode using this input DAR, etc" would be good, I would really like to get some background info too.

Actually I have some more questions, the answers of which I beleive will help my sort some basic things out.

So, first, let me state what i THINK I've understood from what I've read all over the places so far. If I'm missinformed (the most probable case) please by all means correct me.

• DAR (Display Aspect Ratio) is only meaningful to the digital world (i.e. AVIs, MPEGs, etc)
• SAR (Sample Aspect Ratio) is only meaningful to the analog world, but equals to PAR (Pixel Aspect Ratio) which is also meaningful to the digital world.
• A typical PAL_BG aerial broadcast signal corresponds to a digital video stream with DAR=4:3 (1.3333) and PAR=59/54 (1.0924)
• All AVI's have PAR=1:1 (square pels), regardless their DAR.
• Since they are intended to be played back on a TV set, all PAL (S)VCD/DVD compliant MPEGs have PAR=59/54, regardless of their DAR (which in turn is kept in a special flag into their video stream, for the player to read it and adjust accordingly the framesize during playback)

For the moment I put aside all the anamorphic, wide, 16:9, etc stuff and I focus on 4:3 only.

So...


Question 1:
What PAR do the DVD officially supported framesizes refer to? (1:1 or 59/54?)


Question 2:
a) Does the analog PAL_BG signal correspond to the non-square pels digital framesize 720x576 or to the digital square pels framesize 768x576. Or are these two framesizes/pelsizes equivallent?

b) If the above framesizes are equivallent, where does this leave the framesize 704x576?

c) Which one of the above three framesizes is referred to as "D1". Judging form the name "half D1" for 352x576, I would say D1 refers to 704x576... doesn't this sound the logical thing? Why DVD standards include both the 720x576 and the 704x576 framesizes (what's their fundamental difference/purpose of usage?)


Question 3:
In general, should I capture the specific TV footage in the officially supported DVD framesizes (assuming they are meant to contain non-square pels) or should I calculate their respective square pel framesizes and capture in them? (btw, my card does not allow me to capture at 768x576... it maxes out to 720x576).


Question 4:
Is it safe to capture at Half-D1, or do I lose too much data of the original PAL_BG signal? I gave it a try and I wasn't disappointed at all. It looked quite good. How come?


Ok, I stop here for now... I've already thrown way too many stuff onto the.. table

Best regards,

[trevlac]

Hi

Mind if I give it a go?

Question 1:
What PAR do the DVD officially supported framesizes refer to? (1:1 or 59/54?)

DVD has nothing to do with PAR. PAR is a viewing device thing (and an artifical concept that removes us from the real facts). The device/player must adjust.

Question 2:
a) Does the analog PAL_BG signal correspond to the non-square pels digital framesize 720x576 or to the digital square pels framesize 768x576. Or are these two framesizes/pelsizes equivallent?

Analog pal is 702x576. If you show a 720x576 DVD on a TV, it is effectively cropped by the player to 702x576. If you show a 720x576 DVD on a pc, it is effectively cropped to ~ 702x576 and resized to 768x576 by the PC player.

b) If the above framesizes are equivallent, where does this leave the framesize 704x576?

704x576 is a cropped version of 720x576. Less is cropped if you do the above.

c) Which one of the above three framesizes is referred to as "D1". Judging form the name "half D1" for 352x576, I would say D1 refers to 704x576... doesn't this sound the logical thing? Why DVD standards include both the 720x576 and the 704x576 framesizes (what's their fundamental difference/purpose of usage?)

I blelive D1 is a form of digital video tape. The reference to D1 is to a 720x576 frame size that is specified in the standard D1 follows. So all this D1, 1/2D1 talk is a bit loose.

Question 3:
In general, should I capture the specific TV footage in the officially supported DVD framesizes (assuming they are meant to contain non-square pels) or should I calculate their respective square pel framesizes and capture in them? (btw, my card does not allow me to capture at 768x576... it maxes out to 720x576).

Easy way: capture at your intended output frame size. Harder but more correct: Capture at your standard output sample rate (like 13.5 for DVD) and pad/crop to hit the standard frame size. This means you have to know the line length your card samples.

Question 4:
Is it safe to capture at Half-D1, or do I lose too much data of the original PAL_BG signal? I gave it a try and I wasn't disappointed at all. It looked quite good. How come?

Pixels are directly related to the number of samples take from the source. If the source does not contain more detail than can be contained in a 352x576 frame, 720x576 does not have advantages. Broadcast allows more detail than 352x576 can contain (both NTSC and PAL). Hard to say about a given signal and program.

[migf1]

Hi

Mind if I give it a go?

Hi travelac, thanks for answering!

DVD has nothing to do with PAR. PAR is a viewing device thing (and an artifical concept that removes us from the real facts). The device/player must adjust.

Meaning that I don't have to care about PAR at all? Neither during the capture phase nor during the MPEG Encoding phase?

Analog pal is 702x576. If you show a 720x576 DVD on a TV, it is effectively cropped by the player to 702x576. If you show a 720x576 DVD on a pc, it is effectively cropped to ~ 702x576 and resized to 768x576 by the PC player.

Thank you, that was precious info!!!

However it loops back to my original question(s). It now goes like this:

Unless we put PAR back to the discussion, neither 720x576 nor 702x576 (nor 704x576, btw) are of DAR=4:3... right? Now, my Philips SAAA7108 chipset capture-card (nVidia GeForce4) does not allow me to capture at 768x576 (DAR=4:3), but it maxes out at 720x576. So, do you confirm once more that I don't need to care about PAR? If so, what framesize should I capture the standard PAL_B/G 4:3 TV aerial broadcast at? 720x576 or 704x576 (the latter comes closer to the useful 702x576 part of the digitized signal)?

I used to be under the impression that the standard aerial 4:3 PAL TV analog signal becomes a video stream with fs (framesize) 720x576 (after the capture-card digitizes it) of which only a window of 704x576 contains usuful data. So I thought the safe side for capturing the complete analog PAL signal was to always capture at 720x576 (collecting the extra grabage too, and cropping them out if I wanted to, before feeding the MPEG Encoder with it).

But once I started reading more & more, more capturing-fs came along (like 768x576 and 704x576), along with DARs and PARs. This, coupled with the fact that (if I recall corectly) Lordsmurf indicates somewhere on his website that standard TV 4:3 analog PAL signal coressponds aproximately to 340x576 (after digitization), ruined my whole world !!!!

In a nutshell, and because I dont really know how to:

[snip]
Capture at your standard output sample rate (like 13.5 for DVD) and pad/crop to hit the standard frame size. This means you have to know the line length your card samples.

(although I have read all kinds of technical docs I've found -English is not my native language, and certainly NOT all that technical terminology)
should I keep capturing PAL TV 4:3 broadcast at 720x576 or not? From your answer (which seems to contradict Lordsmurf's info) I got that 352x576 is not enough, but it also seems that 704x576 is not either... right? (remember that we're having this conversation in our quest to achieve the BEST & the MOST ACCURATE result we can get in our final (S)VCD/DVD player).

Best regards,

PS. I didn't quote the rest of your answer, because either I somehow already knew the provided info or I covered what I didn't know in this new post. However, I feel the need to thank you once more for taking the time to answer my original post, and I certainly hope you do the same for this new one

[trevlac]

Hi again. You may already know this, but I just wanted to fill in the gaps.

By the way: this was all a big mystery to me not too long ago.

Meaning that I don't have to care about PAR at all? Neither during the capture phase nor during the MPEG Encoding phase?

PAR is a 1 step removed way of talking about sample rate. If you are only interested in TV viewing, PAR just confuses the matter.

Unless we put PAR back to the discussion, neither 720x576 nor 702x576 (nor 704x576, btw) are of DAR=4:3... right? Now, my Philips SAAA7108 chipset capture-card (nVidia GeForce4) does not allow me to capture at 768x576 (DAR=4:3), but it maxes out at 720x576. So, do you confirm once more that I don't need to care about PAR? If so, what framesize should I capture the standard PAL_B/G 4:3 TV aerial broadcast at? 720x576 or 704x576 (the latter comes closer to the useful 702x576 part of the digitized signal)?

702x576 is in fact 1.333 on a PAL TV. This is because the measure is 702 (13.5 MHz) samples x 576 lines. So, since the values are not in the same unit, you can not divide. PAR is then used as an adjustment factor to allow you to divide. However, this is misleading. Pixels and samples do not cover an area. They are points. Or better yet, bites out of a line of continuous information.

Strangely enough, 720x576, 704x576, and 352x576 can also be 4:3. This is because the first 2 simply sample a wider line than the analog 702 (but at the same 13.5MHz rate), the last uses a different sample rate (6.75). So again, the division does not work.

To get it perfect, you must know how much of the signal width is capture by your card. If you know this, you can resize to the proper sample rate. BTW: Resize is often called resample because that is what it really does. The true width of the sample area is not being changed. Just the number of samples. Crop/pad changes the sample area.

Lordsmurf indicates somewhere on his website that standard TV 4:3 analog PAL signal coressponds aproximately to 340x576 (after digitization), ruined my whole world !!!!

In a nutshell, and because I dont really know how to:

[snip]
Capture at your standard output sample rate (like 13.5 for DVD) and pad/crop to hit the standard frame size. This means you have to know the line length your card samples.

(although I have read all kinds of technical docs I've found -English is not my native language, and certainly NOT all that technical terminology)
should I keep capturing PAL TV 4:3 broadcast at 720x576 or not?

Smurf and I don't agree on this point. The number he gives is a TV Line number. To go from TVLines to 4:3 frames, you need to adjust by 4:3. So the number should be 340*1.333 = 454x576. Also, 340 seems a bit low to me. As a ball park number, PAL bcast is probably at least a 520x576 frame size.

The find out what your card does, read this