Forum Archive Home -> Camcorders (DV/HDV/AVCHD) -> Can someone EXPLAIN the whole "720x480" thing to me?
Can someone EXPLAIN the whole "720x480" thing to me? | ||||
| CursedLemon posted 2009 Nov 17 06:49 | ||||
| My DV camera obviously captures its video in 720x480, and I'm just curious what the thought process was behind this whole idea. That is, why capturing a 4:3 video will end up as 720x480, which is obviously NOT 4:3 and has to be filtered to play correctly (or so it appears to uneducated me). | ||||
| guns1inger posted 2009 Nov 17 06:57 | ||||
| DV uses non-square pixels, and these are adjusted by your player on playback. NTSC DVD also uses 720 x 480. Just to add to the fun, 16:9 (widescreen) images are also 720 x 480 (NTSC) or 720 x 576 (PAL).
Don't know what filtering you are talking about, unless you mean de-interlacing. In which case it is only required for playback on your PC. Your TV will take care of it for you. If you want details, try wikipedia. Start your searches with DV video, interlacing, and Aspect Ratios | ||||
| CursedLemon posted 2009 Nov 17 07:01 | ||||
| Yeah, I meant deinterlacing.
Why are non-square pixels used (or why does it end up that way, maybe)? I guess to someone who doesn't understand it, it seems like an unnecessary complication. | ||||
| jagabo posted 2009 Nov 17 08:05 | ||||
| Interlacing is unrelated to resolution.
The choice of 720x480 had to do with early digital video for broadcast. 704 (with the frame padded to 720x480) pixels were deemed necessary to match the horizontal visual resolution of high quality studio analog video. 480 was chosen because it's the nearst mod16 size that can capture all the resolution of the 486 scan lines of NTSC video (6 are cropped away). | ||||
| CursedLemon posted 2009 Nov 17 08:21 | ||||
| So the "square pixels" thing is only for progressive displays like a PC monitor, whereas interlacing is used to make 720x480 images into coherent 4:3 or 16:9 resolutions? | ||||
| jagabo posted 2009 Nov 17 09:03 | ||||
Interlacing is unrelated to pixel aspect ratio. Interlacing had to do with reducing bandwidth requirements for analog broadcast. To get smooth motion TV needed 60 pictures per second. But too much bandwidth was required to broadcast 60 full images every second. So they decided to broadcast 60 half pictures every second instead. One half picture (field) scanned all the even scanlines, 1/60th second later the second half pictured scanned all the odd scanlines. By the time the second field is being displayed the first has faded away. That is, you see only one field at a time. This gives the appearance of a full 480 lines of resolution but at the expense of a picture that flickers when there were sharp horizontal edges. When these half pictures are stored as digital video two halves (fields) are woven together into complete frames. But to properly view the video you need to display one field at a time. The pixels that make up digital images don't have to be square. When they are square (pixel aspect ratio, PAR = 1:1) the final display aspect ratio (DAR) matches the storage aspect ratio (SAR). The general relationship of DAR, PAR, and SAR is: DAR = PAR * SAR So, in general, any frame size can be used to create any display aspect ratio by specifying the shape of individual pixels. | ||||
| Alex_ander posted 2009 Nov 17 09:07 | ||||
720x480 are numbers of the stored samples for horizontal and vertical image dimensions, while 4:3 (or 16:9) is an instruction to display picture using those stored samples (often stored as an additional number corresponding to one of the standard AR's). That is why 720x480=345600 and 4:3=1.3333.. :) . Only in a special case ('square pixels') can you directly derive image proportions from pixel numbers (by definition). | ||||
| CursedLemon posted 2009 Nov 17 09:14 | ||||
| Thanks, guys. :) Stuff like this is so much to wrap your brain around when you only dabble in editing video but every new piece helps build the picture! | ||||
| jagabo posted 2009 Nov 17 10:24 | ||||
The concept of aspect ratios is quite simple:
In the top row a 6 pixel by 4 pixel image drawn with very large square pixels. Its display aspect ratio matches its frame size aspect ratio, 6:4 or 3:2. The second row the pixels are narrower than they are tall. The resulting display aspect ratio is ~4:3. In the third row the pixels are wider than they are tall. The resulting display aspect ratio is 16:9. | ||||
| CursedLemon posted 2009 Nov 17 18:31 | ||||
| So is 720x480 a square-pixel representation of a 480i video? | ||||
| jagabo posted 2009 Nov 17 19:05 | ||||
No. Standard definition 480i is 4:3. 720:480 = 3:2. The pixels are 10 percent taller than they are wide (PAR = 10:11). Note that a 720x480 video actually contains the 4:3 image in a 704x480 portion of the frame. There are 8 pixels added to each side for padding. So the full 720 pixel wide frame is slightly wider than 4:3. Using just the 704x480 part: DAR = PAR * SAR 4:3 = 10:11 * 704:480 4/3 = (10/11) * (704/480) 4/3 = (10 * 704) / (11 * 480) 4/3 = 7040 / 5280 1.333 = 1.333 | ||||
| CursedLemon posted 2009 Nov 17 19:15 | ||||
| Haha, wow. I never would've guessed you could figure it mathematically like that. I think I understand now, thanks a bunch. :) | ||||
| ZQX posted 2009 Nov 18 00:21 | ||||
| Yes thank you jagabo, that was succinctly explained. | ||||
| Alex_ander posted 2009 Nov 18 07:36 | ||||
As already mentioned, no. An example of 'square pixel' video: full resolution HD 1920x1080 video (1920/1080=16:9). Some other BD resolutions like 1440x1080 are not 'square pixel' but the video is still stretched to fullscreen which is always 16:9 (no additional number stored). In case of NTSC DVD the 704x480 part of the stored 720x480 samples represents a 4:3 (or 16:9) image. Its visible part (unless intentionally cropped to 704 at pre-processing) is 711 horizontally, so on a display without overscan NTSC image is actually not 4:3. Those numbers describe the thing completely. | ||||
| jagabo posted 2009 Nov 18 08:08 | ||||
DAR = PAR * SAR DAR = 1:1 * 1920:1080 DAR = (1/1) * (1920/1080) DAR = 1 * 1.778 DAR = 1.778 ==> 16:9 widescreen | ||||
| MOVIEGEEK posted 2009 Nov 18 10:47 | ||||
Math is too hard for me but this picture makes sense :P
 | ||||
| edDV posted 2009 Nov 18 12:02 | ||||
| A few more details for US domestic digital video.
ATSC SD digital video is 704x480. It can be either 4x3 or 16x9. Neither are square pixel. ATSC HD 1920x1080 or 1280x720 as transmitted are always 16x9 and square pixel. The cameras and video recorders used in HD production are often non-square. They are converted to square pixels for transmission. All these formats are 16x9. HDCAM or HDV are 1440x1080i or 1280x720p. XDCAM-HD can be 1920x1080p, 1920x1080i, 1440x1080i or 1280x720p DVCProHD (often used for news/reality) is 1280x1080i or 960x720p. AVC-Intra is 1920x1080p, 1920x1080i or 1280x720p HDCAM-SR is 1920x1080p (RGB) or 1920x1080i | ||||
| CursedLemon posted 2009 Nov 18 16:37 | ||||
| Am I correct in saying that PAR is a quality of the display type, not of the video? | ||||
| edDV posted 2009 Nov 18 16:45 | ||||
Not exactly. The video is sent with a flagged or implied PAR. The display processor reads the flag or offers user options to scale horizontally. We are talking an HDTV here. Computer monitors are dumb as dirt. They don't read flags or offer aspect options. That is the job of the display card and/or software player software. | ||||
| CursedLemon posted 2009 Nov 18 17:17 | ||||
| Okay, wait.
Earlier in the thread I wanted to know if the 720x480 representation of my recorded 4:3 DV files was due to them being displayed in a 1:1 PAR environment. Am I actually wrong in assuming that? | ||||
| edDV posted 2009 Nov 18 17:47 | ||||
"720x480 representation of my recorded 4:3 DV files" ? Your DV camera could have been any sensor resolution. If you switched the camcorder to 4:3 DV mode (or it defaulted to 4:3 mode) then the camcorder processor scaled the sensor resolution to 720x480 with implied 0.9091 (NTSC DV) PAR. 720x480 is not square pixel 4x3. If you delete the 16 side pad pixels and multiply 704 by 0.9091 you get 640, or 640x480 square pixel 4x3 for display on a square pixel monitor. If you switched the cam to 16x9 wide mode, the camcorder processor would have scaled the sensor resolution to 720x480 with implied 1.2121 (NTSC DV wide flag) PAR. If you delete the 16 side pad pixels and multiply 704 by 1.2121 you get ~854, or 854x480 square pixel 16x9 for display on a square pixel monitor. On second read of your post, it can be interpreted a different way. If 720x480 is displayed with 1.0 square pixel PAR (a lazy player?), then it isn't being displayed 4x3 aspect, it is being horizontally stretched to 3x2 aspect ratio. | ||||
| CursedLemon posted 2009 Nov 18 18:03 | ||||
| Okay, so either way NTSC DV files are recorded as 720x480, and all my camera is doing is changing the PAR, yes? I guess the thing I'm having trouble understanding is how any particular display type deals with showing a video of a different PAR type.
EDIT: Yes, that's kind of what I meant. Sorry. 8( | ||||
| CursedLemon posted 2009 Nov 18 18:19 | ||||
| Okay, I think I'm kind of looking at video information the wrong way, in that I'm not thinking of pixels as simply "assignments" of data, but rather the data itself, which I assume is incorrect. | ||||
| edDV posted 2009 Nov 18 18:24 | ||||
We have been talking display more than internal camcorder processing. DV video comes with two aspect ratio options. No flag = 4x3. Wide flag tells the player to display 16:9. MPeg2, ATSC SD and DVD work the same way. It is the player's job to process the incoming video to 4x3 or 16x9 for that particular display. Most players require you to define your display in preferences as 4x3 or wide. If the display is 4x3 and the video is flagged wide, you get letterbox. If the display is wide and the video is 4x3, you get side pillars. Camcorder internals are a different story. We can go through that if you want. | ||||
| edDV posted 2009 Nov 18 18:38 | ||||
Think of a DVD analogy. If you buy the "full screen" DVD version and play it on a wide TV, you get a centered 4x3 picture with side pillars. You can tell the player to "stretch" or fake wide but everybody gets fat. Or, you can buy the wide DVD which plays fine on a wide TV but for a 4x3 TV you need to select letterbox or "pan/scan" which chops off part of the wide picture. Both the "full" and "wide" versions of the DVD are 720x480. | ||||
| CursedLemon posted 2009 Nov 18 18:57 | ||||
| So the PAR is completely independent of the actual video data, correct? | ||||
| jagabo posted 2009 Nov 18 19:02 | ||||
| Think of it like this: pixels are logical constructs, points that have no physical dimensions. It's when they are displayed on the screen that they take on a physical size and shape. Without any PAR or DAR information most programs assume pixels are square. Since most physical displays have square pixels you end up with a final display aspect ratio that matches the frame size aspect ratio. So a 720x480 video without (or ignoring) PAR/DAR information will be displayed as 720x480 pixels (or some other scaled size that matches that 3:2 aspect ratio like 360x240) on the physical screen.
When PAR information is included the program should scale the image to compensate for the non-square pixels. A 720x480 4:3 DAR video should be displayed as 640x480 (or some other 4:3 frame size) on the screen; or with 16:9 DAR as ~853x480. But many programs do not pay any attention to the PAR/DAR flags and continue to assume square pixels. Editors often show pixels of the source 1:1 with the display so you don't see artifacts generated by digitally scaling to adjust for the PAR/DAR. | ||||
| CursedLemon posted 2009 Nov 18 19:36 | ||||
| Yeah, that understanding of pixels definitely clears my head about some of this. Thanks a bunch. | ||||
| edDV posted 2009 Nov 18 19:42 | ||||
| You may be missing that the video recorded 720x480 by the camcorder is very different when in 4x3 vs 16x9 mode.
In 4x3 mode the video as seen 720x480 square pixel is horizontally stretched. In 16x9 mode video as seen 720x480 square pixel is horizontally compressed. Here is an example of "wide" 720x480 (horizontally compressed). 
Here is the same picture horizontally stretched to 16x9.  | ||||
| edDV posted 2009 Nov 18 20:03 | ||||
| A full size example
720x480 (DV wide) horizontally squeezed 
854x480 (16x9) square pixel  | ||||
| CursedLemon posted 2009 Nov 18 20:18 | ||||
| Now wait. I think through all this I don't ever actually recall understanding why non-square pixels are used at all. | ||||
| edDV posted 2009 Nov 18 21:01 | ||||
A few reasons for standard definition. - 704x480 (720x480 with pads) dates back to CCIR-601 (1985) for digital broadcast (aka D1). The 13.5MHz horizontal sample rate was chosen for compatibility for both NTSC and PAL. That allowed common NTSC/PAL equipment and eliminated horizontal resampling for NTSC to PAL conversion. Analog NTSC had 486 active lines. This was reduced to 480 lines for memory efficiency. Thus a 4x3 square pixel frame became 640x480. Since PAL has 576 lines, a square pixel SD PAL frame became 768x576. - CCIR-601 was extended in the late 80's to allow 4:3 and 16:9 programming using the same transmission and recording equipment. The data stream is similar except for the wide flag. Display equipment looks for the flag so it knows to display as 4:3 or 16:9. - In the early 90's MPeg2 and later DVD adopted the same resolutions and sample rates as CCIR-601 broadcast. DVD added half D1 (352x480 and 352x576), MPeg2 scaled down from there to 352x240, 352x288 and lower for internet and telecom video. - In the late 90's ATSC and DVB digital broadcasting also used the CCIR-601 (ITU-Rec-601) spec for standard definition. Later VC1 and h.264 AVC adopted the same resolutions. All this was done by the broadcast and telecommunications industry. It wasn't until ~Y2K that the computer industry looked up from their 1MByte RGB square pixel frame buffers. | ||||
| edDV posted 2009 Nov 18 21:16 | ||||
| HD broadcasting could have gone to non-square pixels for transmission efficiency. For example few could tell the difference between 1440x1080 and 1920x1080 with current equipment. 1440x1080 uses two thirds the bit rate for equivalent compression or would show higher quality at the same bit rate. Consumer video quality is limited more by compression formats than resolution. Still the standards bodies went with 1920x1080 to allow headroom for quality improvement under the assumption that compression quality will continue to improve.
A major accomplishment for the HD standards committees has been world wide agreement on square pixel 1920x1080 and 1280x720 high definition resolutions. Frame rate disparity still exists between "NTSC" and "PAL regions due to the need for standard definition interoperability. The international film and TV program producers are converging on 24p (23.976p) as an international media exchange standard for "movies" and low motion TV drama. | ||||
| CursedLemon posted 2009 Nov 18 21:56 | ||||
| Boy, this feels like all of my audio education hitting me like a sack of potatoes all over again. :D Thanks for being so informative. | ||||
| 2Bdecided posted 2009 Nov 19 04:09 | ||||
| @edDV,
There are plenty of anamorphic HD broadcasts (e.g. BBC HD is 1440x1080 in the UK, and AFAIK several in the USA are 1440x1080 or 960x720, depending on the platform) and anamorphic HD recording formats (HDCAM (not SR), DVC-Pro HD, and of course HDV). Cheers, David. | ||||
| 2Bdecided posted 2009 Nov 19 04:25 | ||||
Rec. ITU-R BT.656-4, which defines SDI (and an obsolete parallel interface) i.e. ways of transporting D1 video - does define the vertical blanking - but still includes 488 lines for NTSC. (It includes the CGMS-A and CC lines. I think previous issues of the spec missed the CGMS-A lines, making it 486 exactly). I think it's only the emergence of MPEG (DVD, digital broadcasting) which brought us to 480-lines for NTSC video - though PCs were there much earlier. It's a multiple of 16, which works well for both. Much simpler for PAL - it's always been 575 analogue, 576 digital. Cheers, David. | ||||
| lordsmurf posted 2009 Nov 19 05:08 | ||||
| I don't know that I'll ever find use for this information. Interesting, however. | ||||
| totalz posted 2009 Nov 24 21:53 | ||||
| What about a NTSC DVD with DAR of 2.35:1? Is it still encoded in 720x480? In which case the display resolution would be 1128x480? And the PAR would be 47:20? | ||||
| jagabo posted 2009 Nov 24 22:00 | ||||
| There are no DVDs with a DAR of 2.35:1. 2.35:1 movies are letterboxed in 16:9 DAR 720x480 frames. | ||||
| poisondeathray posted 2009 Nov 24 22:01 | ||||
It still has a frame of 720x480, but there are black borders encoded into the top and bottom (letterboxing). The PAR is the same 32:27 (or 40:33 if using MPEG4 specification). The frame will display as 852x480 (or 853x480) in most media players, but there will be letterboxing whoops jagabo too quick :o | ||||
| jagabo posted 2009 Nov 24 22:04 | ||||
| Sometimes 2.35:1 movies are letterboxed into 720x480 4:3 DAR frames. Yuck! | ||||
| edDV posted 2009 Nov 24 22:30 | ||||
ATSC currently is square pixel MPeg2 for HD. I considered going into QAM and DVB but that would only confuse the OP who is in North America. I explained the production formats above but for ATSC HD broadcast square pixels rule. | ||||
| edDV posted 2009 Nov 24 23:09 | ||||
I was trying to avoid details. Analog NTSC was 486 lines plus 2 vertical interval. There was a big spat in the mid 80's whether broadcast of 480 lines with 3 line black crops top and bottom violated FCC rules. Computer frame buffers of the day were 640x480 and the ram cost to go 486 lines was costly. This affected most character generators, weather machines, frame syncs and CGI full frame effects. The second issue related to A/D of analog video. It was argued previous production required 486 lines because of creative intent of the director. The digital TV geeks claimed a slight letterbox crop was in the overscan area and should be allowed. The FCC studied the issue for many many months causing much pain for us digital equipment makers. Finally they wimped out saying letterbox was a programming decision. 480 line letterbox with 6 fill lines was in program space and thus not regulated. Later the standards were amended to allow 480 lines. I previously posted a link detailing even more digital video vs. legacy controversy. I'll try to find it. | ||||
| edDV posted 2009 Nov 24 23:31 | ||||
| PS: Here it is and it has some updates. Proves that everything we know is wrong.
http://lipas.uwasa.fi/~f76998/video/conversion/ More useful reading http://www.bbc.co.uk/commissioning/tvbranding/picturesize.shtml | ||||
| totalz posted 2009 Nov 25 01:15 | ||||
| Thanks jagabo & poisondeathray for the info...
This may be off-topic, but if I want to transcode a 16:9 720x480 NTSC dvd, do I keep the same setting? Or 854*480? I prefer quality :p | ||||
| Alex_ander posted 2009 Nov 25 02:47 | ||||
If you want to transcode to something 'square pixel', then 852x480 (mod4) is a possible final size. However by ITU, in original video only 704x480 samples of those 720x480 represent the 16:9 AR part of video, so for correct proportions you need to crop to those numbers just before resizing. The visible part of image within 720x480 may be wider than 704 (like 711x480 for NTSC DV/DVD). If you don't want to crop off any visible bit, here's another option: first resize 720x480 to 872x480, then crop horizontally to visible (rounding to mod4). The 872 number comes from 853x720/704 proportional compensation. Apart from AR considerations, take care of interlacing, telecine or whatever type of the original video. | ||||
| totalz posted 2009 Nov 25 03:33 | ||||
Thanks for the mod4 reminder : ) ITU - International Telecommunication Union??? Probably not!? Googled it : Results 1 - 10 of about 36,600,000 for ITU [definition]. (0.16 seconds) Does it improve quality with square pixel? I mean 1:1 is always good, but since the source is not, does square pixel works better with H264? How can I find out the actual image width, in the case as you've mentioned above? | ||||
| Alex_ander posted 2009 Nov 25 04:02 | ||||
Yes, that's from its standards. http://www.itu.int/en/pages/default.aspx Does it improve quality with square pixel? Only in case your resize filter is better than that of a playback device. The stretched video will need higher bitrate for the same encoding quality. But it is compatible with devices which don't support AR flags. Otherwise you can use a container like matroska which supports those flags, and use original resolution. How can I find out the actual image width, in the case as you've mentioned above? Actual image width depends on how the video was produced. In theory, it is about 711 just after capture but it may have been cropped to 704 at production. If 711, then the visible part will be 711x872/720=860, not exactly 16:9 of course (same as in source). | ||||
| 2Bdecided posted 2009 Nov 25 04:52 | ||||
http://forum.doom9.org/showthread.php?p=1072530#post1072530 http://forum.doom9.org/showthread.php?p=1100187#post1100187
Angels on a pin head anyway - most production treats 720x576 as defining the active picture now - an error of 2.3% (16 pixels) - it makes these single pixel errors kind of irrelevant. Cheers, David. | ||||
| totalz posted 2009 Nov 25 05:33 | ||||
[quote="Alex_ander"]
Great, I guess I will stick to the concept of anamorphic, less bitrate, smaller file size : ) According to wiki, http://en.wikipedia.org/wiki/Dvd_video. A full D1 NTSC is 720x480, so, is there such a thing as 720 visible width? Cause I found the PAR for NTSC 16:9 is 40:33, which is perfect for 704x480! So, is there a software I can use to determine the actual visible width? Or I will have to do as you suggest "first resize 720x480 to 872x480"? | ||||
| Alex_ander posted 2009 Nov 25 06:33 | ||||
PAR can't be a reason for anything (it can be a result of smth. at best). The things like PAR are artificially derived from real things, defined in standards. The standard method of conversion analog NTSC to digital (720 samples per TV line, 13,5 MHz sample frequency) leads to such a position of samples in time, that ~711 of 720 belong to visible part of analog TV line, the rest are in adjacent analog blanking intervals. Those 711 correspond to horizontal dimension of a 4:3 (16:9) analog image. Vertically there are 485 analog lines defining the second visible dimension of that image. Since only 480 lines (= vertical samples) are used (for rounding numbers to mod 16 etc.) and the rest are dropped, the 711x480 can't be 4:3 (16:9) while 704 can, so that number was adopted (and both are mod16) and that is why we have 704x480 samples responsible for the declared AR's. Combining those numbers with displayed AR, one can get those 'PAR's' (if desired). The rest of 711 are present and visible within 720 (unless intentionally blanked at further digital processing) but they don't define the nominal AR. So, is there a software I can use to determine the actual visible width? Or I will have to do as you suggest "first resize 720x480 to 872x480"? You can use a cropping tool in AVSP+AviSynth or Vdub and look at the crop numbers while watching the screen, or use some screen measuring tools. | ||||
| jagabo posted 2009 Nov 25 06:40 | ||||
Where are you getting 575 lines? Yes, the top and bottom scanlines are half scanlines, but they are half width, not half height. If you look at the analog signal on a scope the top scanline starts half way across the width of the picture, the bottom scanline ends halfway across the picture. If you capture those lines you capture a full line of pixels for each, 576 lines total. If you underscan the picture on a CRT you'll see 576 scan lines with only the right half with picture information on the top scanline and only the left half on the bottom scanline. In a digitized NTSC signal the active 4:3 picture with is ~711 pixels and the active signal height is 486 pixels. Since we throw away 6 scanlines from the height we must throw away some pixels from the width. 711 * 480 / 486 ~= 702. But we use 704 to keep a mod16 frame size. | ||||
| Alex_ander posted 2009 Nov 25 08:34 | ||||
ITU-R BT.1700 (2005, Russian version) (table 4) The number of active lines---------------------------- 575. The basic analog AR can't be defined for a different number of lines. If you count analog lines in only left or right part of the screen, it will be 575 (without the second half-line in the opposite part). In case of DV camera it is possible to capture 576 full digital lines due to 1 line shift in DV standard from ITU for PAL and narrower analog field blanking. But image proportions still come from analog line count and if you use 576 to define AR in digital form, the horizontal number of samples will be also different from analog line length (which is 702). | ||||
| 2Bdecided posted 2009 Nov 25 09:30 | ||||
They look like this: http://www.pembers.freeserve.co.uk/World-TV-Standards/Raster-Top.jpg from here: http://www.pembers.freeserve.co.uk/World-TV-Standards/Line-Standards.html In an analogue video signal, each sync pulse is there to drive one or other circuit in a TV to ensure the vertical scan (50Hz) and horizontal scan (15.625kHz) are synced to the incoming signal, such that the picture ends up in the right place, with the correct field alignment and interlacing. The circuits in conventional TVs have very little "intelligence" - it's the analogue video signal itself that defines these things. And that signal drives those circuits to give lines where each line's height is proportional to its length - and in a very real sense, a picture with a half line at the top and the bottom is shorter (or longer!) than a picture with a full line at the top and bottom, and also closer to being "square" than one with a full line at the top and bottom. Don't forget: they could easily have not had those half lines in the analogue standard - but they chose to put them there. Why? Well, there are lots of things they help with post-fact - but one undeniable feature is that they make the picture a rectangle, rather than a parallelogram. Now, the idea that any of this really mattered (down to the equivalent of one modern pixel) in the analogue world is a bit fanciful - but the system is built properly to have a theoretical "perfect" alignment and adjustment and signal timing that gives a picture, in digital parlance, that's exactly 702x575. Does it matter? No. Especially not now everything is digital. But that page by Jukka Aho takes great delight in pointing out this "dirty little secret". Whereas, as long as you excuse people for rounding to the nearest pixel (and what else do you expect them to do?!) there is no dirty little secret. Just a fundamental misunderstanding on that page, which created an apparent pixel or two of error that doesn't really exist! So the shocking conclusion is that most of the international standards are correct, and do work. Cheers, David. | ||||
| jagabo posted 2009 Nov 25 11:31 | ||||
OK, I understand where this argument comes from now. Given a rectangular 4:3 display the lines are displayed slanted (a simple linear ramp is used for the vertical position of the electron beam) such that the top and bottom lines are half outside the display. Hence they are "half height" on the physical display resulting in a total of 575 lines. But by this same argument, the individual scanlines are slightly longer than the 4 part width of the physical display. And the height of that slanted image (along its own, slanted, axis, not the axis of the physical screen) is slightly taller than the 3 part height of the physical display. In any case, we all agree that these small details aren't worth worrying about. With digital video files you treat 704x480 and 704x576 as the 4:3 or 16:9 image. The extra 16 pixels used to fill a 720 pixel wide frame may or may not be displayed by any particular device but don't count as part of the 4:3 or 16:9 image. | ||||
| edDV posted 2009 Nov 25 11:39 | ||||
So long as 704x480/576 is 16x9 and the extra pixels represent additional picture width. This is necessary for proper upscale/downscale to/from 1920x1080 or 1280x720. In a production context all players need to follow common rules or graphics, mattes and object layers fail to line up in the composite forcing a rescale or worse a recode for compressed layers. These issues cause work rejection and lost work hours. I refuse to start a project unless picture standards are agreed in the contract. If the client wants 720x480 to be 16x9 he needs to say so up front and understand the issues. For example, 720x480 gets cropped to 704x480 for ATSC or cable SD broadcast and/or the sides will be cut off in a 1080i or 720p upscale. | ||||
| 2Bdecided posted 2009 Nov 25 17:06 | ||||
I'm all for saying that 704x576 and 704x480 are the correct aspect ratio. That's what I've been saying since I worked through it last year. It's that website, claiming a dirty little secret, that says otherwise. And the point is, that's wrong - 704x576 and 704x480 really are correct to the nearest pixel, so they're both convenient and accurate. Shame the world is now often using 720 instead! Cheers, David. | ||||
| jagabo posted 2009 Nov 25 17:34 | ||||
But the lines are scanned diagonally -- so they are longer than the width of the 4:3 screen. Although, given the ~0.1 degree angle, the difference is far, far less than a pixel. (I made a mistake in my first back-of-the-napkin calculation and thought it was about 1 pixel but it's closer to 1/1000 of a pixel.) So I'll have to agree, the top and bottom scanlines can be considered half height under this scenario. | ||||
| manono posted 2009 Nov 25 18:03 | ||||
Not always. Classic 'fullscreen' films, for example, are 1.37:1 rather than 1.33:1, and using the full 720x480/576 on DVD is closer to being accurate. http://en.wikipedia.org/wiki/Academy_ratio | ||||
| edDV posted 2009 Nov 25 18:30 | ||||
That is the usual argurement that 720 doesn't matter for DVD but it means a certain level of incompatibility with digital broadcasting or most telecommunications which chop to 704 before scaling. I don't know for sure but I'd bet most display scalers do the same. | ||||
| manono posted 2009 Nov 25 19:03 | ||||
Yeah, but I specifically mentioned DVD. I know nothing (and care even less) about what the broadcasters do.
If you're saying a DVD player and/or TV set will crop 8 off of each side before resizing to, say, 1440x1080 with pillarbars then, at least in my own case, I know that to be untrue. | ||||
| edDV posted 2009 Nov 25 19:12 | ||||
I don't think DVD players do. The analog outputs seem to carry the full width. But I wonder if all "wide" 720 pixels survive flat screen plasma or LCD-TV "just scan" modes. Almost all sets otherwise overscan H and V more than 16 horizontal pixels. This calls for an appropriate test pattern. PS: I see your point on 4x3 to pillarbox. I was thinking 16x9. I need to test that too. | ||||
| manono posted 2009 Nov 25 20:27 | ||||
Many DVD players - perhaps the majority of them out there - crop the image at least somewhat before sending it to the TV set.
Like I said, I can only speak for myself. My Sony Bravia 1080p set doesn't overscan at all and doesn't crop anything from the picture before resizing. I can prove that if jagabo would point me to his overscan picture. I tested it in my TV once and then posted the pic from a camera here, but I have no idea where that post is now. | ||||
| jagabo posted 2009 Nov 25 21:47 | ||||
Here's a 720x480 crop test image. It's only marked to 20 pixels, not enough for overscan testing, but enough for testing DVD player cropping.
And a 704x480 version if you want to test that too. 
I think I've done this test before and found that some DVD players cropped the 16 extra columns, some didn't. | ||||
| 2Bdecided posted 2009 Nov 26 04:18 | ||||
| jagabo, are they yours, or copied? I have something similar on a test DVD.
On your 704x480 image, the scale on the right side it out by one pixel. What you've labelled 0 is actually 1. On their SD analogue outputs, all the DVD players I've tested remove a little of the 720-pixel image, windowing it down to ITU/SMPTE width (approx). On the BBC, their HD programmes (1440x1080) are resampled down to 720x576 to fill all pixels - so assuming 720 = 16x9 - i.e. incorrectly as far as most of us are concerned. As you say edDV - you need to make sure everyone working on a project agrees on this! Cheers, David. | ||||
| jagabo posted 2009 Nov 26 07:44 | ||||
The 720x480 image was downloaded from somewhere long ago. I think I modified it a bit to suit my needs.
Thanks, I'll fix that. | ||||
| edDV posted 2009 Nov 26 16:00 | ||||
| My initial tests on the Vizios show near perfect edges in 16x9 Wide but in 4x3 pillarbox mode vertical is OK but horizontal is cropped over 20 pixels each side. There are many combinations of settings for the Phillips 5992 and TV. This will take some time but it is nice to have the chart. | ||||
| manono posted 2009 Nov 27 00:02 | ||||
Here's mine using the 720x480 pic encoded as 4:3:
and as 16:9: 
The DVD player is an Oppo DV-981HD. The TV is a Sony Bravia KDL-46VL130. That's what Costco called it. It's called something else most other places. They're connected via HDMI. In case anyone wants to try this themselves to test for cropping and/or overscan, here's a link to an ISO of an NTSC DVD I made (4.3 MB): http://www.mediafire.com/?mmmcy2mznno There's a rudimentary menu with button links to the 4:3 and 16:9 versions of the picture, both of which play for 10 seconds. Use ImgBurn to burn it to DVD-RW, DVDR, or CD if your player can play Mini-DVDs. |
Login/Register to our forum to be able to post here.