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DVB 544x576 mode - pixel aspect ratio and centre point?
The DVB standards (in particular, ETSI TR 101 154) define that
544x576 is a valid MPEG-2 luminance sampling grid resolution for DVB broadcasts. The standards also suggest that if 544x576 is to be converted (upsampled) to ITU-R BT.601 pixels, the figure 544 needs to be multiplied by 4/3. However, this calculation will give 725.333... ITU-R BT.601 pixels, which is a tad more than the typical framebuffer width of 720 or 704 pixels. Using all those pixels in real world equipment would mean extending into the horizontal blanking period, not to mention using a wider framebuffer than is usually used! Obviously, something needs to give in. Now, is the intention of the 544x576 format that only a 540 pixel wide area of the sampling grid is actually used, and maps to 720 ITU-R BT.601 pixels (and the extra 4 pixels are there just to satisfy the MPEG-2 needs-to-be-evenly- divisible-by-16 technicalities, and can be cropped off before the conversion?) This would make sense, since 540 * 4/3 = exactly 720 (and 540 is not evenly divisible by 8 or 16, but 544 is.) If the above assumption is correct, where is the centre point located in this 544x576 image? For example, if I want to convert a 720x576 (13.5 MHz) image to the 544x576 format, do I first downsample to 540x576 and then add 2 pixel columns to the left and the right side (making the middle point in the original picture the middle point of the 544x576 picture), or do I just add a 4 pixel column on the right side (in which case the idea would be that the rightmost 4-pixel column is just slack space for the MPEG-2 divisible-by-16 requirements)? Is there any established, standard way of doing it? (I am asking this because it has been suggested that I should add this common DVB resolution to my aspect ratio guide & conversion table at http://www.iki.fi/znark/video/conversion/, but I am not exactly sure if my above-mentioned assumptions about this sampling grid format are valid.) -- znark |
"Jukka Aho" wrote in message ... The DVB standards (in particular, ETSI TR 101 154) define that 544x576 is a valid MPEG-2 luminance sampling grid resolution for DVB broadcasts. The standards also suggest that if 544x576 is to be converted (upsampled) to ITU-R BT.601 pixels, the figure 544 needs to be multiplied by 4/3. Am I right in thinking this is the resolution for a 4:3 picture broadcast 12P16? |
Mat Overton wrote:
544x576 Am I right in thinking this is the resolution for a 4:3 picture broadcast 12P16? 544x576 is supposed to be stretched over the full width of 16:9 or 4:3 screen, so this is more like a bandwidth-saving, slightly lower resolution format for the penny-pinching broadcasters. :) -- znark |
Mat Overton wrote:
"Jukka Aho" wrote in message ... The DVB standards (in particular, ETSI TR 101 154) define that 544x576 is a valid MPEG-2 luminance sampling grid resolution for DVB broadcasts. The standards also suggest that if 544x576 is to be converted (upsampled) to ITU-R BT.601 pixels, the figure 544 needs to be multiplied by 4/3. Am I right in thinking this is the resolution for a 4:3 picture broadcast 12P16? I believe this is close to being the case. However this resolution is ALSO used as a full-width DVB standard, indeed quite a few DTT and DSat services use it for both 4:3 and 16:9 services. (AIUI all ITV1 variants on DSat are 544x576 resolution - and they broadcast full frame 4:3 and full frame 16:9 on this platform) On DTT I think ITV1 is 720/704x576 (can't recall which) but the ITV News Channel (4:3 only) is 544x576, as is bid.up TV I think (16:9 only) Steve |
Jukka Aho wrote:
The DVB standards (in particular, ETSI TR 101 154) define that 544x576 is a valid MPEG-2 luminance sampling grid resolution for DVB broadcasts. The standards also suggest that if 544x576 is to be converted (upsampled) to ITU-R BT.601 pixels, the figure 544 needs to be multiplied by 4/3. However, this calculation will give 725.333... ITU-R BT.601 pixels, which is a tad more than the typical framebuffer width of 720 or 704 pixels. Using all those pixels in real world equipment would mean extending into the horizontal blanking period, not to mention using a wider framebuffer than is usually used! Obviously, something needs to give in. Now, is the intention of the 544x576 format that only a 540 pixel wide area of the sampling grid is actually used, and maps to 720 ITU-R BT.601 pixels (and the extra 4 pixels are there just to satisfy the MPEG-2 needs-to-be-evenly- divisible-by-16 technicalities, and can be cropped off before the conversion?) This would make sense, since 540 * 4/3 = exactly 720 (and 540 is not evenly divisible by 8 or 16, but 544 is.) If the above assumption is correct, where is the centre point located in this 544x576 image? For example, if I want to convert a 720x576 (13.5 MHz) image to the 544x576 format, do I first downsample to 540x576 and then add 2 pixel columns to the left and the right side (making the middle point in the original picture the middle point of the 544x576 picture), or do I just add a 4 pixel column on the right side (in which case the idea would be that the rightmost 4-pixel column is just slack space for the MPEG-2 divisible-by-16 requirements)? Is there any established, standard way of doing it? (I am asking this because it has been suggested that I should add this common DVB resolution to my aspect ratio guide & conversion table at http://www.iki.fi/znark/video/conversion/, but I am not exactly sure if my above-mentioned assumptions about this sampling grid format are valid.) My gut instinct would be to centre the 540 within the 544, in the same way that the 702 samples are centred within the 720. (702 samples being the 4:3/16:9 active area - with the extra 18 adding a little extra width in digital systems) I believe that the 702 samples are centred within the 704 sample variant as well? Steve |
Stephen Neal wrote:
My gut instinct would be to centre the 540 within the 544, in the same way that the 702 samples are centred within the 720. Yes, this would sound logical. The digital active image area of 544 pixels @ 10.125 MHz being even wider than 720 pixels @ 13.5 MHz just made me suspect that maybe there is some special way of handling these images. (Also, I have seen a couple of 544x576 DVB streams where the image had a wider right border than the left one, but maybe this was just coincidental.) I believe that the 702 samples are centred within the 704 sample variant as well? Can't say I would have seen any official guide as for how to handle this, but as there is an EBU recommendation about centering 702x576 in the middle of 720x576, I have always believed that the same principle applies to 704 pixel wide 13.5 MHz sampling grids, too. Who writes these standards, anyway, and why don't they document these things better? What do they get paid for? :) -- znark |
"Jukka Aho" wrote in message
... Can't say I would have seen any official guide as for how to handle this, but as there is an EBU recommendation about centering 702x576 in the middle of 720x576, I have always believed that the same principle applies to 704 pixel wide 13.5 MHz sampling grids, too. Who writes these standards, anyway, and why don't they document these things better? What do they get paid for? :) The real question is what do you have to smoke to devise standards whether the ratio of pixels in the horizontal and vertical directions is neither 4:3 nor 16:9? Does it not make eminent sense to have the same resolution (pixels/mm) in both directions at least for *one* of 4:3 and 16:9? But then the obscure numbers that have become enshrined in standards have always intrigued me. Why are audio tape speeds not integer numbers of inches per second (eg 2, 4, 8, 16 etc) or else round numbers of cm/sec? Why is the UK railway gauge 4' 8½" rather than 4' 6", 4' 9" or 5' (or else round numbers of centimetres)? Why are computer drives either 5¼" or 3½" rather then 5" and 3"? Why is there not an integer number of cubic inches in a gallon (so as to relate linear and volumetric measurement)? Why are all imperial units related by factors other than ten (eg inches/foot, yards/mile, yards/chain, pounds/stone etc)? Why is cinema film shot at 24 frames/sec rather than 20, 25 or 30, given that 50Hz (halved to derive 25 Hz) and 60Hz (halved to derive 30 Hz) are the standard mains frequencies? The weirdest one is the spec for high-definition TV which uses 1080 lines - neither 2xPAL (1250) nor 2xNTSC (1050) lines, so *both* standards will have to be interpolated using weird conversion factors (leading to loss of sharpness) when showing old low-definition material on high-definition. Maybe I just like numerical simplicity: if I had a clean sheet of paper to devise a standard, I'd always ensure that the numbers that it used were nice and simple in whatever system of units (presumably SI) that I used. Going further off-topic, why are many rev counters in cars labelled with numbers like 10, 20, 30 x 100 rpm rather than the engineering standard of expressing small or large numbers as powers of 1000 (eg 1, 2, 3 x 1000 rpm) - so as to express them as numbers in the range 1-999.999 x 1/1000000, 1/1000, 1, 1000, 1000000 etc? |
On Sat, 28 Feb 2004 18:50:00 GMT, "Martin Underwood"
wrote: | Why is the | UK railway gauge 4' 8½" rather than 4' 6", 4' 9" or 5' (or else round | numbers of centimetres)? That one at least is easy, it depends on the width of the bums of two horses. The original ?railway? trucks were ordinary carts, with wheels 4" 8 1/2" pulled by horses, along wooden rails. Wooden rails were much smoother than cart tracks. As improvements like iron rails, and steam locomotives the gauge stayed the same. Roman roads have ruts of much the same gauge. http://www.spartacus.schoolnet.co.uk/RAgauge.htm Isombard Kingdom Brunell had a great idea when he designed the Great Western Railway. He built it in the technically much better Broad Gauge 2.2m but the standard won out against the better. You can see a replica of the broad gauge Iron Duke locomotive at National Railway Museum in York. All standards happened in a similar sort of way :-((((((((( -- Dave Fawthrop dave hyphenologist co uk Killfile and Anti Troll FAQs at http://www.hyphenologist.co.uk/killfile. |
Martin Underwood wrote:
The real question is what do you have to smoke to devise standards whether the ratio of pixels in the horizontal and vertical directions is neither 4:3 nor 16:9? Does it not make eminent sense to have the same resolution (pixels/mm) in both directions at least for *one* of 4:3 and 16:9? Well, I am afraid everything has its reasons... it is just that the reasoning behind the choices is not necessarily written down in the standards (which does not help in understanding them), and implementation hints are scarce! Here is quite an interesting article about those ITU-R BT.601/656 numbers: http://groups.google.com/groups?selm...Gy%40hpqmoea.s qf.hp.com More of the same... http://groups.google.com/groups?selm...%2475a048c0%24 712b7c0a%40pc-l301385.wn.bbc.co.uk .... and yet more! http://groups.google.com/groups?selm...%24ef84a7e0%24 652b7c0a%40pc-234866.wn.bbc.co.uk (The URLs above have been split on two lines. If your newsreader cannot handle them automatically, please copy them into your web browser in two parts.) -- znark |
"Martin Underwood" wrote But then the obscure numbers that have become enshrined in standards have always intrigued me. Why are audio tape speeds not integer numbers of inches per second (eg 2, 4, 8, 16 etc) or else round numbers of cm/sec? Early wire recorders ran at 60ips (and were VERY dangerous for the operators if the wire broke, which happened all too frequently). Speeds of tape followed, and halved as technology allowed, only going "wrong" below 15ips. The weirdest one is the spec for high-definition TV which uses 1080 lines - neither 2xPAL (1250) nor 2xNTSC (1050) lines, so *both* standards will have to be interpolated using weird conversion factors (leading to loss of sharpness) when showing old low-definition material on high-definition. FWIW, the comparison with 1080 should be 1152 for PAL (2x576 active lines) and 960 for NTSC (2x480), so from NTSC 1080 = 9/4 * 480, and from PAL 1080 = 15/8 * 576. Not such weird conversion factors, so maybe the standards guys did know something. John Howells |
"John Howells" wrote in message
... "Martin Underwood" wrote But then the obscure numbers that have become enshrined in standards have always intrigued me. Why are audio tape speeds not integer numbers of inches per second (eg 2, 4, 8, 16 etc) or else round numbers of cm/sec? Early wire recorders ran at 60ips (and were VERY dangerous for the operators if the wire broke, which happened all too frequently). Speeds of tape followed, and halved as technology allowed, only going "wrong" below 15ips. Fair enough. If modern tape speeds are derived from successively halving 60 ips, that makes sense. I wonder how the original figure of 60 was arrived at. And how do vinyl record speeds of 16 2/3, 33 1/3, 45 and 78 rpm originate. 33 1/3 is exactly 1/3 of 100 rpm, but how about the others? The weirdest one is the spec for high-definition TV which uses 1080 lines - neither 2xPAL (1250) nor 2xNTSC (1050) lines, so *both* standards will have to be interpolated using weird conversion factors (leading to loss of sharpness) when showing old low-definition material on high-definition. FWIW, the comparison with 1080 should be 1152 for PAL (2x576 active lines) and 960 for NTSC (2x480), so from NTSC 1080 = 9/4 * 480, and from PAL 1080 = 15/8 * 576. Not such weird conversion factors, so maybe the standards guys did know something. Ah, the 1080 "lines" is the number of active lines, is it, not the total number of line-periods as for PAL/625 and NTSC/575? Fine. I'm surprised that, given the US/Japanese dominance in the field, the standard isn't 960 active lines, with Europe having to make whatever it can of the resulting mess ;-) |
"Dave Fawthrop" wrote in message
... On Sat, 28 Feb 2004 18:50:00 GMT, "Martin Underwood" wrote: | Why is the | UK railway gauge 4' 8½" rather than 4' 6", 4' 9" or 5' (or else round | numbers of centimetres)? That one at least is easy, it depends on the width of the bums of two horses. The original ?railway? trucks were ordinary carts, with wheels 4" 8 1/2" pulled by horses, along wooden rails. Wooden rails were much smoother than cart tracks. As improvements like iron rails, and steam locomotives the gauge stayed the same. Roman roads have ruts of much the same gauge. http://www.spartacus.schoolnet.co.uk/RAgauge.htm Isombard Kingdom Brunell had a great idea when he designed the Great Western Railway. He built it in the technically much better Broad Gauge 2.2m but the standard won out against the better. You can see a replica of the broad gauge Iron Duke locomotive at National Railway Museum in York. All standards happened in a similar sort of way :-((((((((( Yes, I've heard this one. The width of two average horses' bums gives the approximate gauge, but why not then round the gauge to the nearest whole number of inches, centimetres, or other measurement system? Surely not all horses' bums were exactly the same size to the nearest half inch! I understand that different collieries used slightly different gauges - it just so happened that the one that George Stephenson modelled his railways on had a gauge of 4' 8½" - which says to me "why did that colliery use this gauge rather than a round number of inches?". And why did Brunel choose 7' 0¼" rather than 7' exactly? In one of the sheds at Didcot Railway Centre I've seen a "derivation" which involves pi, though I'm not sure whether this is actually Brunel's. |
"Martin Underwood" wrote Fair enough. If modern tape speeds are derived from successively halving 60 ips, that makes sense. I wonder how the original figure of 60 was arrived at. No idea. But it was probably a compromise, as are most standards, with an arbitrary value chosen that gives acceptable quality and still give useful performance. And how do vinyl record speeds of 16 2/3, 33 1/3, 45 and 78 rpm originate. 33 1/3 is exactly 1/3 of 100 rpm, but how about the others? No idea. Ah, the 1080 "lines" is the number of active lines, is it, not the total number of line-periods as for PAL/625 and NTSC/575? Yes. With a digital system there is no continuous line frequency clock, so for digital systems the resolution numbers are more like computer displays - WYSIWYG. Fine. I'm surprised that, given the US/Japanese dominance in the field, the standard isn't 960 active lines, with Europe having to make whatever it can of the resulting mess ;-) Quite! But I doubt it's an accident that 576 = 480 * 6 / 5. John Howells |
And why did Brunel choose 7' 0¼" rather than 7' exactly?
4' 8½" rather than 4' 6", 4' 9" or 5 Originall 4ft8in and 7ft the extra was added on when it was found the gauge was a bit too tight so rather than shorten axles widen gauge |
Martin Underwood wrote:
Fair enough. If modern tape speeds are derived from successively halving 60 ips, that makes sense. I wonder how the original figure of 60 was arrived at. And how do vinyl record speeds of 16 2/3, 33 1/3, 45 and 78 rpm originate. 33 1/3 is exactly 1/3 of 100 rpm, but how about the others? I seem to remember that around 100 years ago, records companies used different standards varying from 72 rpm to 84 rpm. 78 works out to be the average. Some turntables come with a speed adjustment which will allow for the exact speed to be played. The difference between speeds came down manufacturer and the ideal speed for the old handcranked mechanical gramophones. -- John |
Why is cinema film shot at 24
frames/sec rather than 20, 25 or 30, given that 50Hz (halved to derive 25 Hz) and 60Hz (halved to derive 30 Hz) are the standard mains frequencies? The rates for film have absolutely nowt to do with mains power. And like most of the other answers given here have more to do with practicality. Silent film was run at anything from 8 up to around 20 fps, eventually 18fps was standardised upon as the ideal rate to give smooth motion without jerkiness (When each frame was show twice, giving an effective frame rate of 36fps). With film costs of the time, anymore would have been too expensive. Most film was also 70mm, but was cut in half by film makers (to give twice the length) who couldn't afford the cost. When sound came along 18fps wasn't enough length to squeeze in good enough optical or magnetic sound quality, and the best quality / cost compromise was 24fps (Again each frame is still shown twice). Film being mechanical could easily be any speed regardless of the mains voltage or cycle. (Indeed some modern projectors require 3-phase). 25fps only came later when interlaced electrical television was invented as the easiest way to work with the 50 / 60Hz cycle of modern power. You might say why not standardise film at 25fps or 30??? Well Hollywood would never bow down to television, and most projectors can't be easily adjusted and are fixed at the correct speed. The beauty of their mechanical nature is that I work today with machines which were built in the 1940s, are still going strong with spares still readily available. I'd like to see the correct incarnation of digital / DLP projectors still running in a theatre in 2060...... |
Martin Underwood wrote:
Ah, the 1080 "lines" is the number of active lines, is it, not the total number of line-periods as for PAL/625 and NTSC/575? Fine. I'm surprised that, given the US/Japanese dominance in the field, the standard isn't 960 active lines, with Europe having to make whatever it can of the resulting mess ;-) You're closer than you realise. 1080 is the number of active lines in Japanese 1125-line HDTV, which has been on air since the early 1980s. Yup, they were there 20 years ago, and UK broadcasters still regard it as too new-fangled to contemplate. Still, at least everyone is agreed on 1920x1080 square pixels for HDTV, which might not have happened if earlier European efforts had borne fruit. -- Richard Lamont OpenPGP Key ID: 5ABEC9C3 http://www.stonix.demon.co.uk/key.txt Fingerprint: 9DEE 7113 DF02 A516 404C 22AC 1FF6 185D 5ABE C9C3 |
Still, at least everyone is agreed on 1920x1080 square pixels for HDTV, which might not have happened if earlier European efforts had borne fruit. Square pixels WOOOHOOOO!!!!!! You don't know how happy that makes me.... No more pratting about with graphic aspect ratios in Photoshop ;) |
In article m, Martin
Underwood wrote: Fair enough. If modern tape speeds are derived from successively halving 60 ips, that makes sense. I wonder how the original figure of 60 was arrived at. And how do vinyl record speeds of 16 2/3, 33 1/3, 45 and 78 rpm originate. 33 1/3 is exactly 1/3 of 100 rpm, but how about the others? I've been told (though I can't quote a reference) that the speed of 33+1/3 came about because it simplified the gearing for turntables that were linked to cameras and projectors for the early sound films, which had their sound tracks on disk. Rod. |
In article , Mat Overton
wrote: Film being mechanical could easily be any speed regardless of the mains voltage or cycle. (Indeed some modern projectors require 3-phase). 25fps only came later when interlaced electrical television was invented as the easiest way to work with the 50 / 60Hz cycle of modern power. It's nothing to do with interlace. Television frame rates were chosen to be the same as local mains frequency to minimise the effect of inadequate smoothing of power supplies, which would cause hum bars on the screen. It was thought that if the hum bars were stationary or only moving very slowly, they would be less annoying. Modern power supplies can use many more active components (because they don't have to be thermionic valves), and so power supplies can be much more effectively regulated and smoothed, making hum bars practically non-existent, so monitors can display at virtually any scan frequency. You might say why not standardise film at 25fps or 30??? Well Hollywood would never bow down to television, and most projectors can't be easily adjusted and are fixed at the correct speed. The beauty of their mechanical nature is that I work today with machines which were built in the 1940s, are still going strong with spares still readily available. I'd like to see the correct incarnation of digital / DLP projectors still running in a theatre in 2060...... I'm told that many cinema projectors are actually run at 25fps, though whether this is just in Europe or America as well, I don't know. Rod. |
"Mat Overton" wrote in message
... Why is cinema film shot at 24 frames/sec rather than 20, 25 or 30, given that 50Hz (halved to derive 25 Hz) and 60Hz (halved to derive 30 Hz) are the standard mains frequencies? The rates for film have absolutely nowt to do with mains power. And like most of the other answers given here have more to do with practicality. Silent film was run at anything from 8 up to around 20 fps, eventually 18fps was standardised upon as the ideal rate to give smooth motion without jerkiness (When each frame was show twice, giving an effective frame rate of 36fps). With film costs of the time, anymore would have been too expensive. Most film was also 70mm, but was cut in half by film makers (to give twice the length) who couldn't afford the cost. When sound came along 18fps wasn't enough length to squeeze in good enough optical or magnetic sound quality, and the best quality / cost compromise was 24fps (Again each frame is still shown twice). Film being mechanical could easily be any speed regardless of the mains voltage or cycle. (Indeed some modern projectors require 3-phase). 25fps only came later when interlaced electrical television was invented as the easiest way to work with the 50 / 60Hz cycle of modern power. So someone consciously decided to standardise on values of 18 (rather than 20) and 24 (rather than 25). I can't understand the reasoning behind that. People in days gone by seem to have shunned multiples of 5 and 10, don't they? Of course everything in computing is based on powers of 2 or 16, but at least there's a very logical explanation for that. I hadn't realised that early film was 70 mm rather than 35 mm. I'd assumed that 35 mm double-sprocket came first, then this was halved to 16 mm double-sprocket which could be sliced lengthways to give two lengths of Standard 8 single-sprocket, and that 70 mm was developed about the time that widescreen formats were devised in the 50s as being twice the width of 35 mm. By the way (going further off-topic) what was the format for still photography that gave a frame size slightly larger than the 24x36 of 35 mm? I have some slides from the early 60s (when my parents first got married) which are fractionally larger than normal 35 mm and have only a couple of sprocket holes per frame, and only on one side of the film. The frame is about 38.5 x 26.5, still mounted in the standard 50 mm x 50m cardboard mount. How is the speed of a projector governed: I'd always assumed that in the old days the motor was locked to the mains frequency like in an electric clock and that now it's controlled by quartz crystal. What a shame that the world didn't standardise on a single worldwide frequency and voltage in those early days of mains power - preferably 48 Hz to sync with existing film standards! What frame rate was the original EMI 240-line TV system (the rival to Baird's system). Was that 25 Hz. Was there any equivalent pre-cursor to the 525/30 American system. Interesting that all the standards for the dimensions of film (apart from the overall length of cine film) are measured in mm, given that the first cine film was shot in Leeds and presumably a lot of the development would have been in the UK and America. |
Martin Underwood wrote:
[snip] The weirdest one is the spec for high-definition TV which uses 1080 lines - neither 2xPAL (1250) nor 2xNTSC (1050) lines, so *both* standards will have to be interpolated using weird conversion factors (leading to loss of sharpness) when showing old low-definition material on high-definition. Aaah - you are confusing active with total number of scanning lines there Martin. The 1080 HD standard refers to the number of active lines (not the total number of scanning lines), and so compares with 480 active (or 488) for 525 and 576 active (or 575) for 625 lines, which if line doubled would give 960 active (1050 total) or 1152 active (1250 total) assuming the blanking stayed in the same proportions. Given that transmission systems no longer need to transmit the non-active lines, and non-scanning displays (non-CRT devices like LCDs, Plasmas, DLPs etc) don't either, the active line structures are more important than the total number of lines (including vertical blanking etc.) HOWEVER I believe that the 1080 standard is actually closest to the Japanese 1125 total system (which most HD kit on sale in the 80s and 90s was built for - apart from the odd bit of bespoke 1050/1250 stuff) which had 1030 or 1050 active lines (can't remember which) and someone must have taken the pragmatic approach partially. In fact I believe that quite a lot of stuff still has 50 or 30 lines missing ?? The 1080 number in square pixel terms makes loads of sense - 1920x1080 is 16:9 with square pixels - though 1080 is not completely divisible by 16 (so 1088 is often used in MPEG transmission terms?), 1920 is, and is a nice number! Using a common image scannning structure irrespective of frame rate also means that many frame rate conversions will require no re-sampling of the image area (and thus no interpolation) at all, and also allows cameras to work on multiple standards using the same image sensors (It is common for HD cameras to be able to run at 24,25 and 30p or 50 or 60i). If 1080/24p is used (as it increasingly is) as the production format, it can be converted to 1080/25p or 1080/50i by a slight frame rate conversion (i.e. speeding up by 4%), and it can be converted to 1080/60i by using a 3:2 pulldown, neither of these requiring any significant interpolation. Your argument for downconversion is understandable - but given that the frame rate conversions are more difficult than basic image scaling the loss of quality when going from 1080 rather than 1152 or 960 to 576 or 480 is pretty negligible - as in the case of NTSC/480 regions you are starting off with a sharper image, and in PAL/576 regions only 72 fewer lines (and given that you are throwing between 505 and 600 away in the downconversion that probably doesn't matter massively) Basically the choice of a common 1920x1080 (or two standards if you include 1280x720 - though 720p stuff in 50Hz regions is pretty unheard of I think) scanning structure for all frame rate flavours of HD is more significant than the choice of one that is linked to a legacy resolution based on 525 or 625 analogue scanning standards. Steve |
wrote in message ... On Sun, 29 Feb 2004 15:03:03 GMT, as the pitiful wreck that had once been "Martin Underwood" was cut from the mess that had once been his life, he managed to utter: I hadn't realised that early film was 70 mm rather than 35 mm. I'd assumed that 35 mm double-sprocket came first, then this was halved to 16 mm double-sprocket which could be sliced lengthways to give two lengths of Standard 8 single-sprocket, and that 70 mm was developed about the time that widescreen formats were devised in the 50s as being twice the width of 35 mm. Are you perhaps forgetting 9.5mm centre sprocket film? I was drawing a veil over this oddity. Putting the sprockets in the centre of the film seems a very strange decision because you have to space the frames further apart to avoid them encroaching into the frames. Mind you, I suppose each of those frames can then go right up to the edges of the film, rather than losing a "sprocket strip" down one side. Perhaps it wasn't such a bad idea after all... |
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In article , Ashley Booth wrote:
Are you perhaps forgetting 9.5mm centre sprocket film? I was drawing a veil over this oddity. Putting the sprockets in the centre of the film seems a very strange decision because you have to space the frames further apart to avoid them encroaching into the frames. Mind you, I suppose each of those frames can then go right up to the edges of the film, rather than losing a "sprocket strip" down one side. Perhaps it wasn't such a bad idea after all... Not a good idea when the sprocket slips! Not really. Actually, all the 9.5mm projectors and cameras I remember used a claw, not a sprocket, and it could make quite a mess if it missed the holes. What they used to do was have much longer holes, almost frame height, for the first few inches of film in the cartridge so that the claw would have a greater chance of going through one of them instead of punching extra holes. The leading edges of the long holes had the same spacing as the proper ones, so after the camera had been run for a few seconds, everything lined up. Lacing a projector properly took a bit of care, but you could see the film so it was relatively easy. You don't have this problem with a sprocket mechanism as in 35mm of course, because the pins are permanently engaged with the holes. 9.5mm was technically quite a good idea, as the force pulling the film through the gate was balanced with no sideways component tending to make it jam as in 16mm or 8mm, and the picture area was nearly the same as 16mm, even for sound film which had a slightly smaller frame to accommodate the sound track. Technically good, just different from the rest of the world, but that's the French for you. Rod. |
In Roderick Stewart writes:
In article , Mat Overton wrote: Film being mechanical could easily be any speed regardless of the mains voltage or cycle. (Indeed some modern projectors require 3-phase). 25fps only came later when interlaced electrical television was invented as the easiest way to work with the 50 / 60Hz cycle of modern power. It's nothing to do with interlace. Television frame rates were chosen to be the same as local mains frequency to minimise the effect of inadequate smoothing of power supplies, which would cause hum bars on the screen. There was also the problem of magnetic interference. Parts of Los Angeles were still 50Hz in the late 30's, early 40's. RCA made a special 50Hz version of the pre-war TRK-12 television receiver that had shielding around the kinescope, for use in LA. I'm told that many cinema projectors are actually run at 25fps, though whether this is just in Europe or America as well, I don't know. For PAL film-to-tape transfers the film is run at 25fps, even though it was shot at 24fps. The slight speed-up is preferable to the bizarre pulldown you'd have with 24/25. -- Tim Mullen ------------------------------------------------------------------ Am I in your basement? Looking for antique televisions, fans, etc. ------ finger this account or call anytime: (212)-463-0552 ------- |
On 1/3/04 10:48 pm, in article , "Tim Mullen"
wrote: [snip] I'm told that many cinema projectors are actually run at 25fps, though whether this is just in Europe or America as well, I don't know. For PAL film-to-tape transfers the film is run at 25fps, even though it was shot at 24fps. The slight speed-up is preferable to the bizarre pulldown you'd have with 24/25. Yep - though film for European TV is often shot at 25fps to remove the need for speed up (though presumably is more expensive to shoot, as it requires an extra frame of stock and processing every second...) Similarly HD stuff shot for the European market is often shot using 25p and slowed down to 24p for the US (where it may then be converted to 60i using 3:2 pulldown) Steve |
In article m, Martin
Underwood writes Putting the sprockets in the centre of the film seems a very strange decision because you have to space the frames further apart to avoid them encroaching into the frames. They *do* encroach into the frames. It makes the pulldown smooth though, as there are no twisting forces, and, as you say, the frame area is much bigger than 8mm. 9.5mm still has a rather anoracky following, and you can still get the stock. The later cameras are supposed to give very good results. The other oddity of 9.5mm is the ability to freeze frames for a finite time, for titles, etc., following notches cut in the edge of the film. Sounds like the origin of those hackneyed shots of the film catching fire to me, but I've got a few old reels (cartoons, I think) with the notches. Not every projector could do it though, the rest just ploughed on regardless, I assume. Regards, Simonm. -- simonm|at|muircom|dot|demon|.|c|oh|dot|u|kay SIMON MUIR, UK INDEPENDENCE PARTY, BRISTOL www.ukip.org EUROPEANS AGAINST THE EU www.members.aol.com/eurofaq GT250A'76 R80/RT'86 110CSW TD'88 www.kc3ltd.co.uk/profile/eurofollie/ |
It's so long since I've seen a non-Imax 70mm print in the cinema, I've
no idea if they've successfully adopted an optical audio format for 70mm or not, digital or otherwise. Apart from DTS timecode no. Although Dolby did patent it's sprocket hole spaces, but then they did the same for the opposite side of 35mm film, but that was just to protect their interests. Mind you who needs it - When you've seen a brand new mag striped print of 2001 on the Cinerama screen in Bradford you wonder what they'd bother with anything else. (Apart from the envoronmental issues of mag striping, the wearing out of the track, the degrading over time, the-...... etc, etc.!) |
Who writes these standards, anyway, and why don't they document
these things better? What do they get paid for? :) The real question is what do you have to smoke to devise standards whether the ratio of pixels in the horizontal and vertical directions is neither 4:3 nor 16:9? Does it not make eminent sense to have the same resolution (pixels/mm) in both directions at least for *one* of 4:3 and 16:9? I think it already is the same resolution in both directions if you take into account the Kell factor due to interlace. The effective number of active lines in 625 analogue is more than a field (287½) but less than a frame (575), because fine detail at the full 575 line resolution flickers violently as it is only refreshed at 25 Hz. The effective vertical resolution is 575 lines times the Kell factor, which is usually taken as about 0.7, making it almost exactly 400 lines. Multiply this by 4/3 and you need 533 pixels horizontally, that is 267 cycles in 52 microseconds, which works out at 5.1 MHz. This was how the vision bandwidth for the original 625 line monochrome System B was arrived at. (It was rounded down to 5 MHz.) Taking the Kell factor into account, the pixels are squa 400 x 533. The only trouble is that these square pixels do not exist in any still picture, field or frame. The pixels are virtually square when you take an average between field resolution and frame resolution, as the eye does when watching an interlaced TV picture. |
In article , Mat
Overton writes It's so long since I've seen a non-Imax 70mm print in the cinema, I've no idea if they've successfully adopted an optical audio format for 70mm or not, digital or otherwise. Apart from DTS timecode no. Although Dolby did patent it's sprocket hole spaces, but then they did the same for the opposite side of 35mm film, but that was just to protect their interests. Mind you who needs it - When you've seen a brand new mag striped print of 2001 on the Cinerama screen in Bradford you wonder what they'd bother with anything else. (Apart from the envoronmental issues of mag striping, the wearing out of the track, the degrading over time, the-...... etc, etc.!) I agree that it's stunning. It's a shame though that the vast majority of film production is on 35mm. printed up to 70. You don't get the full benefit. IIRC, making a 70mm print is a 3-stage process: Make the optical print, stripe it, record the sound. In consequence, 35mm showprint = around 100UKP, 70mm showprint = 10,000UKP. That and (as you mention), the fact that they wear out much faster than comopt (of whatever sort), are why you never see 70mm in the cinema normally over here - it's just uneconomic to fly prints across the Atlantic. Regards, Simonm. -- simonm|at|muircom|dot|demon|.|c|oh|dot|u|kay SIMON MUIR, UK INDEPENDENCE PARTY, BRISTOL www.ukip.org EUROPEANS AGAINST THE EU www.members.aol.com/eurofaq GT250A'76 R80/RT'86 110CSW TD'88 www.kc3ltd.co.uk/profile/eurofollie/ |
It's a shame though that the vast majority of film production is on
35mm. printed up to 70. You don't get the full benefit. IIRC, making a 70mm print is a 3-stage process: Make the optical print, stripe it, record the sound. In consequence, 35mm showprint = around 100UKP, 70mm showprint = 10,000UKP. That and (as you mention), the fact that they wear out much faster than comopt (of whatever sort), are why you never see 70mm in the cinema normally over here - it's just uneconomic to fly prints across the Atlantic. ISTR there is now only one European company who can print mag striping due to the serious environmental issues involved (waste chemicals etc). Of course America doesn't give a damn about that and many labs over there will happily print 70mm mags when they are (rarely) needed. Some 70mm prints where made are now DTS only. It's also fortunate that the extra sprocket hole per frame means that the timecode is much larger, and therefore much more reliable than the (still very reliable) 35mm version. Unless Dolby used the opposite side as a 100% back up track, I couldn't see DD ever appearing on a 70mm print. Without wanting to get too off topic. There has been the recent change to remove silver from optical print soundtracks, which makes film printing more environmentally friendly, however I bet that was more to do with the price of silver and the cost of the chemicals..... http://palimpsest.stanford.edu/byfor.../msg00050.html |
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