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Confused!!! DVCAM 4:3 or 3:2 aspect ratio?
Posted by Eurie Jennings
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Re: Confused!!! DVCAM 4:3 or 3:2 aspect ratio? September 04, 2008 09:01AM |
It is pretty confusing if you're not used to how the ratios work. You shouldn't go wrong if you just use the presets (NTSC DV if you're working with NTSC DV).
To add to Martin's explanation, note that video pixels are not displayed as square pixels. In the SD world, they are usually slightly rectangular when displayed. When displayed as square pixels, they work out to either 4:3 or 16:9 (DAR).
In terms of video pixels, NTSC DV is 720:480 (total number of pixels), which can be brought down to 3:2. Same goes for PAL, 720x576, can be reduced to 5:4. It's a little confusing, but they aren't meant to be displayed in that ratio.
For web, however, computers natively display pixels as squares, so it's always a good idea to convert your video pixels to square pixels when you're doing the final encode if you are going out to web (640x480 or 320x240, depending on the size you want to use).
Hope this helps.
www.strypesinpost.com
To add to Martin's explanation, note that video pixels are not displayed as square pixels. In the SD world, they are usually slightly rectangular when displayed. When displayed as square pixels, they work out to either 4:3 or 16:9 (DAR).
In terms of video pixels, NTSC DV is 720:480 (total number of pixels), which can be brought down to 3:2. Same goes for PAL, 720x576, can be reduced to 5:4. It's a little confusing, but they aren't meant to be displayed in that ratio.
For web, however, computers natively display pixels as squares, so it's always a good idea to convert your video pixels to square pixels when you're doing the final encode if you are going out to web (640x480 or 320x240, depending on the size you want to use).
Hope this helps.
www.strypesinpost.com
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Re: Confused!!! DVCAM 4:3 or 3:2 aspect ratio? September 04, 2008 09:44AM |
It really makes a lot more sense if you remember that standard-definition television is not a digital medium. It's purely analog, made up of scan lines. There are 525 scan lines in a raster, of which 486 are used to draw the picture. You can basically imagine standard-definition television as a purely linear signal, just like an audio recording. The TV itself breaks the signal up into lines, and arranges the lines into fields.
Digital SD is merely an approximation of the analog signal. The CCIR-601 standard calls for an analog television signal to be represented digitally as 720 luma samples and 360 chroma samples per scan line.
But here's the thing: Your color television (CRT-style, analog, NTSC) has red, green and blue phosphor points arranged in an array of vertical stripes. Since the 601 digital TV standard calls for 720 horizontal luma samples per inch, you'd expect there to be 720 phosphor stripes across your TV screen, right?
Yeah, no. Try somewhere between 300 and 400, depending on the television. The signal coming in off your antenna is modulated with a greater frequency than your screen is capable of displaying. That's just how NTSC is.
Like Strypey said up there, computer screens use square pixels. Or at least, they do today; I'm not positive, but I think the Macintosh was the first computer to use square pixels on its screen, back in the mid-80s. Before that, computers usually drew pixels that were narrower than they were tall. But today, computer screens use square pixels. So if you take a digital television signal, with 720 horizontal samples and 486 lines, and draw it on a computer screen, it'll look stretched out. That's where pixel-aspect-ratio correction comes in. Almost all computer programs that handle video can do pixel-aspect-ratio correction automatically, so you never even have to think about it.
The "640x480" thing is kind of an abomination, really. If you take a digital signal in the 601 format and resample it, mathematically, so it displays at the proper aspect ratio, the raster you end up with is 648 pixels wide by 486 pixels high. For reasons that I will never understand, somebody decided, once upon a time, that that was close enough to the old-fashioned computer screen resolution of 640 by 480 pixels. Because a 640x480 frame can be handled as a matrix of 16x16-pixel squares, it was possible to use various technical tricks to help the computer move all that information around to draw it to the screen at 30 frames per second.
But those days are long gone, and the whole "640x480" thing needs to die a horrible, painful death. Computers are more than capable of doing pixel-aspect-ratio correction on the fly; in fact, it's built in to Quicktime.
Digital SD is merely an approximation of the analog signal. The CCIR-601 standard calls for an analog television signal to be represented digitally as 720 luma samples and 360 chroma samples per scan line.
But here's the thing: Your color television (CRT-style, analog, NTSC) has red, green and blue phosphor points arranged in an array of vertical stripes. Since the 601 digital TV standard calls for 720 horizontal luma samples per inch, you'd expect there to be 720 phosphor stripes across your TV screen, right?
Yeah, no. Try somewhere between 300 and 400, depending on the television. The signal coming in off your antenna is modulated with a greater frequency than your screen is capable of displaying. That's just how NTSC is.
Like Strypey said up there, computer screens use square pixels. Or at least, they do today; I'm not positive, but I think the Macintosh was the first computer to use square pixels on its screen, back in the mid-80s. Before that, computers usually drew pixels that were narrower than they were tall. But today, computer screens use square pixels. So if you take a digital television signal, with 720 horizontal samples and 486 lines, and draw it on a computer screen, it'll look stretched out. That's where pixel-aspect-ratio correction comes in. Almost all computer programs that handle video can do pixel-aspect-ratio correction automatically, so you never even have to think about it.
The "640x480" thing is kind of an abomination, really. If you take a digital signal in the 601 format and resample it, mathematically, so it displays at the proper aspect ratio, the raster you end up with is 648 pixels wide by 486 pixels high. For reasons that I will never understand, somebody decided, once upon a time, that that was close enough to the old-fashioned computer screen resolution of 640 by 480 pixels. Because a 640x480 frame can be handled as a matrix of 16x16-pixel squares, it was possible to use various technical tricks to help the computer move all that information around to draw it to the screen at 30 frames per second.
But those days are long gone, and the whole "640x480" thing needs to die a horrible, painful death. Computers are more than capable of doing pixel-aspect-ratio correction on the fly; in fact, it's built in to Quicktime.
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Re: Confused!!! DVCAM 4:3 or 3:2 aspect ratio? September 04, 2008 10:15AM |
>Computers are more than capable of doing pixel-aspect-ratio correction on the fly; in fact, it's
>built in to Quicktime.
They can, but I won't advise it, especially when you're going out to the multitude of end-user codecs, with all sorts of frame sizes, aspect ratios. It's a jungle. Eg. A 360x240 mpeg4 file (how do we know it originates from NTSC DV?), and the whole plethora of custom display aspect ratios that are possible.
>It really makes a lot more sense if you remember that standard-definition television is not a
>digital medium.
A very good point. If you also understand that because of these standards, it allows for smoother transition of video technology across the ages- from black and white to color, then analog to digital, and now to HD and web formats. A certain amount of backward compatibility is maintained, so that accounts for some of the extremely diverse standards across the world.
www.strypesinpost.com
>built in to Quicktime.
They can, but I won't advise it, especially when you're going out to the multitude of end-user codecs, with all sorts of frame sizes, aspect ratios. It's a jungle. Eg. A 360x240 mpeg4 file (how do we know it originates from NTSC DV?), and the whole plethora of custom display aspect ratios that are possible.
>It really makes a lot more sense if you remember that standard-definition television is not a
>digital medium.
A very good point. If you also understand that because of these standards, it allows for smoother transition of video technology across the ages- from black and white to color, then analog to digital, and now to HD and web formats. A certain amount of backward compatibility is maintained, so that accounts for some of the extremely diverse standards across the world.
www.strypesinpost.com
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Re: Confused!!! DVCAM 4:3 or 3:2 aspect ratio? September 04, 2008 12:03PM |
Well, that's why I referred to Quicktime specifically. I have no idea if the MPEG-4 specification, being a really small subset of Quicktime, supports on-the-fly aspect-ratio correction.
And I agree about standards and backward compatibility ? 1080i60 is 59.94 Hz, not 60 Hz ? but the whole digital thing really opened the floodgates. There are a zillion digital video formats that have absolutely nothing to do with video, as we all learned the first time somebody sent us a link to a Youtube page and asked for it to be cut into their show.
And I agree about standards and backward compatibility ? 1080i60 is 59.94 Hz, not 60 Hz ? but the whole digital thing really opened the floodgates. There are a zillion digital video formats that have absolutely nothing to do with video, as we all learned the first time somebody sent us a link to a Youtube page and asked for it to be cut into their show.
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