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Digital TV Tricks
12 years ago
General
Guess what we did a few weeks ago? We set our standard definition sub-channel to broadcast in wide-screen. I'll let that sink in for a moment.
Yes, I know. SD doesn't do widescreen. SD was 4:3 and that's all it ever was. You're absolutely correct, SD can't broadcast anything but 4:3 aspect video. And the key word in that sentence is broadcast.
Remember all the fine metadata (like PSIP) that is sent along with a digital broadcast? One of the things you can send for each channel is a hint to tell the decoder what format the picture is in. You can say, "Hey, this is 16:9 widescreen" and wide-screen HD TVs will display it full frame. Digital converter boxes for older SD TVs have their choice of letter-boxing or center-cutting (usually at the preference of the user). You might be able to report your video as "16:9 not protected for a center cut" in which case a smart decoder would letter-box it regardless of preference.
And the opposite is true: video can be tagged as 4:3 and a widescreen TV would center it ("pillarbox" is the official term, even if I think it sounds... dumb) or it may zoom in on it, cutting off the top and bottom or just stretch it out and make everyone look fat. Again... probably user preference.
There is a third hint you can send. You can tell the decoder that your video is 4:3 Anamorphic Widescreen.
You start with a widescreen video and squeeze it down horizontally until it fits a 4:3 frame and broadcast it that way. When the picture arrives at the decoder in your TV, the decoder knows to stretch it back out to fit the entire widescreen display and the viewer is none the wiser. In fact, we've used this method for our newscast remotes since before I began working at the station. The photogs shoot with a 16:9 DVCam that outputs 4:3 anamorphic. That is broadcast over an SD microwave link back to the station, where the video switcher stretches the image back out to 16:9.
You may have realized by now that this squeezing and stretching causes a reduction in resolution and you'd be completely correct. SD is still 720x480 no matter how you slice it, so taking an 853x480 image and squeezing it down to 720x480, you're losing 133 lines that you'll never get back. It's a compromise that has to be made. Plus... hey, it's still only SD, it was never that sharp to begin with. But at least it's full-frame widescreen SD!
One of you smarties is gonna ask why we don't broadcast our sub-channel in HD? That's because of the dreaded "B" word: bandwidth. Remember all the sub-channels are on a single transport stream broadcast on a single TV frequency and there's only so much bandwidth allotted to each frequency. You can mix and match 'em as you see fit, but you still only have so much total bandwidth to use.
So the satellite receiver that feeds our sub-channel does anamorphic widescreen SD. Our playout servers will do anamorphic widescreen SD for commercial breaks. But we have one stubborn part of the chain: we have a paid programming server for our sub-channel that only does 4:3 SD (it can be upgraded to do 16:9 HD, but that's a hardware re-tooling and it would have to be sent back to its owners for that... plus we'd still have to downconvert the HD to SD). Which means right now when we go to the paid programming server, it outputs regular 4:3 that is broadcast with the anamorphic hint and... results in stretched-out video.
(One engineer has remarked the last thing you want to stretch out are the diet commercials... the "after" images will still look chunky and the "before" images will look even fatter!)
Now it is possible to change the format hint on the fly and most television decoders will handle that without issue. One of the ones that doesn't happens to be in our master control for monitoring the sub-channel... it has to be forcibly told to re-check the channel format information (although I forget if it's as easy as changing channels or as bad as a total power-down).
Unfortunately, our encoder is not that fleet-of-foot: changing the format hint is a change in the configuration file and to re-load the file is essentially a re-boot of the encoder. Not something you want to do each time you change from the satellite receiver to the paid programming server and back again.
One unanticipated consequence of this change was noticed when recording our local high school sports broadcasts. The gear associated with high school sports has always been SD and it's better that way. Running a single triax cable to each camera totally outweighs any advantages that widescreen cameras might have. Especially when you're doing 500+ foot cable runs...
So we figured out how to cut the 4:3 image to fit 16:9, we hung wire strands in the camera viewfinders so the photogs can frame up in 16:9 and then it all goes out in anamorphic widescreen for the viewers at home. Widescreen high school sports! Awesome!
What we failed to remember is that up to the viewers' TVs, the picture is anamorphic. That includes all our internal video routing. So when we set up our usual recordings of the game for coaches and for use in the 11pm news... yeah. The football really was a sphere and the players looked like willowy giants.
We're working on a fix...
Yes, I know. SD doesn't do widescreen. SD was 4:3 and that's all it ever was. You're absolutely correct, SD can't broadcast anything but 4:3 aspect video. And the key word in that sentence is broadcast.
Remember all the fine metadata (like PSIP) that is sent along with a digital broadcast? One of the things you can send for each channel is a hint to tell the decoder what format the picture is in. You can say, "Hey, this is 16:9 widescreen" and wide-screen HD TVs will display it full frame. Digital converter boxes for older SD TVs have their choice of letter-boxing or center-cutting (usually at the preference of the user). You might be able to report your video as "16:9 not protected for a center cut" in which case a smart decoder would letter-box it regardless of preference.
And the opposite is true: video can be tagged as 4:3 and a widescreen TV would center it ("pillarbox" is the official term, even if I think it sounds... dumb) or it may zoom in on it, cutting off the top and bottom or just stretch it out and make everyone look fat. Again... probably user preference.
There is a third hint you can send. You can tell the decoder that your video is 4:3 Anamorphic Widescreen.
You start with a widescreen video and squeeze it down horizontally until it fits a 4:3 frame and broadcast it that way. When the picture arrives at the decoder in your TV, the decoder knows to stretch it back out to fit the entire widescreen display and the viewer is none the wiser. In fact, we've used this method for our newscast remotes since before I began working at the station. The photogs shoot with a 16:9 DVCam that outputs 4:3 anamorphic. That is broadcast over an SD microwave link back to the station, where the video switcher stretches the image back out to 16:9.
You may have realized by now that this squeezing and stretching causes a reduction in resolution and you'd be completely correct. SD is still 720x480 no matter how you slice it, so taking an 853x480 image and squeezing it down to 720x480, you're losing 133 lines that you'll never get back. It's a compromise that has to be made. Plus... hey, it's still only SD, it was never that sharp to begin with. But at least it's full-frame widescreen SD!
One of you smarties is gonna ask why we don't broadcast our sub-channel in HD? That's because of the dreaded "B" word: bandwidth. Remember all the sub-channels are on a single transport stream broadcast on a single TV frequency and there's only so much bandwidth allotted to each frequency. You can mix and match 'em as you see fit, but you still only have so much total bandwidth to use.
So the satellite receiver that feeds our sub-channel does anamorphic widescreen SD. Our playout servers will do anamorphic widescreen SD for commercial breaks. But we have one stubborn part of the chain: we have a paid programming server for our sub-channel that only does 4:3 SD (it can be upgraded to do 16:9 HD, but that's a hardware re-tooling and it would have to be sent back to its owners for that... plus we'd still have to downconvert the HD to SD). Which means right now when we go to the paid programming server, it outputs regular 4:3 that is broadcast with the anamorphic hint and... results in stretched-out video.
(One engineer has remarked the last thing you want to stretch out are the diet commercials... the "after" images will still look chunky and the "before" images will look even fatter!)
Now it is possible to change the format hint on the fly and most television decoders will handle that without issue. One of the ones that doesn't happens to be in our master control for monitoring the sub-channel... it has to be forcibly told to re-check the channel format information (although I forget if it's as easy as changing channels or as bad as a total power-down).
Unfortunately, our encoder is not that fleet-of-foot: changing the format hint is a change in the configuration file and to re-load the file is essentially a re-boot of the encoder. Not something you want to do each time you change from the satellite receiver to the paid programming server and back again.
One unanticipated consequence of this change was noticed when recording our local high school sports broadcasts. The gear associated with high school sports has always been SD and it's better that way. Running a single triax cable to each camera totally outweighs any advantages that widescreen cameras might have. Especially when you're doing 500+ foot cable runs...
So we figured out how to cut the 4:3 image to fit 16:9, we hung wire strands in the camera viewfinders so the photogs can frame up in 16:9 and then it all goes out in anamorphic widescreen for the viewers at home. Widescreen high school sports! Awesome!
What we failed to remember is that up to the viewers' TVs, the picture is anamorphic. That includes all our internal video routing. So when we set up our usual recordings of the game for coaches and for use in the 11pm news... yeah. The football really was a sphere and the players looked like willowy giants.
We're working on a fix...
DVD video tends to be a bit strange in general, as the format still dates from the mostly-analog and pre-HD era. This is reflected in things like the need for anamorphic video and the lack of a non-interlaced mode. There's a flag to specify the correct display ratio, but the video resolution is fixed. There's a complication even there though: DVD players don't have digital outputs, so even though the stream specifies a format there is no way for the player to tell the TV! So the viewer usually has to manually select the correct display ratio.
Except over SCART. That monstrosity devotes a whole pin to it: +5V for 16/9, +12V for 4/3. No-one likes SCART though. The connector that does everything - poorly.