of ATSC and aspect ratio???
 

Folks,

With US due to phase out analog broadcasting very shortly and the need for set top boxes etc to convert digital ATSC broadcasts to NTSC one issue I have not read about is the whole matter of 4:3, letterbox and 16:9 aspect ratios.

Here in Australia we use DVB-T which uses an anamorphic squeeze to deliver 16:9 images and when this is displayed on a 4:3 set the whole image is displayed looking something uncorrected like CinemaScope. Set top boxes have settings to provide 16:9, letterbox or a 4:3 center segment of the 16:9 image. (I assume the same options are available on US set top boxes).

But as I understand it ATSC does not use the anamorphic scheme ...so how do 16:9 images appear on a NTSC if 16:9 is set in the set top box.

What lead me to ask this arcane question was a comment on another board that complained many cable companies are simply using the 4:3 center section of the ATSC image rather than providing a proper letterbox version of 16:9 network broadcasts.

Most of the ATSC set top boxes I've seen simply generate a letterbox 4:3 picture when presented with a 1080 program.

A couple of non coupon boxes present a full screen anamorphic picture, but unless the tv is capable of a 16:9 mode by dropping the vertical height, the result isn't very viewable.

My Sony PVM 2950 accomodates this mode quite nicely when I use a samsung HD STB.

The 4:3 programming on stations that transmit in 1080i means issues with aspect ratio -you get a small square picture about 14" on a 29" crt.

I can set my converter box to put out either a letterboxed 16:9 on the 4:3 CRT, or it will overscan the sides to fill the screen vertically. There were a few stations here doing a 16:9 stretch on all their 4:3 stuff but they've stopped doing that for the most part.

I have the cheap RCA and mid-line Zenith boxes, and both have selectable aspet ratios. But none seems to be quite true on a standard 4:3 tube. The closest I can get is to select 16:9, which seems to be the closest (neither fat & short nor thin & tall). That sacrifices about 20% of the picture viewing area in the form of insufficient vertical height. The whole subject stinks; from A to Z.

Charles

No problems like that here. Of course if you put the converter box in 16:9 you lose the "20%" as black bars just like any other letterboxed source material. At least it isn't 2.35:1! Geometric distortion is zero in 4:3 mode too. I have two Zenith boxes in use now, not a problem with those. When it scales to a 4:3 output I lose the sides of the picture on a 4:3 CRT set- big deal. There's usually nothing happening there out in the sides anyway.

Personally I love the picture quality and lack of line noise interference from the 50 year old AC power lines around my neighborhood, no more co-channel interference from ducting or inversion, zero ghosting, no aircraft flutter and fading as I live in close proximity to 3 airports with many aircraft flying over, so not having that is great, and the razor-sharp picture quality on my old CRT sets.

What you described is not "an anamorphic scheme", but is simply filling the transmitted 16:9 digital raster with the 16:9 picture. Any anamorphic effects are due to the receiver box not properly formatting the output for the display. As you say, you can properly select letterbox or cropped center when feeding a 4:3 display. The "tall, thin" anamorphosis occurs if the receiver box is set for a 16:9 display but viewed on a 4:3 set. The situation is the same for both DVB and ATSC. This mistake could occur either through customer settings or through wrong settings at the transmitter in some cases.

Thats because when they produce shows they compromise it by filming everything in the 4:3 safe area - at least some of the shows are starting to push it out to the edges of it rather then sticking to a title safe area within the 4:3 area.

Thats the reason that all the analog channels here are broadcasting in 13:9 or 14:9 so that old tvs with overscan problems are not missing out on the captions and titles on current shows - LCDs etc dont suffer from overscan and thats what things are made for these days.

Hi,

This is my first post to audiokarma even though I have followed the threads for years.

I am a broadcast engineer and the issue of SD aspect ratio is coming back to burn broadcasters. I do not like to use the term "anamorphic" in the context of DTV since the ATSC standard originally made provision for 4X3 and 16X9 SD.

The ATSC pixel array for SD is 480 X 704. US SD video has never had square pixels: for 4X3 the pixels are a vertical rectangle and for 16X9 the pixels are a horizontal rectangle. Therefore the difference between SD 4X3 and 16X9 is the way the video is captured and presented in a 480 X704 pixel array.

In transmission almost all American broadcasters set their MPEG2 encoder with the signaling flag set to 4X3. This flag is included in the broadcast video stream and tells the DTV receiver how to properly fill the screen. Hence 4X3 video with the aspect ratio flag set to 4X3 will mean that 4X3 screens will be fully filled and 16X9 screens will be provided with side bars.

If on the other hand a broadcaster decided to broadcast 16X9 SD, they would need to present 16X9 video to their MPEG2 encoder and set the aspect ratio flag to 16X9. The flag would tell 4X3 receivers to either “letterbox” or “center crop” the video (viewer choice) and 16X9 receivers would display full 16X9.

Unfortunately an arbitrary decision was made to limit SD to 4X3 only at the beginning of the DTV conversion. Satellite and Cable digital carriage of broadcaster’s SD has been similarly limited because they followed the broadcaster’s arbitrary decision. However, if a broadcaster decided to send out 16X9 video with the aspect ratio flag properly set, it would not affect cable, satellite or home over-the-air reception because all DTV receiving equipment will automatically properly adapt. There would be no fat or skinny video images.

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PBS has the problem of not producing programs 4X3 safe. That is, the video must be letterboxed on cable to display the entire picture otherwise ssential elements will be cut off. The proble is that when an older 4X3 program comes along, it has black borders all aroung the video.

AFD (Automatic Format Description) is supposed to fix this. But most sets are not fitted with AFD.

I personally prefer to have some of the picture cut off as opposed to having black bars and a tiny picture. But the bottom line is the broadcasters are not handling this properly.

sounds familiar
 

seems US broadcasters have the same issues as here.... a bit of mess for the consumer ..and trying to explain letterbox versus 16:9 to my buddies ....you can see their eyes glaze over!

This can usually be fixed by picking the "zoom" or similar option on the converter box. The best boxes (Zenith/Insignia and maybe others) can remember the zoom setting when you change channels.

Bravo to PBS! It is about time that programmers and TV directors used the 16:9 screen properly. 4:3 viewers can watch the show in letterbox mode. Imagining that all programming should fill up a screen (whether it is 16:9, 4:3 or another aspect ratio) is no better than wishing every piece of art in the Louvre was the same size and shape. :nono: