Analogue Lines of Resolution: Confused. (1 Viewer)

[Enigmatism415]

I've noticed that the number of signal lines is greater than the number of displayed (visible) lines of resolution in analogue displays. Why is this? (e.g. 525 vs 480, 625 vs 576, 750 vs 720, 1125 vs 1080..or 1035? I fail to see the pattern.) Is this difference only in vertical resolution? Or horizontal resolution as well (720x480 vs 640x480)? Is this issue completely ignorable with digital displays? Any explanation as to where the missing lines end up or why there are lines subtracted in the first place would be of great help. Thank you

 

[Jeff Gatie]

The extra lines are known as the "Vertical Blanking Interval" and are necessary for the timing of the display. It is currently also used for various digital/analog add-ons such as closed captioning, time codes, copy protection (Macrovision) and V-Chip stuff.

 

[chuckg]

This always made me tear my hair out, too.

"Horizontal lines" is what confused me the most. Think of them as "wide dots" and it will make more sense. VHS had the capability to display about 240 wide dots across one line of a TV screen.


Yes, there are 525 scan lines (dots counted vertically) on Broadcast TV. Some of those are not used for the picture - sometimes you can see them on a TV as a bunch of flickering dots at the top. some of those dots are a time code (SMPTE) in digital form - if you are quick you can decode them. Some of the lines are "overscan" and are not meant to be displayed on the screen, but do contain picture content. The overscan is there to let the electronics settle down at the end of the lines. You can reduce the size of the picutre on an old TV and see how messy that area can be.

The "vertical blanking interval" mentioned above is not a part of the line count - that interval is a dark period where the electron gun is moving from the bottom-right corner back up to the top-right corner to begin a new raster scan. Some information is placed there, but it doesn't count as part of the lines of resolution.

I think there is a separation between what is stored on the medium, and what is sent out the connections. A frame of video might be stored on a DVD as a 640x480, but then upscaled and output as 720x520 or so.

My 1920x1080 rear-projection TV does do overscan. I'm not sure how much I miss, but not more than 3 or 4 percent. Computer monitors generally do not overscan, or the full resolution of the image is reduced in size so that it does fit on the screen.


I know this is incomplete and perhaps inaccurate, but I hope it helps some!

 

[Enigmatism415]

Thank you for clarifying that for me. So, if these "wide dots" on NTSC displays actually add up to 525 lines, visible or not, Does this mean that if DVD has 500 native lines of resolution (or so i've often read) that all 500 can be displayed on a "480i" TV? Or will 480 lines and below always be displayed regardless of what the media is or whether its analogue or digital? (A theory I had was that DVD has 500 possible lines of resolution, but studios usually only encode 480 to be compatible with NTSC displays)

 

[Allan Jayne]

For 525 line analog TV (NTSC) approximately 480 of the scan lines contain picture content. All of such (480 or so) lines are part of the presentation except that many CRT TV sets are unable to show the first few and last few without severe geometric distortion. Digital 480i and 480p TV signals and DVD's made from the aforementioned analog video utilize exactly 480 of those scan lines for picture content.

Relative to the source material as already recorded, the 480 active scan lines yield 480 lines of resolution.

For the 480i analog video as broadcast (NTSC), each scan line can be subdivided into about 530 equal parts (pixels) of which about 440 contain picture information. The unviewable pixels represent the part of the scan line drawn (more correctly blacked out or blanked) when the beam has to be returned to the left side of the screen (horizontal retrace interval). Formatting information including for black reference (black pedestal) and color synchronizing (color burst) occupy this time. Many more pixels per scan line could be represented using higher video frequencies exceeding NTSC broadcast allotments and the standard is 720 visible pixels for DVD. TV sets may or may not reproduce dots that small and also the first few and last few pixels as displayed on CRT's may take on irregular spacing so as to result in distortion of the extreme left and right edges of the picture.

Video resolution is traditionally measured across the largest circle that fits in the space being referred to. For 4:3 DVD pictures the 720 pixels translate into 540 lines of resolution horizontally. For 16:9 pictures the same 720 pixels give a horizontal resolution of 405. For VHS tape, spots can't be reproduced as narrow, and only about 240 lines of horizontal resolution result from the same analog video scan line format.

Overscan is used on CRT's to, among other things, hide the distorted edges.

Extra information for formatting, closed captions, etc. also occupy bytes between the rows of picture content pixels and between fields/frames of pixels in digital video and these blocks of information are sometimes called horizontal retrace intervals and vertical retrace intervals, respectively. The vertical retrace interval for 480i digital video is not exactly 22-1/2 rows of pixels worth of data (525-480, half of that after the odd field, half after the even field).

DVD and HDTV are not stored/transmitted exactly pixel by pixel. Instead compression is used to save space, for example 5@blue instead of blueblueblueblueblue (oversimplified example). So digital video frames as stored on the DVD or broadcast do vary in size. If a digital video signal is displayed on a CRT, real horizontal and vertical retrace intervals are created and inserted by the TV set. When analog video is displayed on an LCD or other digital display, the scan line is chopped up into however many pixels are needed. It is not out of the question for what would be half of one analog pixel and half of the next to end up as one of the digital pixels so produced. Thus there can be some loss of horizontal resolution relative to the subject matter.

The regular (non-blue-ray) DVD standard allows for 576 scan lines for PAL programs. Other standards could exist too, including HDTV formats, but the limit is the total number of bytes. Much more resolution per frame would result in a very small playing time per disk on regular DVD.

Video hints: Television and Video Resolution

 

[chuckg]

I think Allan has covered the whole thing pretty well!

I've basically decided not to get worried about it...here at work we've gone around in circles trying to create video, and having all sorts of "square pixels versus non-square" issues. We've settled on one method, and we've stuck to it.

One tiny thing I think is slipping away from us: "lines" versus "scan lines" A crummy example:

- - - - -
- - - - -
- - - - -
- - - - -

The above text has four "scan lines" and five "lines" in each scan line. I hope that helps!

 

[Enigmatism415]

Thank you Allan! I think... I understand now, I just might have to study your post for a while. But cheers! I'm sure I'll figure it out.

 

[Allan Jayne]

Psst! (The above text) exemplifies nine lines for horizontal resolution. In both digital and analog video, the white spaces between the black dots or lines count as well. The finest horizontal detail reproduced by DVD (720 pixels across) would be a pattern of 360 thin upright black lines with 359 thin white spaces in between (and a thin white gap at the far right).

But the black gaps between scan lines (if the CRT electron beam is focused that finely) do not count as lines of resolution vertically. So for 480 lines of resolution there must be at least 480 actual scan lines of picture information. The finest show of vertical resolution would be 240 black scan lines and 240 white scan lines alternating.

Given the 720x480 format for regular DVD, the pixels are not square for either 4:3 or 16:9 pictures. But for both 1080i (1920x1080) and 720p (1280x720) HDTV formats, the pixels are square for 16:9 which is their only standard aspect ratio.

Square pixels make it easier to work with computer generated video programs, although any standard can be used if used consistently. Whether or not pixels are square does not matter for live video.

 

[chuckg]

(thanks for picking my nits, they are so awfully annoying!) I only left a gap to make it easier to read - Allan is absolutely correct.

 

[JeremyErwin]

Lines of resolution are also expressed in terms of "lines across the largest circle that can fit on the display"-- which may explain why (the frame buffer of) a DVD is sometimes quoted as having about 500 lines of resolution. 75% of 720 is 540...

It's an analogue concept that doesn't always square well with digital displays and sources. And since digital media is lossily compressed, the actual resolution varies from frame to frame.