Question about digital video and resolution

[Tony Hall]

Am I the only one that finds video resolution confusing? I had a thread going about this on DVXuser, but it ran out of steam. I thought I'd bring up the topic here and see what we can figure out.

Here's what I think I know about video resolution:

Scan lines are the 525 horizontal lines in NTSC video of which DV uses only 480. So, 480 lines is the maximum vertical resolution in DV because a pixel cannot sharply display more than one color at a time.

TV lines is the measure of how many black and white stripes or dots can be sharply displayed 3/4 of the picture horizontally. The maximum horizontal resolution is 540.

So, when people say that there's 525 lines in NTSC and 625 in PAL, they're talking about scan lines

Whey people say that DV is 480 lines, they're talking about scan lines

When people say my DVX has 500 lines of resolution, they're talking about TV lines... because it's impossible for DV to have 500 lines of vertical resolution if it doesn't even use that many.

When they say that this monitor has 800 lines, they must be talking about TV lines because that seems to be what people are talking about when they're talking about resolution. Plus, it's a big number and how could you possibly need that much vertical resolution.

In the American Cinematographer video manual, there's a list of digital formats and their resolutions. It's apparent that they're talking about scan lines since they list the HDcam format as 1080i and 720p. However they list DV as having 500 lines of resolution... that matches more closely with the TV lines of the DVX than the 480 of DV. I'm sure that they're talking about scan lines though.

This is what I mean. Why are we expected to just *know* what kind of lines they're talking about... especially when it could go either way. One thing that I've noticed is that when speaking of a device, it's usually TV lines, or the horizontal resolution... and when they're speaking of a format or a broadcast, they usually mean the number of horizontal scan lines (vertical resolution).

Anyone have any input? Am I wrong about anything?

[Glenn Chan]

1- Manufacturers fudge specs, which is really confusing. Anyways, just ignore them because they're generally not useful. To be useful, all resolution specs have to be measured the same way and not put under some sort of reality distortion field. Sony's camcorder specs for example are fudged to make the numbers look nice and are not very useful.

2- Scan lines refer to the broadcast format and doesn't have too much to do with resolution. In the extra lines you get extra information carried needed to maintain sync and for things like closed captioning. I don't know too much about the exact details of this, so someone else would be able to fill stuff in on this.

3-

In the American Cinematographer video manual, there's a list of digital formats and their resolutions. It's apparent that they're talking about scan lines since they list the HDcam format as 1080i and 720p. However they list DV as having 500 lines of resolution... that matches more closely with the TV lines of the DVX than the 480 of DV. I'm sure that they're talking about scan lines though.

1080i and 720p refer to specific formats. Did they specify the lines of resolution each format can deliver?

As far as DV goes, 540 lines of horizontal resolution (TV lines) is the *theoretical* maximum. In real world situations you would definitely see less. How many lines of resolutions you see depends highly on how you measure, and what the output format is. Also... the 500 lines of resolution spec is a little incomplete without specifying what the contrast between white and black lines is (I'm missing the technical term for this).

In analog formats, you will see that things get fuzzier and fuzzier as the alternating black and white lines on a test chart converge. There comes a point where the black and white lines start turning into greys. If you specify the 'contrast' between the two very low then you will get more lines of resolution.

[Boyd Ostroff]

Couple other threads on this topic...

http://www.dvinfo.net/conf/showthrea...threadid=17642
http://www.dvinfo.net/conf/showthrea...threadid=27834

[David Kennett]

Tony

I can see why you are confused. You seem to have a pretty good handle on most things. Ive worked in broadcasting for years, and have grown up with TV. Sometimes that helps to clarify things. You must know what kind of equipment you're talking about when you read the specs. Is it standard TV or HD? My response to the following question assumes it is a standard TV monitor.

"When they say that this monitor has 800 lines, they must be talking about TV lines because that seems to be what people are talking about when they're talking about resolution. Plus, it's a big number and how could you possibly need that much vertical resolution."

Back in the old days, there were no pixels to define resolution (horizontal or vertical). Vertical res was fixed. There are 21 lines in each field that have no picture info. So subtract 42 (2x21) from 525 and you have 483. Since that is so close to VGA 480x640, they just kinda forget those extra three lines. See how pixels equate to lines of resolution! Therefore, standard TV has 480 lines (or pixels) vertical resolution MAXIMUM. In reality it's always a little less, depending upon equipment quality.

Still back in the old days - horizontal resolution was yet another matter. If you had better high frequency response, you could get better horizontal resolution. And this is where competitors tried to excel. Keep in mind that there were not yet pixels (a digital term) to define resolution. So horizontal resolution was DEFINED in terms of scan lines, even though scan lines are irrelevant in terms of horizontal res. So it's entirely possible that a quality monitor could have 800 lines of HORIZONTAL res. Everyone already knew what the vertical HAD to be.

In the analog world horizontal res was limited by the frequency response, and it varied, depending on the medium.

HORIZONTAL resolutions:
VHS - 250 lines
S-VHS - 400 lines
Laserdisk - 400 lines
Broadcast - 360 lines (limited by the 6MHz broadcast channel)
DV - 540 lines

Lines of resolution DO equate to pixels, but there is one more tradition to clear up. Since the measure of resolution was LINES, and the REAL lines always refered to VERTICAL res, horizontal res was always given it terms of height This meant you had to multiply the hor-res by 4/3 (the aspect ratio) to get true hor-res. Today digital res is usually given in real pixels ie. 1280x720 HD.

Notice how the DV 540 lines hor-res, when multiplied by 4/3 equals 720, the actual number of hor pixels in DV.

24 bits of data is usually assigned to each pixel (24 bit color)

Hope this helps a little

[Tony Hall]

Thanks, that does help a lot. Those two links above were pretty hardcore and Dave, your post was helpful too. I'm going to let all that sink in now.