Resolution

Resolution is defined as the number of picture elements one can discern within a given space.  For example, monitor resolution is typically given as 72 ppi (Pixesl Per Inch) and printer resolution may be 300 dpi (dots per inch).  Television completely confuses the issue, because of the way it scans horizontal lines across the screen.  There are no scanned vertical lines, but many sources quote television horizontal resolution in terms of vertical lines.  What's that?  Horizontal resolution in terms of Vertical lines ??  

Yes. The number of lines defines the resolution, but in the opposite (perpendicular) direction !!! Always remember :

Horizontal Lines = Vertical Resolution

Vertical Lines = Horizontal Resolution

The Kell Factor ( .7 )

This states that the actual visible resolution is about 70% of the number of physical lines.  So for broadcast TV, you only get an actual visible vertical resolution of approx 330  (.7 x 480 = 336).  The Kell factor is also used for horizontal resolution.

The "Line Pair" Myth

- where 1 Line = 2 Lines (Black/White)

This myth appears in many places.  It is completely ridiculous and not applicable to TV.  It comes from using an old photography concept - where alternating black and white lines are used to describe resolution - but then is completely misstated.  Yet many of the sites state "you must have 1 black line and one white line to represent one line.  Huh ??  They also say you need two B/W pixels for a resolution of 1.  That's all they say - no explanation.

For some unfathomable reason they say you need two lines (one black and one white) to represent a line resolution of 1.  Since this is fully embedded in the literature - we must talk about it.  Henceforth we will call these "line pairs" to avoid confusion.  For example, the screen has 480 visible horizontal lines, and most say that means it has resolution of 240 lines, which is very confusing.  

Looking a bit deeper into this, what is the vertical resolution of these images?  Assume that each is a portion of 4 lines on a TV screen.

It is 4 obviously.  But the myth says it is 2.  They claim that without the black "background" there is no way to resolve the white.  Ridiculous.  In that thinking the second image would not have any resolution since there is no black !!  Yet - according to these websites "you must have alternating black and white lines" !!!

I have seen several websites that go into great detail to explain it - but their explanations, at least to me - do not cut the mustard of clearing this up. Here is a typical explanation taken from http://www.evdemo.com/page.asp?id=15 that tries to show why only line pairs can be counted when figuring resolution.  He uses an analogy of placing an image of a ladder onto a screen :

"It would seem that 483 scan lines would give you a vertical resolution of 483 vertical lines of resolution (483 rungs on the ladder). This is not the case. If one scan line saw one rung, the next scan line would need to see the space between the rungs, and the following line would see the next rung in order for the rungs on the ladder not to merge together. Put another way, if each scan line saw a rung, then the ladder would look like it was made of solid rungs with no spaces. Thus, an image that goes "rung-space-rung-space" is defined as 4 lines of vertical resolution and it took four scan lines to do it. Thus, 483 scan lines can show 120 actual rungs on the ladder.  

Hmmm, actually he is even farther off than the line pair myth - he is saying you need four lines to get one line !!

First off - if you have each successive line showing rung-space-rung-space, then with 483 lines you would see 241 rungs - not 120.  Why he divides 483 by 4 I cannot comprehend.  But the theory he is describing is that you must have black spaces in between to see a ladder - black-white-black-white, etc - that these "pairs" are what you count.  Sure, but you still have info in the black line - it shows the space between the rungs - so count it as part of the picture resolution!!  And what if it wasn't a ladder?  What if it was a tree trunk?  Then you have brown-brown-brown-brown.  Each line contributes to the resolution - not each line pair !!!  So long as each line is visible, and contributes to the picture - then lines=resolution.

BUT - this perfect scenario, where total horiz lines = vert resolution, can only occur if the actual image just happens to line up exactly with the scan lines.  So there is a limitation that does reduce the resolution, which we explain next.

Vertical Resolution (Horizontal Lines) Limitations

Well, actually it "could be 480" - if you send the TV a signal that consists of 480 unique colored lines, and they are are in the exact same position of the 480 physical scan lines of the TV.  Then you will be able to see every line (close up with a magnifying glass, at least).  

However, they never line up precisely in the real world, just as any image placed on a TV will have edges that will not line up precisely with the physical scan lines.  

For example, if the image with 480 alternating B/W lines and is positioned so that the lines are halfway between the TV scan lines, then every line will show up as gray, because it will have to interpolate the two half-lines that lie on it's space.  When it averages white with black it displays gray.  

This is a good example of Kell's theorem.  It is also a good example of the line pair myth - since with image "a" the squares are reproduced exactly - therefore the resolution is defined as the numer of squares - not the number of "pairs of squares".

But you typically can only resolve at maximum about 70% of the actual physical scan  lines.  Therefore, the input image cannot have more alternating vertical lines or picture elements than 336, when placed onto a 480-line screen.  This is why broadcast TV is rated at approx 330 lines.

Horizontal Resolution limitations

Going across the scan line, you have a much tighter limitation  .  .  .  bandwidth.  For example - the following image will require a square wave for exact reproduction :

The sharp edges on a square wave require about 15 harmonics of the base 4 MHz carrier signal, or 60 MHz.  So the Television simply can't handle it.  Therefore, instead is does the best it can with the 4 MHz, and uses a quick but gradual change for the edges.  This causes a small amount of blur - but it is generally not enough to notice.  The following shows the actual luminance used, since a square wave cannot be reproduced at that bandwidth:  The horizontal blur effect is exaggerated to show to effect :

Again - many describe the resolution in "pairs", since one pair of black/white pixels = one cycle.  But if each pixer is uniquely visible, then the resolution is the number of pixels - not the number of pixel pairs.

So how many pixels across can a TV accurately reproduce??  Well, there is an advantage here - we are no longer contrained by having to reproduce the image using evenly spaced lines - as we are in the vertical direction.  The horizontal luma signal is analog, and can lay down images anywhere on the line.  It's only limitation is the 4.3 Mhz bandwidth of the luma carrier.  Again the Kell factor comes into play, but this is a topic of debate and you will see many values mentioned.  The horizontal scan signal is very much the same. The horizontal scan rate is 525*29.97 or 15,734 Hz. Therefore, 63.6 uS are allocated per line. Typically about 10 uS of this is devoted to the blanking line on the horizontal scan, leaving 53 uS.  

There are so many numbers quoted - 427, 455, 330, and 220 pixels per horizontal scan line, etc.  Some quote pixels, some quote pixel pairs.  Until I find this number listed in an ITU, NCSA, or CCIR recommendation, I have decided to leave this alone for now.

The Resolution of VHS and other Formats

VHS uses the same vertical resolution as the TV's horizontal visible lines  .  .  .  240.  What you lose in VHS is the horizontal resolution - the detail *within* each scan line. I'm going to use a nonstandard term here, and say that broadcast video is supposed to be able to resolve about 440, uh, "details" within a scan line. (Note that this applies to the luminance signal only; chroma is much worse.) This is usually quoted as 330 lines per picture height: "lines" because these things are tested with test charts having vertical lines at different densities (and having nothing to do with scan lines), and "per picture height" meaning 330 details along a portion of a scan line equal in length to the picture height. Since a scan line is about 4/3 of the picture height, this comes out to about 440 "details".

If you want to talk in terms of light/dark line pairs, which is the way resolution is normally quoted in the photographic arena, cut these numbers in half. ie 330 lines of resolution means about 165 pairs of light/dark lines.

(Why was the number 330 chosen for NTSC? Because that's about the number of "details" that can be reliably resolved in the vertical direction with 525 scan lines, taking Nyquist, Kell, and others into account. So with vertical resolution fixed by the number of scan lines, there is no point in making the horizontal resolution any better.)

VHS cuts this to about 220 "details" per picture height, or about 280 "details" per scan line

Here are some other common formats followed by their typically stated resolutions in Horiz x Vert (remember - these vary depending on the source !! ) :

Broadcast: 330x482
CABLE: 330x482
BETA: 250x482
SUPER BETA: 285x482
Laser Disc: 425x482 (4:3)
VHS: 240x482
S-VHS: 400x482
DVD 4:3: 540x480 (progressive scan)
DVD 16:9: 405x480 (progressive scan)
HDTV Standard Res: 704x480 (16:9)
HDTV Hi Res: 1280x720 (16:9)
DBS: 364x240
ED-Beta: 525x482
TVRO: 525x482 (Live Backhaul)

The many Meanings of Resolution

The word "resolution" when applied to TV screens is not the same as the word resolution when applied to computer monitors.  Nevertheless, we have no other way of comparing them - so the practice of using them interchangeably continues.  Here are a few different meanings for the word :

• PC Monitor Resolution - the measure of width x height in pixels.
• TV resolution in physical lines - 
  Horizontal Lines - the number of visible lines (approx 485, or 480 for simplicity - this is also the "maximum" resolution", since no matter what signal you feed into the TV, it can do not better than this.  
  Vertical Lines - there are really only horizontal lines (vertical resolution) since the beam scans left-to-right.  Nevertheless, the horizontal resolution is sometimes stated as vertical lines, but it is actually just the max number of allowable signal phase changes as the beam moves from left to right. Some refer to it as pixels, but that also is incorrect technically.
• TV resolution in visual lines - the actual visible resolution in "line pairs" - for some unfathomable reason you need two lines (one black and one white) to represent a resolution of 1.  Therefore the vertical screen resolution is 240, since you have 240 pairs of lines.  However, the actual screen resolution could be 150 lines, if you sent a signal to the television with 150 line pairs - it would still be displayed using the 240 physical line pairs, but the picture would be 150 line pairs visually.  Each visual line pair would be spread out across 1.6 physical line pairs (1.6 = 240/150)

NOTE:  the visual lines are created from the the signal feeding the TV.  This is about 320x240 for VHS and 400x320 for SuperVHS or Hi8.  Not that in both cases, the screen's aspect ratio is 4:3  (for 320x240 the pixels are square and for 400x320 the pixels are rectangular, or squashed together a bit tighter in the horizontal direction).

For the next section - physical lines is the actual scanned lines of the television, while visual lines is the actual lines sent into the television by the input device or via the airwaves.

Horizontal Physical Lines - Vertical Physical Resolution

• there are 525 horizontal scan lines in every television frame - no matter what signal you feed in.  
• There are 40 invisible lines used for flyback (vertical retrace) and 485 visible lines
• we approximate the number of horizontal lines to 480.
• so there are 480 visible lines in one frame, which consists of two fields, each with 240 lines
• this creates 240 line pairs (see line pairs myth) - which gives us 240 lines of vertical resolution  - this is the maximum possible resolution according the the myth.  In reality, take the Kell factor of .7 and multiply it time the 480 lines, for a vertical resolution of 336 - rounded off to 330.

Horizontal Visual Lines - Vertical Visual Resolution

• The number of visual lines cannot exceed the number of physical lines, so the max visual vertical resolution is 240 horizontal lines

Vertical "Lines" - Horizontal Resolution

• vertical lines - many use this term - but there really are no physical vertical lines
• you can measure horizontal resolution in pixels instead of lines to avoid confusion
• the horizontal resolution is measured by determining how many black/white pairs of pixels (or vertical lines) we can squeeze into the picture from left to right. Many prefer to view this as lines, but there are no vertical lines !!
• physical horizontal resolution (max resolution) is a function of the signal bandwidth
• visual horizontal resolution is dependent on the input signal, which varies deppending on the platform used
• typical broadcast TV and VHS horizontal visual resolution is approx 320 pixel pairs (640 pixels)
• the S-VIDEO connector is capable of a horizontal physical resolution of 500 (but no format has an input signal with 500 lines)
• Hi8, SuperVHS and LaserDisc horizontal visual resolution is approx 400 pixel pairs (800 pixels)

Visual resolution can only be as good as the weakest link in the chain - so the max resolution of your Television will not be attained if your input device and media does not support it.