interlaced scanning

8/13/2019 Interlaced Scanning

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Interlaced Scanning

- the most in-depth discussion of scan lines you will find - anywhere -

If you Forget everything else on this Page - Remember These facts . . .

Fields and Frames

Moving Pictures (video) appears the same as real life - but in reality is a quickly displayedseries of images, or pictures

Television camera's take 30 pictures each second

Movie camera's take 24 pictures each second - which are converted to 30 per sec for TV

using Telecine TV calls one picture a Frame - it breaks each frame up into two interlaced fields

The Television screen displays 60 fields each second

We define the 60 fields as 30 pairs of fields, and each pair is called a FrameTherefore the Television screen displays 30 Frames each second (the exact number is 29.97 -

rounded off)

In reality, physically, there are no frames - it is a concept to denote one full image- what isscanned onto the screen is fields

our eyes and brain meld the fields together so that we perceive it as smooth, flowing video

Scanned Horizontal Lines

Each Field is comprised of 262.5 horizontal lines which are scanned onto the screen, left to

right, each line is scanned below the previous line

There is one odd Field (Field 1) and one even Field (Field 2)The odd field scans lines 1, 3, 5, etc and the even field scans lines 0, 2, 4, etc - hence the

term interlaced The two field's interlaced lines mesh perfectly to create one full frame of lines 1,2,3,4, etc

There are 525 horizontal lines total in each frame, 262.5 lines per field - but only 91% of

them are visible

the scanning beam is turned off for all invisible lines40 lines are for vertical retrace and are invisible, 2 lines are for closed captioning info and are

also invisible

483 lines are active - meaning the scanning beam is turned on

A few of the active lines (3 to 6 lines) are croppedThe final number of visible lines after cropping is approximately 480.

No one except television Engineers needs to know this stuff. However, the TV

scanning algorithm is particularly fascinating. The method is so clever and

ingenious. At the same time, it is extremely difficult to sort out the details.

When you look at a TV screen, you see smooth, flowing motion. What you don't see

is the high-speed scanning of horizontal lines, one on top of the other, to quickly


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display images, one after the other, which in turn are melded by your eyes and brain

into motion.

Every 30th of a second, two interlaced fields are scanned onto the screen to display a

full image, called a frame. Every frame is composed of two fields. Every field is

composed of alternating horizontal lines - field 1 scans the odd numbered lines, andfield 2 scans the even numbered lines. That way, the two fields are like two

interlocking pieces of a puzzle, and since they are painted onto the screen so quickly,

all you see is the resulting frames which are created by the pairs of fields. Your eye

and mind melds the 60 fields per second, which is 30 frames per second - into

smoothly moving video.

Basic Scanning

Color TV Scanning - three high-power pinpoint beams of electricity are slamming

into the back of the screen, which is covered with Red, Green, and Blue dots. The

beams rapidly scan across from left to right, lighting up the dots, creating any color in

the rainbow, depending on how much power is sent to each dot (Red, Green, and Blue

are primary colors - which means they can be combined to create any color). Then

the beams blank off for a split second while they retrace back to the left and slightly

lower. They then turn back on and scan across again - creating another line of colors

below the previous one.

Once the last line is scanned at the bottom, the beams continue scanning left-right, but

turn off, moving up - returning to the top of the screen to scan another field. Thisvertical retrace takes 20 horizontal scan cycles. The next field begins it's scanning in

the middle of the screen, so that this next set of lines are placed exactly halfway

between the previous set.

Monochrome TV Scanning - (Black and White) - uses the exact same process -

except that is uses only one beam. We will describe the monochrome scan process in

detail, since it is a mirror of the color process.

From this point on - all discussion refers to monochrome TV

Color TV uses the exact same algorithm but has 3 beams

(RGB)


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This is the only complete explanation of television line scanning on the web. As a

former TV repairman, I took a particular interest in this page. Since there are

hundreds of often contested facts about this process - this page will remain a work in

progress for quite some time.

Due to the vast disagreement from multitudes of sources, this page has required

hundreds of hours of research - much of it involved verification of facts. Apperently,

this is the only TV scanning description page that includes all of the following in

the li ne budget :

half lines - 1st field ends scanning in the middle, 2nd field begins scanning in the

middle actual full lines , or cycles per field - where Field 1 is 262 5/12, and Field 2 is 262

7/12 full lines two closed captioning lines - they occur immediately after the vertical retrace lines the first line of each field is a vertical retrace line - begins at the bottom - not at the

top

cropped lines are included - the number of cropped lines may vary

Without these items, it is impossible to fully comprehend the process. As always,

there may be inaccuracies, since the sources do not agree on virtually anything. So

feedback is always welcome.

Deflection of the Beam

Sawtooth waveforms are sent through both the left/right pair of coils and the

top/bottom coils around the deflection yoke of the CRT. These waveforms cause the

beam to bend - moving from left to right, and top to bottom. The retrace portion of

the waveform is much more sudden and steep and this causes the beam to retrace back

(to the left for horiz retrace and to the top for vert retrace). Here is a simplified

diagram - there are actually approximately 480 visible lines and exactly 20 retrace

lines:


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Full Lines (cycles) vs Visible Lines

Here is a full line. Which is the same as one complete horizontal scan cycle. It

includes both visible scanning and retrace :

The complete horizontal scan cycle is 63.4 uSec (15,750 cycles per sec). 53 uSec for

the left-to-right scan, and 10 uSec for the right-to-left retrace. The common divider is

12, which breaks up the cycle into segments of 5.25 uSec. Therefore the visible scan

is 10/12 of one cycle and the retrace scan is 2/12 of one cycle. One-half of a visible

line is 5/12 of one cycle.

The weird, 262.5 Lines per Field Number

Now, why is it that each field has 262.5 lines ?? First off - Field 1 begins scanning

from the left, while Field 2 begins scanning from the middle to insure that it's lines fit

exactly in between the lines from Field 1. From the diagrams above, we see that:


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Field 1 has a half line at the end. After that Field 2 begins scanning.

Field 2 has a half line at the beginning, and a retrace line. After that it scan lines normally,

beginning at the left.

Therefore, both fields have one visible half-line. But looking closer, we see that Field

2 includes a retrace line, whereas Field 1 does not. So if we look at the two timewise,as a proportion of one entire cycle - it comes out to 5/12th of a cycle for Field 1 and

7/12th of a cycle for Field 2.

So if you define a full line as a complete cycle, then Field 1 has 262 5/12 full lines and

Field 2 has 262 7/12 full lines.

Now, looking at the diagrams below, keeping in mind that each Field begins scanning

with the invisible vertical retrace:

Field 1 begins scanning at the left of the screen. The last "half line" actually goes from the leftto the middle - which is 5/12th of one cycle.Field 2 begins scanning in the middle of the screen. The first "half line" goes from the middle

of the screen to the right (5/12), and includes the retrace back to the left (2/12), for a total of

7/12th of one cycle.Both Fields - the two half lines of each field = 5/12 + 7/12 = one cycle

Field 1 - each line begins at the left - the last line of the odd field moves halfway

across the screen, which is 5/12 of a cycle - for a total of 262 5/12 cycles - or full

lines. Since only 1/2 line is visible at the end, there are 262.5 visible lines.

Field 2 - each line begins in the middle - the last line of the even field begins halfway

across, moves to the right (5/12 cycle) and then retraces all the way back (2/12 cycle),


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for a total of 7/12th of one cycle. Therefore Field 2 has a total of 262 7/12 cycles -

or full lines. Since only 1/2 line is visible at the end, there are 262.5 visible lines.

NOTE: here we are showing the first horizontal line (line 0) at the top of the screen,

just to familiarize you with how this works. BUT in the final section - we show how

the actual NTSC scan lines are numbered - starting at the bottom (the beginning of thevertical retrace). Fortunately even though it begins from the bottom - the first line for

Field 2 will still start in the middle of the screen horizontally, just as it does here, so

you can make the transition from this model to the actual model easily.

Rounding to a Half Line

Since the scanning to the right takes 53 uSec, and the scanning to the left takes 10

uSec - from this point on we will ignore the horizontal retrace, since it is a small

fraction of the cycle. We can safely say that if we ignore these values, our numbers

will never be OFF by more than 2/12 of a cycle . . . close enough. Besides, in both

fields, the final scanned visible line is a half line.

Those %$#* Half-Lines - How to Number Them ??

Field 1 begins scanning normally at the left of the screen, although it ends scanning in

the middle. Field 2 begins scanning in the middle of the screen, and ends scanning at

the left. What? The left? Yes - the full horizontal scan cycle begins at the left,


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moves to the right, and then retraces back to the left. So you must consider a full

cycle to define the beginning and end !! Therefore, a normal scan line begins at the

left, and ends at the left.

But these half lines !! They are needed so that each field scans lines that fit directly in

between the other field's lines. But the resulting complexity has driven many sites tosimply ignore the half-lines altogether !!

How do you number lines, when the first line begins at 1/2 ?? You can assume that

all lines in field 2 begin halfway across the screen so that line 1 starts at 0.5 and ends

at 1.5. Or you can simply omit all references the the half-line as most other sites do.

We take another approach. The half lines will be fully included in all discussion of

the scanning process.

Each half line is numbered as a normal line - and we simply make note of the factthat it is a half line. For example, a frame has lines numbered from 0 to

525. This is 526 unique line numbers, but line 0 and line 525 are half lines, so the

total is still accurate at 525 lines.

Timed Line (consecutive) vs Field Line (odd/even) Numbering

To further confuse things - scan lines are numbered in two different ways:

Timed Line (consecutive) numbering - this confusing method is used by some explanations

of scanning. It is the exact order the lines are scanned (1,2,3, etc) onto the screen, where field1 is scanned, and then field 2. This method corresponds to the sequential timing of the scan

lines. Unfortunately, as you can see, the resulting Frame line numbering is all messed up

!! Using this method, Field 1 scans lines 1,2,3,4, . . . 262, 263. Then Field 2 takes overwhere field 1 left off, and scans the lines 264, 265, . . . 523, 526. Note that this totals 526 lines

- not 525 !! Not really - you must realize that line 1 of Field 1 is a half-line, and line 526 of

Field 2 is a half line !!


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Field Line (odd/even) Numbering - this is the most common method. It is also slightlyconfusing because the line numbers do not go in the order that they are scanned onto the

screen. However, it does result in a frame with consecutive numbers. Each field's visible linesare numbered using either odd line numbers (Field 1) or even line numbers (Field 2). This

method corresponds to the physical placement, or where each line is. It does not correspond to

time, since all the odd lines are scanned first - then all the even lines. Field1 scans lines 1, 3, . .

. . 523,525 and Field 2 scan lines 0, 2, 4, . . . . . 522,524. Note that this totals 526 lines - not525 !! Again, you must realize that line 525 of Field 1 is a half-line, and line 0 of Field 2 is a

half line !!

Why is the first Line of Field 2 numbered Zero ??

The first line of Field 2 is a half line that begins in the middle of the screen and scans

to the right. This places it physically above line 1 of Field 1. If we assign it as line 2,

then the lines would be numbered, from top to bottom as 2,1,4,3,6,5, etc. If we assign

it as Line 0, then the lines are numbered in succession as 0, 1, 2, 3, etc.


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Why are the Half Lines assigned a Full Line Number ??

The process is already confusing enough, not to have to deal with fractions or

decimals for line numbers. You just have to remember that the last line of Field 1 is a

half line, and the first line of field 2 is a half line. So we will refer to the half lines by

numbering them with integers, as if they were like any other line. However, infiguring the line budget - it is important to add them up as half lines !!

This section shows a very common model - where the line

numbers begin at the Top. This is 100% accurate - Line

Numbers actually begin at the bottom with Vertical Retrace(shown in the following section)

Here is a method of modeling the line scanning, that simplifies the process. This is a

common model - and is included here so that you will not be confused when going to

other sites. BUT beware - it begins the line numbering at the top !! This model is

more intuitive, and that is why it is used to describe the process. But is can causeconfusion when compared to the NTSC model, where scanning begins at the bottom

with the retrace lines.

The model below uses Field Line (odd/even) Numbering but only numbers the visible

lines !! In truth, the first line of an NTSC display that is visible is approximately line

47 of field 1 (see next section). The beam actually moves downward at a constant rate

- therefore the scan lines have a very slight downward angle. The following diagram

illustrates this - it shows only a few of the scan lines and a few of the flyback lines, so

the angle is exaggerated - with 525 lines, the actual angle is so slight that it appears to

be straight :


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Note that the second field begins halfway across the screen - that way it will not

simply scan over the exact same positions as the field 1 scan lines. This ingenious

method causes the field 2 scan lines to be traced exactly between the field 1 lines !!


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This section shows the actual NTSC Line numbering methods,

where Scanning begins at the bottom with the Vertical Retrace

First, a Simplified, 7-Line Model

Let us begin with a simplified example (we omit cropped and closed caption

lines). Since NTSC television uses an odd number of total lines per frame (525) and a

number of lines per field ending with .5 (262.5) - we willapproximate the scanning

process by using a frame that has an odd number of lines - 7 lines, and fields that

have a total number of lines ending with 1/2 - each field is 3.5 lines.

7 Lines total - Field 1 (1 retrace line, line1, line3, line5 (half)) and Field 2 (1 retrace

line, line0 (half), line2, line4)

5 Visible Lines - Field 1 (line1, line3, line5 (half)) and Field 2 (line0 (half), line2,

line4).


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The following diagrams show both frame numbering (same as "timed" numbering - in

green) and field numbering (in black) - and include the half lines. Since we are only

using 7 lines - the angle is much more severe than a 525 line screen !!


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Why does the Scanning begin at the Bottom ??

The scanning is cyclical, and so it does not matter where you define the beginning and

end. However, when you first turn the TV on, it must synchronize before it can scan

anything. During the scanning of the visible lines, a constant horizontal synch pulse is

issued at a rate of 15,750 cps. But the only portion of the signal that contains both

horizontal and vertical synch pulses is the vertical retrace period. During that time,

vertical synch pulses are clocked at exactly twice the rate of the horizontal synch

pulses - or 31,500 per second, and the horizontal synch pulses continue normally at15,750 cps.

Since this is the period where everything is synched up together - it was decided to

view this as the beginning on the scanning process for Field 1. If you disagree, look

at the one clue - the 2 closed caption lines - they are defined as the 21st lines for both


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fields. Since there are exactly 20 vertical retrace horizontal scan lines per field - it is

obvious that they come before the CC line. Therefore the order of scanning a field is:

1. vertical retrace horizontal scan lines (invisible - the beam is turned off) - goingfrom bottom to top

2. Closed Caption line (invisible - the beam is turned off) - at the top3. cropped lines - on top4. visible lines - going from top to bottom5. cropped lines - on the bottom

What do you mean by "Vertical Retrace Horizontal Scan Lines"

The vertical retrace moves the beam from the bottom to the top. This is much slower

than horizontal retrace, and during that time - 20 invisible horizontal lines are

scanned. Therefore it is a vertical retrace, during which time horizontal scan lines are

being scanned.

The Real Deal - the NTSC Line Budget

Finally - we will stop with the samples, and will now show you how the interlaced

scanning line are numbered for standard Television !!! We will include all details as

closely as possible. Please realize that there are several ways to number the lines. We

will use field numbering to make sure each field is detailed - as opposed to

consecutive line numbering that describes the frame as a whole.

The screen scans the lines in an "interlaced" pattern. To paint one image on thescreen, which is called a "frame" - two fields are painted. First the TV scans field 1

(odd, or lower field), where it scans all the odd lines. To be accurate, the following

elements must be included :

cropped scan lines - there are a few lines at the top and bottom that wouldnormally be visible, but they are cropped by the TV frame - more severe

cropping occurs at the sides

visible scan lines - each line is scanned from left to right, which takes about 53uS

Retrace - invisible - the beam returns back to the left but is blanked out duringthat time - this takes about 10 uS

vertical retrace (flyback) uses horizontal scan lines 1-39 (odd) for Field 1, andlines 0-40 (even) for Field 2. For example, after the last half line for Field 1

(line 525), on the bottom is scanned - the beam returns to the top, which takes a

number of scan cycles - just as it is during horizontal retrace, the beam is

blanked out during vertical retrace


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half lines - line 525 of Field 1 is a half line, and line 0 of Field 2 is a half line closed caption lines - the closed caption info is contained within line 21 of each

field, immediately after the last vertical retrace horizontal scan line. For frame

scan-line numbering this works out to lines 21 and 284. For Field Line

(odd/even) Numbering this works out to line 41 for Field1, and line 42 for field

2.

Television actually uses 525 lines, where 486 of these lines are visible (active) -

rounded off to 480. Some of the lines are cut off at the top and bottom, which is

called "cropping". Others are lines that occur as the beam moves from the bottom

back to the top of the screen. Those are called "retrace lines" and they all must be

blacked out by placing a 700-volt charge on the guns during that time.

For each horizontal line - the beam scan from left to right. When the beam reaches

then right end, it must move back to the left (retrace), and move very slightly lower so

that the next line it scans is below the previous one. During the movement back to theleft - the beam is blacked out.

Here is the sequence in scanning the interlaced fields - we are using Field Line

(odd/even) Numbering, and we are including all retrace lines !!! Again - very

important - we are using Field Line (odd/even) Numbering - not timed numbering

!! The actual number of cropped lines varies from TV system to system - but the

number of vertical retrace lines is fixed by the NTSC broadcast signal. The number of

cropped lines is approximated, and can vary from TV to TV !! The following

diagrams detail the scanning process of each field:

*** 262.5 lines = 262 complete lines 1 through 523, and 1/2 line 525


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*** the closed caption line 41 may be part of the retrace, or it may be the first

line at the top - this is not clear - so we can only say that it is indeed line 41,

which is the 21st line of the field

*** 262.5 lines = 1/2 line 0, and 262 complete lines 2 through 524

*** the closed caption line 42 may be part of the retrace, or it may be the first

line at the top - this is not clear - so we can only say that it is indeed line 42,

which is the 21st line of the field

*** although line 44 says "half line", this is only to show that only half of it isvisible. Line 0 is the only half line, and the rest of the lines can be understood as

full lines that begin at the left of the screen. Therefore, the beam, during line 44,

is blanked off for the first half, and turned on for the 2nd half. Line 44 is a full

line.

1) Field 1 is scanned onto the screen

beam is at the bottom left, and begins scanning the vertical retrace, with blanked out horizontal lines

it starts on the bottom on line 1, then line 3, then line 5, up to line 39, where thevertical retrace is completed

line 41 is also blanked out because it contains the closed captioning info lines 43 and 45 are cropped and not visible (the actual cropped lines varies) the lines 47, 49, . . . 521, 523 are scanned and are visible the last line - line 525 is a half line !!! - it is cropped and not visible


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2) Field 2 is scanned onto the screen

beam is at the bottom center, and begins scanning the vertical retrace, with blanked out horizontal lines

the first retrace line - line 2, is a half line !!! each full cycle begins in the middle, goes to the right, retraces back to the left,

and returns to the middle

it starts scanning on line 0 which is a half line !! then line 2, 4, 6, etc - up toline 40, where the vertical retrace is completed

(the retrace goes from 0 to 40, but each cycle begins and ends in the middle of

the screen)

line 42 is also blanked out because it contains the closed captioning info lines 44 and 46 are cropped and not visible (the actual cropped lines varies) the lines 48, 50, . . . 522, 524 are scanned and are visible line 524 is cropped and not visible

Final Line Budget Including Both Fields

lines are numbered from 0 to 525 (526 unique line numbers)

lines 1-524 are full lines = 524 lines

lines 0 and 525 are half lines = 1 line

Total = 525 lines

How to View the Vertical Retrace

- also called Vertical Blanking, or Flyback

You can see these 40 lines by moving the vertical hold control, until you see a black

bar across the center. It is comprised of the 20+20 scan lines used for flyback. The

question is - when you do this, the TV is showing you an image, albiet an off-center

image. Yet, it must somehow be scanning the lines from top to bottom, and

performing flyback to get back to the top. Otherwise you could not view the picture

that you are seeing.

So you have the bottom to top vert retrace, and also the vert retrace showing now in

the center of the screen - how is this possible? Are there now two sets of flyback lines

- those in the middle and those that bring the beam back up to the top?


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interlaced scanning

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