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 displayed series 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 Frame

Therefore 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 is scanned 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 lines

40 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 cropped

The 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 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, and field 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.  This vertical 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)


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 line 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:

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:

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 left to 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), 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 the vertical 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, 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 to simply 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 fact that 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 field 1 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 over where 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 !! 


Field Line (odd/even) Numbering - this is the most common method.  It is also slightly confusing 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 lines are 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 - not 525 !!  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.  

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, in figuring 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 cause confusion 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 :

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 !!


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 will approximate 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).

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 !!



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 at 15,750 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 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) - going from bottom to top

  2. Closed Caption line (invisible - the beam is turned off) - at the top

  3. cropped lines - on top

  4. visible lines - going from top to bottom

  5. 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 the screen, 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 :

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 the left - 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

*** 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 is visible.  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

2)  Field 2 is scanned onto the screen

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?