12/6/07

What exactly is ATSC?

“Advanced Television System Committee” is the name of the technical standard that defines the digital TV (DTV) that the FCC has chosen for terrestrial TV stations. ATSC employs MPEG-2, a data compression standard. MPEG-2 typically achieves a 50-to-1 reduction in data. It achieves this by not retransmitting areas of the screen that have not changed since the previous frame.

Digital cable TV systems and DBS systems like DirecTV have devised their own standards that differ somewhat from ATSC. Their high-def set top boxes (STBs) conform to ATSC at their output connectors. Most of these systems use MPEG-2. (DirecTV and Dish Network are delivering systems that use MPEG-4.)

ATSC has 18 different formats. All TVs must be able to receive all of these formats and display them. Most TV sets will display only 1 or 2 of these formats, but will convert the other formats into these. All 18 formats are shown in the following table.

spec	Horizontal pixels		Vertical pixels	Aspect ratio	Monitor interface	Format name	Frames per sec	Fields per sec	Transmitted interlaced
ATSC	1920		1080	16:9	1080i	1080 60i	30	60	yes
						1080 30p	30	30	no
						1080 24p	24	24	no
	1280		720	16:9	720p	720 60p	60	60	no
						720 30p	30	30	no
						720 24p	24	24	no
	704		480	16:9	480p	480 60p	60	60	no
					480i	480 60i	30	60	yes
						480 30p	30	30	no
						480 24p	24	24	no
	704		480	4:3	480p	480 60p	60	60	no
					480i	480 60i	30	60	yes
						480 30p	30	30	no
						480 24p	24	24	no
	640		480	4:3	480p	480 60p	60	60	no
					480i	480 60i	30	60	yes
						480 30p	30	30	no
						480 24p	24	24	no
NTSC	»640	483		4:3	Note 1	NTSC	30	60	yes

Note 1: Some people refer to NTSC as 480i.

When converting NTSC to digital, about 640 pixels are required to reproduce the image nicely even though the true resolution of NTSC is roughly 400 pixels horizontal.

Interlacing

The term interlacing refers to the practice of drawing all of the odd numbered lines on the CRT, and then drawing all of the even numbered lines, which are drawn interspersed with the odd numbered lines. For 1080i, the 540 odd numbered lines are one field, and the 540 even numbered lines are the other field. When interlacing is employed, there are always two fields per frame. Progressive scan means that interlacing is not employed.

One advantage of interlacing is that, for a given bandwidth, it allows higher resolution (more pixels). Another advantage is that it reduces flicker: A bright white area of the screen will flicker (pulsate rapidly) if that area is drawn only 30 times per second. Drawing 60 fields per second mostly prevents that. A disadvantage of interlacing is that, with only 30 frames per second, it doesn’t portray motion as smoothly. Another disadvantage is that data compression is not as efficient.

1080i and 480i are interlaced formats, while 720p and 480p are progressive formats.

The Monitor Interface

The receiver reduces the 18 formats to 4. The display monitor only has to deal with at most four formats. Most receivers let you select the output format, which you must match to what the monitor can do.

If you look at the second ATSC format in the above table, 1080 30p, you will note that it is transmitted in progressive format, but the receiver will convert it into 1080i, an interlaced format. Why? That is because most CRT TV sets must draw this image interlaced to prevent flicker. (CRT sets that can draw 1080 lines at 60 frames per second are very uncommon.)

Presently there are only four defined interface formats: 480i, 480p, 720p, and 1080i. There could be more, and there can be monitors that can benefit from something else. But presently such a monitor will have to have a built-in receiver. (1080p60 and 1080p24 are becoming a more common monitor interface formats, but the wisdom in them can be questioned.)

Bandwidth

(The term “bandwidth” means “minimum required channel size”. Thus if a random binary data stream is fed through a 2 MHz-wide channel, and if that channel could handle twice that much data, then the bandwidth of that data stream is said to be 1 MHz.)

The bandwidth for NTSC is always 6 MHz. Without data compression, the bandwidth for 1080i would be 300 MHz. With MPEG-2 data compression the bandwidth varies according to how fast the image changes. For 480i the bandwidth rarely goes above 1 MHz. For 1080i and 720p the bandwidth rarely goes above 3 MHz.

Thus it is possible to put six 480i programs or two 1080i programs in a 6 MHz channel. The FCC allows this. Thus terrestrial DTV stations have sub-channels. It is up to the station managers how many sub-channels to have and what programming will air on those sub-channels. Note that a sub-channel showing a static image (e.g. a weather map or bulletin board) requires almost no bandwidth despite being at high resolution.

ATSC is an imperfect standard in that occasionally the bandwidth requirement will exceed the channel size. When this happens, the picture can get blurry or jumpy. Jumpiness occurs when frames are deleted. Blurriness is preferred because it is only momentary and often not noticed. Transmission encoders have improved gradually and hopefully will continue to do so. In the future perhaps they will fail in a completely unnoticeable manner.

Which is better: 1080i or 720p?

· 1080i and 720p require about the same bandwidth when showing live action: A 1080i image has twice as many pixels, while 720p shows twice as many frames per second.

· While showing films at 24 frames per second, 720p requires about half the bandwidth of 1080i.

· A common opinion is that 720p is better for sporting events, while 1080i looks better for documentaries, dramas, and most things that come 24 frames per second.

Unfortunately the networks are picking one format for all their shows. ABC, ESPN, and FOX have chosen 720p. All other networks are using 1080i. Hopefully some day they will choose the format according to the content.

You can find many websites where it is argued that one format is superior. Those who favor 720p are especially strident. They always overlook the fact that many images are stills or have little motion, and will look better in 1080i. They go to great lengths to explain the problems with interlace and flicker. But few people notice these problems (assuming they are sitting at the correct distance, and assuming that rescaling hasn’t introduced gross errors).

1080i and 720p are called High Definition TV (HDTV). 480p is called Enhanced Definition TV (EDTV). 480i is Standard Definition TV (SDTV).

DBS Quality

Present DBS systems (DirecTV and Dish Network) have a bandwidth problem: too many channels. These companies have resorted to some filtering to reduce the bandwidth per program. This allows them to carry more channels, but it gives the images a slightly blurry look. They call it “noise filtering”, but in effect they have reduced the resolution to below 640x480. Exactly what this resolution is has not been stated (550x400? Nobody knows.) On a 17 inch TV this problem is not very noticeable. But the larger the set is, the more offensive it is. You might find it to be a compelling reason to put an antenna on your roof.

This filtering has been applied only to standard-definition channels. The satellite companies claim that the HDTV channels are uncompromised. (Verifying such claims is close to impossible.)

DVD Quality

DVD images are usually 720x480 pixels, 24 frames per second. DVD quality is a step up from NTSC because:

1. digital technology is noise-free.

2. the horizontal resolution is better. NTSC is equivalent to about 400 pixels.

3. when a progressive scan monitor is used, any remaining flicker is eliminated.

4. the colors are better. NTSC has an “overlapping sidebands” problem.

“Overlapping sidebands” is a compromise in NTSC that works most of the time. It will cause wrong colors to appear when showing diagonal lines or fabrics with tweed patterns. Special comb filters improve the image slightly, but DVDs avoid the problem altogether. (Comb filters are only for NTSC.)

Of course, this improvement is lost if the DVD output is converted to NTSC. Many DVD owners have been buying monitors that have component video inputs, thus avoiding NTSC. DVD quality is essentially 480p (EDTV).

What is 1080p?

The term 1080p causes a lot of confusion because people are using it in conflicting ways:

1. The transmission formats 1080 24p and 1080 30p are sometimes called 1080p. But the term 1080p should never be used to refer to a transmission format.

2. If 1080p is to 1080i what 480p is to 480i then 1080p is a 60 frames per second monitor interface format. 1080 60p is becoming a common interface format. But since there are no 1080 60p sources, there is no need for it. (When a 24 frame source is converted to 1080i, no information is lost. A smart monitor can convert this 1080i into a perfect 1080 60p or 120p.)

3. When the maker of a digital display finds a way to improve upon 1080i he will usually say his display does 1080p. The improvement is often just a way to reduce flicker. The sets internally do 1080p but only accept 1080i at the interface.

If you want to be understood unambiguously, you should refrain from using the term 1080p. Instead use 1080p60, etc.

When a 24 frames/second source is converted to 60p, judder (described below) is introduced. A 120Hz display will have to remove that judder, making conversion to 60p seem pointless.

A justification often used for 1080 60p is that if the STB and the monitor can both do the conversion to 60p then the viewer can select the one that does it better.

If Hollywood ever decides to make movies at 60 frames/second then 1080 60p will become an essential monitor interface format. But there is presently no indication that they might do this. In fact they tend to consider the flaws in film to be an artistic enhancement (the “film look”).

Motion Compensated Processing

In this process a computer in the receiver turns a 24 or 30 frames/sec image into a true 60 or 120 frames/sec image. The motion vectors (described below) are used for creating the missing frames. This is probably the best hope for truly smooth motion for 1080i or films. But it requires the networks and studios to make maximum use of motion vectors, which may or may not be the case.

The motion vectors are not sent over the HDMI interface. So motion compensated processing must be done by the receiver or the DVD player. The monitor’s internal motion compensated processing works only with its internal receiver.

Motion Adaptive De-interlacing

Many 1080p monitors employ motion adaptive de-interlacing, which does not use the motion vectors. To create the missing frames, the set first divides the image into regions of motion and regions without motion. Areas without motion are de-interlaced by combining with the previous field. For areas with motion the missing scan lines are created by averaging the adjacent lines above and below. This is all pretty new and some sets are noticeably better than others. When done well, motion adaptive de-interlacing produces an image about as good as 1080i but without the flicker.

Does up-conversion improve the image?

Many receivers can up-convert a 480 image to 720. Others convert a 480 or 720 image to 1080. Does up-conversion result in a better image? Maybe. There are edge enhancement methods that reduce blurriness. But if the original image had no blurriness (de-aliasing) and you enhance it and try to sit closer, you will see false features. In any case, enhancing the resolution by more than 50% usually introduces objectionable false features. An enhanced 480 image will never look like a good 1080 image.

So while there are times when up-conversion will help, it does not permit you to sit at the distance proper for the higher resolution.

Some minor improvements sometimes happen with up-conversion. For example people who sit too close will notice that the scanning lines are gone.

But on the minus side, format conversions sometimes make interlace errors worse.

The 3:2 Pull-down Issue

Theater film is usually 24 frames per second while TV monitors usually operate at 30 or 60 frames per second. 3:2 pull-down, also called telecining, is the process of converting a 24 frames per second image into a 60 frames or fields per second image. It will normally happen one of two ways:

Thus the TV frames are 3 copies of a film frame, followed by 2 copies of the next frame, then 3, then 2, 3, 2, 3, 2, etcetera. This works well. (It has a minor flaw: During each second, 12 film frames are stretched slightly in time, while the other 12 are shrunken. The stretched frames dominate slightly, producing a slightly jerky image when motion is portrayed. This is called 12-cycle judder.)

Thus 3:2 pull-down is nearly the same for interlaced and progressive scan. Given the way the brain works, they will look about the same. Does the progressive scan version look better? Many people believe so. This author believes progressive scan is superior by an increment so small that it is not generally noticeable. (Keep in mind, less than 5% of the face of the CRT is lit at any instant. That it is fully lit is an illusion.)

What is not in disagreement are the problems of de-interlacing an interlaced signal. Nearly all DVDs contain data that is already telecined and interlaced. To convert an interlaced image into a 60-frames/sec image you can simply combine successive fields and display them twice. But if you do this with film material, the following happens:

A moving object will turn blurry 6 times per second, which is quite noticeable. To fix this, the de-interlacer must be smart enough to match fields originally from the same film frame. That process is called reverse 3:2 pull-down, and sometimes cadence detection. Manufacturers sometimes call it 3:2 pull-down detection or sometimes just 3:2 pull-down (which is obviously wrong).

If you want to watch DVDs on a progressive monitor, make sure you have reverse 3:2 pull-down. De-interlacing is also called line doubling or line scaling.

120 Hertz technology

The best monitors show the image 120 times per second. 120 is an exact multiple of 24, 30, and 60. Thus in theory the monitor can show any program without introducing judder. But will the monitor actually do this? Consider the following questions:

1) When using the TV’s internal receiver:

a) Can the monitor detect a 24p program and display it without judder?

b) Can the monitor detect a 24 frames/sec program transmitted in 1080 60i and reorganize it for display without judder?

c) Are motion compensated processing and judder-free display simultaneously possible?

2) When taking input from DVI or HDMI:

a) Can the monitor accept 24p and display it without judder?

b) Can the monitor detect a 24 frames/sec program transmitted in 1080 60i or 60p and reorganize it for display without judder?

c) Are motion compensated processing and judder-free display simultaneously possible?

The answer to question 2c is generally no. Since the motion vectors are not available to the monitor, the DVD player would have to do the motion compensated processing. But monitors that can accept 1080 120p are very rare, and 60p will introduce judder. You will likely have to choose either motion compensated processing or judder-free display, whichever looks better on your system.

120 Hertz technology is often touted as a fix for the slowness of LCDs. This is a dubious claim. The improvement is usually miniscule, often not discernable.