HDTV Set Top Boxes

Buying an HDTV Converter or Receiver

Updated 10/6/00

In a Nutshell

My opinion as of October, 2000. Wait until standards for HDTV broadcasting with anti-copying and anti-piracy provisions in place have been finalized. Otherwise you may end up with a converter that won't pick up many TV shows.

The FCC has issued guidelines about the blocking of high definition television (HDTV) programming from users who do not have equipment with circuits that inhibit copying of the programs. As of October 2000 it is unclear what this blocking will consist of. It is possible that many shows will be scrambled and that the set top box converter you buy today might not be able to receive them at all. More.

My opinion as of October 20, 1999: Unless you just have to have the latest toys to play with, don't buy it. Typical prices nowadays are USD 600. and up. Wait until you can get them for under USD 200.

Here are the questions you need to be thinking about:

1. Does it have the correct output jacks to connect to your TV?

Example: It might have S-video and RGB but not component video jacks.

2. Does it receive all the HDTV broadcast formats?

Example: It might receive 1080i but not 720p.

3. Does it display both standard (4:3) and wide screen (16:9) pictures?

Example: Using the S-video jack it might deliver 16:9 programs only as downconverted letterbox pictures.

4. (A hard question) Does it do all the necessary cross conversions your TV needs?

Example: If the TV accepts only 1080i and 480p, and the box delivers 720p only as itself, you won't be able to see those programs.

5. What is the resolution?

Most set top boxes won't deliver the full resolution of HDTV but you may want to know what a box does deliver.

1. Does it have the correct output jacks?

This is a two part question. The easy part is connecting to your existing TV. The hard part is deciding what you want for connecting to a future TV.

If you are going to use your existing standard TV, including wide screen models, you would want to make sure that the set top box offers the basic output that matches your TV. Set top boxes will usually offer more than one but not necessarily all of these outputs.

The basic (NTSC) outputs are:

Antenna jack delivering RF signal, plays through channel 3 or 4 of your TV,
Video Out (composite video), usually a yellow RCA jack that you connect to the same jack (if any) labeled Video In on your TV,
S-Video, provides better picture quality than composite video but again not all TV sets have it,

All of the basic outputs including S-video deliver interlaced video signals only, NTSC (or PAL or SECAM).

Advanced (HDTV and DTV) Outputs

Component video, three RCA jacks usually colored red, green, and blue, and usually used with a DVD player.
RGB, may use three jacks (RGsB), four jacks (RGBS), or five jacks (RGBHV). Jacks may be either RCA jacks or similarly sized jacks where the plug is twisted on (BNC jacks). RGB may also use a single oval-ish (trapezoid) plug and jack with several pins/holes in the middle, commonly used with computer monitors.
Firewire, a digital signal transmission format whose jacks and usage have not been formally adopted yet.

The signal coming out of the advanced outputs may be NTSC or may be any of the HDTV formats 480p, 720p, or 1080i. Except for Firewire, the video signal is analog as it travels down the cable.

2. Does it receive all the DTV broadcast formats?

The general public is told there are three formats 1080i, 720p, and 480p. This is true for the baseband (demodulated) video signal fed into the HDTV ready TV set. (There is also 480i which is the same as NTSC.) Coming over the airwaves, there are 18 subformats some of which are for each of the formats 1080i, 720p, 480p, and 480i. After being tuned in, demodulated, and decompressed, each signal becomes the baseband video signal of the same format and can be subsequently converted to one of the other baseband formats.

An easy problem to understand is that not all set top boxes receive all the DTV broadcast formats, for example one model of set top box might receive 480p and 1080i broadcasts in general but not 720p broadcasts.

A more difficult situation to identify is not receiving some of the subformats. This will start showing up as individual customer complaints, the set top box works some of the time, perhaps just in the evening. More specifically the box is handling live video as in a newscast but not movies. Technically live video (1080i) has 30 frames per second as broadcast while movies have 24 frames per second, but the set top box always sends (approx.) 30 frames per second to the TV.

You can think of a bicycle with three big sprockets on the pedals but there are also five little sprockets on the wheel. On this bike some chain and sprocket combinations don't work and you have to figure out which ones they are.

Dealing with this question is more difficult if you buy a set top box when some but not all formats are being broadcast in your city. Today (1999) it is hard enough to see a set top box demonstrated in a store and outputting to the TV all four formats let alone a sample from each of the 18 broadcast subformats.

3. Does it handle both standard and wide screen pictures?

A DTV or HDTV broadcast may be either a 16:9 or 4:3 shaped picture with 16:9 predominating. The set top box must provide choices for wide screen and standard TV sets. You might not need all of the possible choices to work with your present TV but you will want to think about a future TV.

Full frame anamorphic: This is for TV sets with both aspect ratios (all 16:9 sets and a few 4:3 sets). The picture occupies the entire video frame, 480, 720, 1080 scan lines. We called this mode "anamorphic" because the TV usually neither knows nor cares what the aspect ratio is. You manually select 4:3 mode or 16:9 mode on the TV to get the best looking picture (not stretched).
Letterbox: This is used for 4:3 TV sets without 16:9 mode (almost all). The DTV or HDTV picture is converted to have 360 scan lines (480p or 480i) and the set top box adds 60 black lines above and 60 black lines below to get the total of 480 active scan lines.
Cropped: The set top box cuts the sides off the picture so if we started with a 16:9 picture, what remains fills a 4:3 screen (480 scan lines) in the correct proportions. You would not select this mode if the program was broadcast in 4:3.

We have heard of at least one set top box whose S-video output was automatically formatted as letterbox when the broadcast was 16:9. This is quite disadvantageous for a small number of wide screen TV sets that have only S-video jacks.

The names letterbox, anamorphic, etc. has not been standardized for HDTV (10/99).

4. Does it do cross conversions to match your TV inputs?

This is also a two part question, first for your present TV, second for a future TV.

You must start with your TV, finding out what video signals it will accept. This can get complicated because some HDTV ready sets might, for example, accept 1080i but not 720p. The set top box will have to take the 720p baseband video signal produced from a 720p broadcast and convert it (using upconversion) to a 1080i baseband video signal. You need to choose a set top box that includes all the desired conversions .

If you have a standard TV and you plan to use the antenna jack, the composite video out jack, or the S-video jack, determining the necessary conversions is easy. These inputs are always 480i only (for NTSC). You just need to make sure all three DTV formats (or all 18 subformats if you prefer) get converted to 480i for NTSC (or 576i for PAL).

Different set top boxes will offer different combinations of signal conversions. If your TV accepts 1080i and 480p but not 720p you will probably prefer that 720p broadcasts are converted to 1080i instead of 480p.

For example the Sony KL-W9000 monitor accepts 480i S-video and 480p RGB. You will want a set top box that outputs 480p broadcasts as-is and downconverts the 720p and 1080i broadcasts to become 480p RGB. The few 480i DTV broadcasts would best be output as S-video although they could be "line doubled" to 480p adding some extra content and expense to the set top box.

The Sony VPL-400Q front projector takes S-video and also 480i and 1080i component video but not 720p. For it the set top box should output 1080i as-is but should upconvert 480p and 720p to be 1080i. Using the video downconverted to 480i would be a possible although inferior alternative for this projector.

5. Bandwidth and horizontal resolution

Like resolution of ordinary TV sets, HDTV resolution will have its share of deceptive advertising and won't be easy to verify except by viewing test patterns.

In most cases a consumer grade HDTV set top box will not deliver the full horizontal resolution of HDTV. (Most sets including HDTV ready ones won't be able to reproduce it even if the set top box delivered it.)

But a good HDTV set and set top box should still deliver HDTV. HDTV implies about a million pixels. The 1080i format is 1920 pixels across by 1080 pixels high. If the horizontal resolution is cut in half due to equipment or programming shortcomings, the remaining 960 across by 1080 high still multiplies out to about a million.

We anticipate that the typical not too expensive set top box will output 480p signals at close to maximum resolution according to spec's, 600 to 720 max. pixels across (MPA), 720p signals may range from 1000 MPA (nearly all according to specs.) down to 640 (half) MPA, and 1080i will have around 1300 (two thirds) down to 640 (one third) MPA.

(Horizontal resolution represents the maximum number of dots or upright lines plus intervening spaces that can be arranged in a row across the screen and still be distinguished.)

The video signal ready to go to the TV requires a higher bandwidth than the incoming broadcast. To reproduce all of its 720 pixels across the screen, a 480i signal requires 6-3/4 megahertz, the 480p signal requires twice that, or 13-1/2 MHz. The 720p video signal requires about 35 MHz to reproduce all of its 1280 pixels across, the 1080i signal also requires about 35 Mhz to reproduce all of its 1920 pixels across. Half the bandwidth means half the horizontal resolution.

TV manufacturers and magazines will continue to say that carrying 35 MHz video from set top box to TV is a stumbling block to delivering the full resolution of HDTV. But computer monitors with that capability (XGA; 1024 x 768 non-interlaced) have been aaround for many years now. The real reason is probably that the TV sets themselves won't be consistently made to have circuits and picture tubes of sufficient quality. Meanwhile the typical DVD player and NTSC TV component input circuits typically have just a 6 MHz bandwidth.

Even though the TV set does not have the full horizontal resolution, 1080i HDTV will still show smoother diagonal lines and edges compared with 480p DTV. Dots and details will still be more precisely positioned along a scan line, in 1920 possible increments as opposed to 720. Vertically there are 1080 increments as opposed to 480 even if scan lines overlap slightly. Vertical resolution does not depend on bandwidth but does depend on the TV's ability to make the scan lines fine enough to not overlap too much, and to center the even scan lines between the odd scan lines.

More on video resolution.

FCC WARNING

It is possible that the HDTV set top box you buy today won't receive some of the shows that are broadcast, leaving you with a limited selection to choose from.

This is due to anti-copying and anti-piracy guidelines from the Federal Communications Commission.

The problem is that no one knows exactly how the guidelines will be used. One method uses embedded codes that tell the set top box whether it can receive the program as HDTV, receive the program only if it is converted to 480p, or block the program (similar to a cable TV set top box) unless the user has paid a pay-per-view fee. Programs may be scrambled so today's set top boxes, without the anti-copy mechanism, won't receive the programs at all.

HDTV will not catch on unless there are shows that people can watch free of charge, just as people watch TV today.

We encourage you to write letters to the FCC to urge them to require that TV stations provide HDTV programming that everybody can receive free of charge in order to meet their (existing) HDTV broadcast requirements.

Other Issues

6a. Line Doublers

A line doubler to convert interlaced NTSC to progressive video in a TV set has nothing to do with HDTV. HDTV ready incoming video is already at or above the doubled scan rate. In fact a TV that requires all input signals to go through a built in NTSC line doubler is not HDTV ready any more than the oldest black and white TV is. Both of these sets require the set top box to downconvert the signal and compromise the picture detail all the way down to the 480i NTSC level before sending it out the output jack.
The set top box does not really need its own line doubler although it may have one for the few digital 480i broadcasts and also to go with a built in NTSC tuner if any. Alternatively it may output 480i broadcasts only from its S-video jack while outputting 480p and HDTV after necessary and/or chosen conversions from any of its jacks.

6b. 29.97 Versus 30 Frames Per Second

This is usually not mentioned to the public although these numbers might be seen in the specifications at the back of the user manual.
When color was added to the NTSC standard, the frame rate was changed slightly from being based on the 60 Hz power line frequency. It did not affect the majority of TV sets back then (1950's) but it probably did affect a few. This idiosyncrasy has survived to this day and HDTV subformats exist for it.
This should not cause problems but occasionally might. Problems might show up as occasional customer complaints that the set top box doesn't receive certain broadcast stations properly.
Set top boxes may output to the TV as 29.97 frames per second (either exclusively or according to the broadcast) instead of 30 fps for 1080i and 480i and 59.94 fps instead of 60 fps for 720p and 480p. There is the slight possibility that some set top boxes may not work with some TV sets for this reason.

6c. Choose Which Method Gives The Better Picture?

After you get your set top box, you may find that together with your TV you have a choice of conversion options for the same picture. An example with a distinct difference to choose from is the Sony VPL-400Q (front projector). It accepts 1080i but treats it as 540p and also crops off the top 30 and bottom 30 scan lines of every field leaving 480p which it displays. You may have a set top box that smoothly downconverts each 540 line field of 1080i into a 480 line field for the benefit of TV sets that accept 480p but not 1080i. Then if you choose 480p or 480i output from the box, you don't have thin slices of the picture top and bottom cropped although there may be some visible artifacts since there is no such thing as a perfectly smooth downconversion.

6d. What Are The Subformats?

Some are:
1080i Video (1920 x 1080 pixels by 30 fps)
1080i Movie (1920 x 1080 by 24 fps)
720p Video (1280 x 720 by 60 fps)
720p Movie (1280 x 720 by 24 fps)
480p Video (16:9) (720 x 480 by 60 fps)
480p Movie (16:9) (720 x 480 by 24 fps)
480p Video (4:3) (640 x 480 by 60 fps)
480p Movie (4:3) (640 x 480 by 24 fps)
The designations 24 fps or 30 fps refer to the amount of material that is transmitted over the air. The tuner or set top box must convert it into odd and even fields 60 (or 59.94) per second for 1080i and full frames 60 (or 59.94) per second for 720p and 480p.

6e. Multiple Camera Angles

Apparently the ability to select one of several video subchannels from an HDTV broadcast station has not been included in today's (10/99) HDTV set top boxes.That capability might not have been standardized yet. Therefore when that capability starts to be used in broadcasts, you will need to buy another set top box. Most likely the subchannels will all be 480p, some will be 480i.

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