HDTV – 720p, 1080i, and 1080p?
It seems that everyone is buying a new HDTV set this year. Conversation around the water cooler is full of techy buzzwords. When the HDTV standard in the U.S. was originated by the ATSC (Advanced Television Systems Committee), they defined eighteen different standards, but not all of the standards are in common use.
The standards that you should be familiar with for HDTV (High Definition TV) are 720p, 1080i, and more recently 1080p. The number in the format (for example “720”) is the vertical lines of display resolution and the letter in the format (for example “p”) stands for the display technique. The letter “p” means “progressive” scan, and the letter “i” means “interlaced”.
HDTV (both 720p, 1080i, and 1080p) has a wide screen aspect ratio of 16:9 (width: height). EDTV (enhanced definition) is a standard definition TV (SDTV the format before HDTV), but with some software “interpolation”. EDTV delivers a picture that is superior to that of SDTV, but not as sharp as HDTV. EDTV uses progressive scanning. EDTV sometimes has an aspect ratio of 16:9 (wide screen).
For reference, standard definition TV (SDTV, the format before HDTV) has 480 lines of vertical resolution and has an aspect ratio of 4:3. The SDTV signal is the U.S. has historically been an interlaced signal. This means that your TV set draws each frame in two passes: once for the even horizontal lines and a second pass for the odd lines. The specification is a maximum resolution for the technology, but often, the signal sources are less than the maximum. For example, VHS tape has approximately 240 lines of vertical resolution, over the air broadcast SDTV has approximately 330 lines of vertical resolution, and a SDTV DVD has approximately 480 lines of vertical resolution.
The televisions in Europe and Japan have been (and are) progressive format. A progressive format makes a single pass for all of the lines. Progressive pictures look a bit smoother than an interlaced image, especially when there’s a lot of movement on the screen, like sports.
To add to the confusion, BluRay DVDs do, and later this year, the satellite TV companies will, broadcast in 1080p. This standard has both the higher number of lines of vertical resolution, and the preferred progressive format. The Dish Network will begin satellite broadcast in 1080p in August 2008 with 150 HDTV channels, and DirecTV will begin both satellite and Internet broadcast by the end of 2008, with about 130 channels of HDTV programming. The satellite companies compress their signal with MPEG-4 techniques, which reduces the bandwidth needed for broadcast. Currently, the only source material for 1080p is movies, especially movies originally made for IMAX. None of the major networks (ABC, ESPN, etc.) has yet announced support for 1080p. In addition, Sony PlayStation 3 and Xbox 360 have 1080p games.
HDTV Video Format: 720p, 1080i, and 1080p, the higher number is better, right? Even though 1080i technically has a higher resolution, it is not the obvious choice. 1080i has 1080 vertical lines of resolution and 1920 horizontal lines of resolution. 720p has 720 lines of vertical resolution and has 1280 lines of horizontal resolution. But the 720p system is progressively scanned. This compensates for the lower level of resolution. The 1080p signal is the best of both worlds, more picture information with a progressive format.
If your signal received, for example for a satellite receiver, is primarily standard definition, a 1080p TV will not significantly improve your picture when compared to a 720p TV. The picture quality is fundamentally limited by the source signal (480 lines of vertical resolution). Similarly, if your signal has a 720 source, then the 1080 TV will not improve your picture. All High Def sports broadcasts from Fox, ESPN, and ABC are broadcast in 720p.
Some networks have opted for the 1080i format because it provides the more picture information, while ABC, Fox, ESPN, and the National Geographic Channel have chosen the smoother pictures of 720p. HDTV programming on CBS, NBC, and other networks is broadcast in 1080i. The new 1080i or 1080p HDTVs will probably make this type of programming somewhat sharper than it would appear on a 720p TV. This becomes more important because as the TV screen gets larger, the visible difference in detail between 720p and 1080i and 1080p displays becomes more apparent. Because of this, most HDTV sets larger than 50 inches are 1080p or 1080i.
1080p HDTV System Capabilities:
A 1080p HDTV displays all inputs signals at 1080p, because this is the normal resolution of the unit. If the system is less than 1080i or 1080p, then a special purpose chip in your HDTV scales the signal. It will interpolate, like the EDTV does with a SDTV signal. If the signal is 1080i, the system will combine the odd and even interlaced frames and display the full signal as 1080p. The resulting performance should be similar to a true 1080p video source.
More than 100 million U.S. homes can receive HDTV from their cable system, and all of the top 100 cable markets in the country have available HDTV programming. Unfortunately, most cable providers carry only a few of the 50+ available HDTV networks.
While HDTV local channel selection is generally better on cable than satellite, the major satellite companies offer more national HDTV channels than almost every cable provider. Both DirecTV and the Dish Network have promised to offer more than 130 HDTV channels by the end of the year. “Satellite’s going to be constrained not so much by how many channels they can carry than by how many they can get,” Bob Scherman, Satellite Business News.
Screen resolution for HDTV has become comparable to microprocessor clock speed for personal computers. Even though clock speed is not necessarily a perfect indication of overall system performance, often the consumer thinks of it that way.
Interestingly, when an expert group, the SMPTE (Society of Motion Pictures and Television Engineers), recently ranked the importance of resolution, it came in fourth. The SMPTE ranked contrast ratio, color saturation, color temperature and grayscale ahead of resolution in importance. The Imaging Science Foundation reached a similar conclusion. However, most HDTV manufacturers bundle quality traits such as improved contrast ratio with higher screen resolutions into their top of the line HDTVs. So if you purchase a set with top of the line screen resolution, you more than likely will receive the top of the manufacturer’s product line for other important screen criteria.
Prices have dropped significantly the last two years, but it appears that the rate of price reduction has been slowing significantly recently. “Television shoppers who visit mass merchants may be more concerned with obtaining a low price and may not be seeking expert advice. However, television shoppers should be aware that the lower prices they find at mass merchants are primarily due to the fact that these stores tend to carry second tier brand TVs, rather than major brands such as Sony or Panasonic. When pricing for a specific television model is compared at each type of retailer, the prices are surprisingly similar,” J.D. Power’s Larry Wu.
And on a final note, be sure to account for the Audio side of your system. George Lucas (Star Wars) once said audio is half the movie viewing experience. It’s also true for a good football or basketball game.