Choosing an HDTV

I think it’s time to upgrade to the world of high definition television.  There are so many choices both in types of TVs and in broadcast formats.  This article documents my attempt to sort through the confusion and make a choice.

The Wikipedia article on HDTV is a good place to start research.

HDTV Formats

There are two main HDTV formats, 720 line and 1080 line.  The screen is rectangular with a 16×9 aspect ratio so the resolutions are:

720 line format: 1280 x 720 pixels
1080 line format: 1920 x 1080 pixels

In addition to the screen resolution, one more variable is important and that is the screen scan rate as well as scan type.  P refers to progressive scan and I refers to interlaced scanning.  The scan type is almost always specified but the scan rate is not but it should be!  This lack of being explicit is somewhat frustrating in determining which way to go.  ABC, Fox, and ESPN are notable networks broadcasting in 720p while CBS, NBC, and several other cable channels are broadcasting in 1080i.

From what I can gather 720p is typically used in 60 frames per second (fps) while 1080i is used in 30 fps.  The advantage of 720p is that the screen is redrawn more frequently which is good for fast action.  The advantage of 1080i is that it has more pixels so stationary or slow moving pictures should look clearer.

Actual raw data rates for each standard assuming 24 bits per pixel is listed as follows:
1080i / 30fps:  data_rate = 1920 * 1080 pixels / frame * 24 bits / pixel * 30 frames / s = 1.49 Gbps
720p / 60fps: data_rate =1280 * 720 pixles / frame * 24 bits / pixel * 60 frames / s = 1.33 Gbps

Both standards have about the same amount of transmitted information, with 1080i being a little higher.  In order to be broadcast, HDTV is sent in a 6 MHz bandwidth.  Obviously it would be very difficult to send 1.5 Gbps of data in 6 MHz of bandwidth, so the data is compressed with MPEG-2 compression to achieve a data rate of 19.39 Mbps.  That’s a compression ratio of over 75 to 1 for 1080i, which seems pretty high.

The high end video from DVDs and some other sources is currently supporting 1080p / 60fps which would be about 3 Gbps of data.  With this you get the best of both worlds (fast frame rate and large number of pixels, but I don’t think it can be transmitted in a single 6 MHz channel.  It doesn’t seem like that will catch on the way television content is currently delivered.  Maybe when we all have fiber connections to our homes or when television is delivered on demand instead of broadcast will 1080p / 60 fps become the norm.

Right now I’m leaning toward 720p / 60 fps for my TV since I like fast action movies, video games, and sports.

Screen Size

Crutchfield has a nice article describing HDTV as well.  This article also provides some recommendations for screen size based on how far away the screen will be viewed from.  For my room the minimum distance is about 9 ft.  This suggests a screen size in the 42 to 50 in. range based on their chart.

Display Technology

There are several display technologies currently available, LCD, plasma, and DLP.  My wife and I both agreed that the plasma screens seemed to look the best during a recent trip to Best Buy.  I sort of wanted to avoid plasma because of the burn in issue, high power consumption, and questionable useful life.  We also both agreed that the DLPs looked a little washed out in side by side comparison to the LCD and plasma screens.  I think Best Buy did not do justice to the HDTV technology because the sources they had displayed on the TVs were not that good.  Also, a friend has DLP and it looked great for the Super Bowl.  LCDs have a good picture quality and are known as reliable and low maintenance, but they only go up to 46″ or so for screen size.  The main advantage of DLP appears to be that it is more cost effective than plasma for larger screen sizes.  The thing that concerns me about DLP is that you have to replace the bulbs at a cost of about $250 fairly often.

Summary

Based on everything I have learned, it seems like an LCD at the top end of their size range, supporting 720p / 60fps is the way I want to go.  I haven’t really price shopped yet though, so I’m not ruling out DLP completely.  From what I can tell though, most DLPs are either 1080i or 1080p and don’t come in the 720p type.