OTA HDTV Reception Q&A

Updates on the DTV transition and how to receive over-the-air digital television for free.

Location: Richmond, IL, United States

Hello there! I created this blog to share the information about over-the-air HDTV reception you have been wearily searching the Web or calling technical lines for, whether you have decided for less expensive means to get your favorite TV shows, or still adjusting from the over the air broadcast DTV transition that occurred on June 12, 2009. After working for a leading antenna manufacturer for almost 5 years, during which time I've shared my expertise with those who asked on the phone and by email at work, I decided to do the same in the Blogosphere! Confused about getting your local HD channels? Just click through the archives, some of the most useful information is in the early posts from 2005-06. If you want to get in touch with me with antenna related questions, just leave a comment anywhere on this site.

Wednesday, October 26, 2005


In a recent post I had mentioned that USDTV, a service providing cable channels over the air for about a $20 monthly fee on unused broadcast DTV bandwidth, had recently received capital for nationwide expansion from several broadcast groups. While trying to research exactly which markets they will expanding to in the next couple months (Dallas appears to a definite, since a lot of Texans appear to enjoy off-air reception), I came across a message board for HDTV viewers in Milwaukee concerned that USDTV’s inevitable presence may reduce the quality of the HDTV provided from a broadcaster partnered with USDTV. The concern is that the presence will reduce the bitrate on the HD transmission, and the picture quality with it.

Nearly a couple years ago I was wondering the same thing, so I contacted a broadcast engineer by email at one of the local area stations. This station is broadcasting NBC in 1080I HD, as well as one multicast channel. This engineer gave me an excellent description on how the bitrate on the broadcaster end works, which I would like to share with you here in my own words. The issue here is based on the video codec being used for DTV broadcasting and reception in the US, MPEG-2, the same codec that allows you to view some streaming videos online.

A digital video stream, be it from DTV reception or from your modem, is like going through a flipbook of still pictures to give the illusion of full motion – let’s say 24 pics per second (24 fps video). If you ever have taken digital pictures with the same camera on the same settings, you would have a few different image files at the same resolution, such as 640 x 480. Yet two pictures of different items on the same resolution from the same camera may have different file sizes. One could be 100 kb, while the other 200 kb. The differences in file sizes would, for the same resolution, be due to the amount of content or detail in the picture. This is because of variations in colors and textures in the photo. If there are few variations from pixel to pixel, then there need not be many variations in the digital data putting the picture together for you to see, and the resulting file size would be small. To put it another way, for the same resolution you would expect a picture of a crowd at a concert to have a noticeably larger file size than a close up of an apple on a yellow tablecloth with a black background.

Now suppose the resolution on the apple pic were 1920 x 1080, the same resolution as a 1080I widescreen HDTV picture. If you took several pictures of this apple in the same location and flipped them together at 24 per second, you’d be making an HDTV video, although there wouldn’t be much happening. If the video were encoded digitally and saved as a Windows Media video file, and played on a PC with a high-resolution monitor, it would be HD. However, because of the still motion, the file size would not be very large because there is no motion, minimizing the variations of digital data (1s and 0s) in the stream, and keeping the file size relatively low. So one could record the video to a DVD at the lowest possible quality, play it on an HDTV monitor, and the quality would APPEAR to be HD, because the file is small, and a limited amount of bits-per-second (video file size divided by length in seconds) results.

If instead of a still life set of photos, the set of flipping photos were actually a NASCAR race. Now with the racecars in quick motion, there becomes more variation in the digital data. If a "pixel" in the screen remains the same color for a long period of time, there need not be a change in the digital data for that point, allowing for video COMPRESSION, a reduction of bitrate. Here though, with a lot more activity, the video file becomes larger, and so to maintain the picture quality, the video stream would have to take in more bits in one second. If the DTV tuner or PC streaming an Internet video clip cannot keep up with the required bit rate, this is because the variations of bits are exceeding the bitrate the tuner or PC are capable of. As a result, to keep up with the variation, the streaming hardware needs to drop some of pixels in the resolution, and this results in "pixelation" on the picture, with a noticeable reduction in that resolution.

An interesting item of note is that HDTV viewers may have seen programming with better HD quality than others. Such shows are filmed with digital 1080I cameras, recording and encoding live action at a very fast bitrate on the order of 1 GB/s! Yet a broadcast HDTV stream of 18 Mbps would be the maximum rate allowed to broadcast the program. Still, at times the viewer may not notice such a drastic reduction in HD picture quality watching the broadcast because the program may have little variation in action. The less action, the more able a broadcaster to reduce the channel bitrate more, and viewers would still be mesmerized by the HD picture. You may have noticed however if a picture is pixelizing, it happens when there is a lot of motion in the video, so the broadcaster would still need a minimum bitrate to maintain the picture quality.

So if you are concerned that multicast opportunities will kill your HD picture quality, you probably will not notice it much except in cases where there is a lot of fast motion. In fact, this issue does not seem to faze CBS, whose HD programs seem to have top-notch HD picture quality. Now there is talk that CBS may launch a nationwide multicast, CBS.2, on their O&O DTV stations initially with more to come. They will devote this channel like DVD extras, a place to get more information and entertainment. CBS is also supposedly planning to "bit-starve" the multicast at times when a HD program with a lot of potential fast action, like NFL football, is airing, and a faster bitrate from the broadcaster is necessary to ensure the 1080I HDTV picture quality for such programming. Perhaps CBS could use its muilticast channel as a place with up-to-the minute scores and stats on a frozen graphical screen, with a small corner picture rotating the NFL action, probably every 10 seconds between games without sound (to fully air a single additional game would probably anger DirecTV with their NFL Sunday Ticket package). Such an information area would require very few bits-per-second on the multicast, maintaining a high bitrate to broadcast CBS HDTV.

I will give a few more numbers comparing bitrates and video quality next time, including encoding an HDTV program onto DVD (whose picture quality is now supposedly inferior to HDTV), and the newest form of pay DTV, Apple’s Video iPod, released earlier this month. Both of these I have been experimenting with on my new Media Center PC. Remember, the so-called signal meter on a DTV tuner is a measure of the BITRATE being decoded by the receiver, and NOT necessarily the amount of signal going into it. DTV should be fine with a strength over 80%, whereas under 70%, the bitrate is less than maximum, and you would see occasional pixelation. Feel free to see my archives for more DTV information if you recently found this on a Blog search. Take care!


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