Poor SD On HDTVs





By Richard Binckley

May 8th, 2005

Introduction
Probably the most asked about issue when a person gets a new HDTV is why is the SD picture quality so bad on my HDTV? This discussion will address this issue, the causes and what can be done to get the best picture possible.

The Causes
Probably the number one cause of a poor SD picture is digital transmission. What’s that you say? I thought digital was supposed to provide crystal clear picture quality. Well the truth is, sometimes it does and sometimes it does not. In order to cram as many channels as possible on a satellite or cable the providers will compress the signal using a method called mpeg2 compression. Like zip files on a computer the original data is put through an algorithm to make the data smaller and allow more data through in given space. Unlike the zip file operation the compression is enough more that some of the original information is lost. When the data received is uncompressed, there are small artifacts that are created due to the algorithm trying to fill in the blanks, so to speak.

Artifacts
Artifacts are errors in the picture after decompression that is a result of the decompression process, the source camera or errors in transmission along the way to your TV. The two main decompression artifacts are the posteraztion and the artifact that results in a “ghost” like outline. It is particularly noticeable around text. Other distortions of the picture include pixelization and digital blurring on fast motion. A good introduction to artifacts in digital images is here: http://www.dpcorner.com/topics/.
While this mainly covers digital photography, the principles are the same, only remember HDTV scaling and decompression has to happen 30 to 60 times per second. In fact you will see stills in that link that describe visually what many of us are complaining about. Also check out the essay HDTV Artifacts.

The cause of the artifacts is twofold. On video that started out analog the video signal is digitized at the source to a digital compressed frame with the resolution of roughly 640×480i. Just like your scanner when compressed to jpeg file there are errors, albeit slight at the native resolution. When you zoom into a photo scanned in at 640×480, you will begin to see some of the errors and pixelization. Somewhere the signal has to be converted (scaled) from the 640×480 original to the 1280×720p or 1920×1080i your HDTV requires. You are sitting at a computer, if you have a photo editor do that to a photo and see how it comes out. Here is an example of compression artifacts with the original image shown as the small inset.
Picture_Compare
Notice the “Ghosts” (sometimes refereed to as mosquito noise) around the building tops that are created by the compression/decompression processes. Also notice the jaggy edges on the sail. Another artifact caused by the enlarged resolution.

TV Size
TV size is a big factor as to whether you notice bad picture quality. The really bad SD pictures seem to be reported primarily in screen sizes larger than 40”. Also the viewing distance will affect how noticeable the artifacts are, so it becomes a combination of screen size and viewing distance. 40” and smaller screen sizes seem to be less objectionable with 34” and smaller barely noticeable at normal living room viewing distances.

If you have a large HDTV already, or are looking to buy a large HDTV try this test. At your seating distance use the PIP function to place two pictures side by side. Most HDTVs today have this feature and it does not matter whether you get a picture in both windows for this test. If the smaller picture becomes acceptable in terms of clairity, then the TV is too big for your seating distance and the only solution for a good SD picture is to get a smaller TV or sit further away from the TV. Obviously this test will be better made before you buy your TV. The bigger the better does not necessarily apply to HDTVs.

Signal Strength
While conventional wisdom says with digital video you either get it or not, there is the possibility of a drop in signal strength for sufficiently short periods of time to not cause a total loss of picture. Loss of signal for very short periods can be error corrected for by your receiver where it will “guess” what the data should be and what you end up with is bad video. If the signal loss duration is long enough that the receiver can not “guess” what the data should be, you get a mpeg2 block or blocks with extreme pixelization. Generally this condition will also show up on the HD picture, but it might not be as noticeable.

TV Settings
There are some TV settings that can aggravate the poor SD picture. These are noise filters and other settings that are intended for the clean up of analog video. When digital video is processed by these filters, it can enhance digital artifacts rather than eliminate them. An example would be the sharpness setting. The sharpness is intended to enhance the edges of objects in an analog video picture. You sure do not want to be enhancing the edges of digital artifacts. I use a setting of no more than halfway, but depending on each particular set, this setting may need to vary. Interestingly enough when I watch SD material on my local DTV stations, the video is so free of compression artifacts I usually set the sharpness to 80 or 90. Play with this and see if it does not make a difference.

Optional Hookups
The one suggestion that is repeatedly said to have some success is the connection of a composite video (yellow) or s-video connection between the receiver and the TV, as well as a second audio L/R connection. Then you can select the SD video input while you’re watching SD programming and the HD input for HD programming. Another possibility if you have analog cable service along with your digital cable service, is to connect the antenna input to your NTSC tuner to the cable. Many TV will scale analog signals to the HDTV screen without the decompression artifacts (because there is no compression with analog) and the picture will look better going through your TV tuner.

Source Scaling
The best SD video will be where the source (DVD Player or TV station) does the scaling. This explains why video from DVD players is better than the video from a SD cable box or satellite receiver. My DTV stations scale the SD picture to the HDTV resolution the station broadcasts before they broadcast it and the picture is about the same as my DVD player. Switch over to my Directv satellite receiver on the same local channel and the picture quality plummets to below that of my VCR.

Directv
Directv has both SD only and HD receivers. My experience is the picture quality of their SD receivers is pretty bad when viewed on big screen HDTVs. Most people report picture quality on SD Directv receivers to be below that of a good VCR. On HD receivers the picture quality is generally much better, with some channels comparing to the picture quality of a DVD. It seems to be somewhat brand dependant though.

Dish Network
Most complaints seem to be that the Dish Network SD picture quality is far worse than Directv. Many people with HD service have threatened to switch back to Directv. The issue seems to be the fault of the 811 receiver. One person was able to get a Dish tech to hook up a 311 receiver that produced as good a picture as the Directv receiver. He was able to talk Dish into letting him keep the 311 in addition to the 811 for a year. This leads to the conclusion that the processor in the 811 may be slower in decompressing the mpeg2 data stream than the 311.

Note On Local Channels Over Satellite
I have read repeatedly that local channels in SD over satellite is far more compressed than the regular satellite channels. I don’t know about this as being fact, but I do know that the picture quality is better on my OTA antenna through my TV NTSC tuner than it is coming through the satellite.

Cable
From what I have read, digital cable has the same problems with SD reception that are found with satellite receivers. There have been many people that have suggested splitting the cable signal and connecting it to the TV NTSC tuner connection. Many people have said the TV tuner has improved their SD reception.

Signal Processors
While they may improve the picture somewhat, most of the artifacts due to decompression will not be dealt with. The best results for the use of a signal processor like the DVDO unit is had when the unit is used to scale the DVDO output to match the native resolution of a display, particularly a projector. One DVDO expert has this to say on another forum:

Quote:

As to whether or not the iScan can clean up your other SD feeds so that they look good when blown up on a large screen, I would have to say that it all depends on what the problem is with the source material. If you have an SD satellite feed which is loaded with compression artifacts or is blurry because of bandwidth limitations, then the iScan (or most other video processors, for that matter) may not do much for you. It may be that the iScan reacts better to this type of problem than your projector’s SD inputs would, but that really depends on the specific projector you choose. If your source is reasonably clean, then the iScan should do a good job of making it look good on the big screen (although the same could be said of other video processors as well). In other words, it all depends . . .

- Dale Adams

Conclusion
In conclusion I would say if you are looking for a HDTV to purchase and you know you will be watching a lot of SD programming, then I would recommend you shop for the TV by watching SD material. Chances are the TV that looks the best to you in SD will be stunning in HD. Also try to match the source you will be using. If you are going to use satellite, then try to view the set with a satellite feed. The same satellite provider if possible. If you are going to use cable, then view the TV with cable.

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Copyright 2005 - Richard Binckley

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