HDTV Artifacts

With Digital Television (DTV) there are several artifacts that can occur due to the digital transmission.   DTV video is compressed at several times throughout the signal path from capture to broadcast.   A broadcast engineer on the highdefforum (member name dmobp) posted this example:

The quality issue depends on how it is produced (What format), how it is transported to us (How much compression) and How much compression we transmit it in. For PBS if the typical program was produced in HD at 1.2gb it will be compressed to 256mb for recording, recompressed for distrubution at 18mb, and then we will un compress it to base band and recompress it to about 13mb. So thats 1.2gb to 13mb. You just got rid of about 90% of the information. Then your home set converts the signal to the native format of the monitor where you could loose more resolution. Some networks distribute it at 45mb, that really helps the quality but the satellite time doubles so it is very costly and PBS is a non profit organization, so every dollar counts.

Now what happens when that amount of detail is lost?  You end up with compression artifacts.   

Mosquito Noise
The most common is mosquito noise or the ghosting look that surrounds text as illustrated here:


The picture on the left is the result of compression/decompression and the picture on the right is the uncompressed version.   As you can see there is quite a bit of video noise around the text in the left hand picture.   This noise is particularly noticeable in digital transmissions of Standard Definition (SD) over cable and satellite.  The larger the screen size the worse this noise artifact will be.  Here the smaller HDTVs (40" and smaller) will not show this artifact nearly as much as the larger screen sizes

Jaggies can be caused by the stair stepping due to the enlargement of SD resolution to the HD resolutions.   The following picture will illustrate why the SD picture on your old 27" set looked sharper than the picture on your new 50" HDTV.

Notice not only the jagged edge of the sail and the top of the buildings, but the more noticeable mosquito noise in the enlarged picture over the buildings. The inset is representative of what you would get on your 27" SDTV.

In trying to smooth out the picture after decompression, some receivers will run the video through various processors.  This can be very effective on the smaller screens, but can be really harmful to the Picture Quality (PQ) in large HDTVs.  Posterization is one of the worse offenders in my estimation.   It is created when the video makes an abrupt transition rather than a smooth one.

(used without permission)

Notice how the boy’s face looks like Claymation?  Posterization can also be caused by cheap displays that do not have enough color variations to allow for smooth transitions.  For more information see:
pcmag.com encyclopedia posterization 

Motion Artifacts
There are actually several artifacts that are caused by fast motion.  Like all moving pictures there is the blur caused by the object moving while the shutter on the camera is open causing multiple or double exposure.   In the next picture an example of this is shown on the right where the quarterback is blurred as he hands off to the running back.

Another artifact caused by fast motion is blockyness caused by overcompressing an area of the picture.   Mpeg2 video is made up of macro blocks and each can be compressed at different levels.   The way mpeg2 works to compress the data stream is by comparing multiple frames and eliminating the redundant areas of the picture in subsequent frames.  Stated another way, over say 16 frames, the only data sent frame by frame is the part of the picture that has changed.  Since the pictures are made up of micro blocks, the end result is a fast moving object looking like it is made up of little squares, like the running back #39 on the left of the picture.  If you have watched much HD sports, I’m sure you have seen this.  It is particularly a problem for the 1080i/30fps broadcasts as that format takes more bandwidth than the 720p/60fps format and therefore needs to be compressed more to fit in the channel bandwidth.

This is not really an artifact, but the result of a drop of signal due to low signal level.  Low signal level can be caused by an insufficient antenna or the station is not operating at full power.

Notice the above picture is an extreme dropout and just after taking this shot the picture blanked altogether.   I used an adjustable attenuator to get these shots on a weak station in my area.  After adjusting to attenuate a bit less I was able to get this dropout:

Notice this is similar to the fast motion blockyness, but is differentiated by the extreme drop out of data in the area that changed.  The background didn’t change, so it stayed intact and only the woman moving suffered data dropouts.  The background is out of focus and was due to the close up of the woman.

Be sure to look at the companion essay HDTV Pictures.

Be sure to check back on this essay from time to time because as I am able to get examples of other artifacts I will add them to this essay.

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