Digital compression of pictures and movies is based on MPEG2 or MPEG4 standards.
These standards cut the picture in small blocs named macroblocks.
In MPEG2 these blocks have a size of 16x16 pixels and are divided by 4 blocs of 8x8 pixels.
In mpeg stream, informations are transmitted to explain the content of each bloc.
2 mains degradations are known on these blocs. We call them artifacts.
1) Bad reception of the digital signal.
2) Limited bitrate of the video stream.
Bad reception of the digital signal
The bad reception of the digital signal causes errors on received datas. The content of the mpeg blocs can be not corrected.
We have on the screen bad blocs in term of content (color, shape) and/or in term of place in the screen.
Only way to resolve this issue : check the antenna ...
Here is a screen shot when there is a bad reception :
Limited bitrate of the video stream
To limit the cost of transmitted datas, providers limit the bandwith of data of each TV channel.
With this limitation, informations of the content of each mpeg macro bloc are altered. We have at reception macro blocs that differ from original picture.
This issue appears when picture has a lot of details and/or with lot of movements. For example : water surface, or sports transmission.
In this case, the information to be transmitted is higher than can be supported by the bandwith reserved by the provider.
We have then limitation on informations which affects all the image even if only part of the picture needs bandwith.
One example : A channel with limited bandwith:
Detailes part of face.:
The background of the picture is moving and request lot of informations. As bandwith is limited, all macro blocs are altered due to limited datas.
The result is that the face is altered.
Nothing can be done to resolve this issue as it is not linked to reception but to a choice made by the provider on settings of transmission and mpeg compression.