Digital TV Radio scanning and measurement



Noise reduction



Introduction
Issue for provider is to sent the maximum of TV channels on a "link" with limited bandwith. This is the case for example on satellite muxes. So they need to limit the size of datas trasmitted with limited alteration on received picture. One of tool available to limit the datas is to use noise reduction of the picture. Reduce the noise on picture decrease the amount of data to be sent on MPEG stream.
But this reduction changes the final picture. There are different ways to reduce noise :
1) Decrease the analog video bandwith of the picture with a filter
2) Doing average on subsequent pictures.
Decrease the analog video bandwith reduce the picture details. Doing average create remanence on movies.
The below channel was transmitted by 2 providers, one choosed to do nothing one the signal, the other decided to do noise reduction.
Here is the screen shot of this channel. :

Channel with noise reduction:


Channel without noise reduction:


It is difficult to show remanence on screen shots. But we can see the differences on the details on the picture on the settee for example.


Please note also, the dark part around the picture. This dark part cannot be seen on a TV screen as it is outside the display. This dark part is also decreasing the amount of Mpeg datas, as this part needs only few data for the macro blocs, but it is not linked to noise reduction.

For these 2 examples, the bitrate for the channel with noise reduction was 3Mb/s and around 4Mb/s for the one without noise reduction. The picture resolution was the same for the 2 channels (720x576), so 25% of bitrate gain.

In summary, it is difficult to know if a channel is using noise reduction without original image comparison. But we can say that decrease picture quality to ensure the bandwith is not the best way to broadcast channels.