This compares the results of allowing the minimum signal level to fall too far before amplification. To isolate the effects to the minimum system signal level, the signal level at the receiving VCR was kept within PAL spec where possible, using booster amplifiers and post-amplifier attenuators. Where post-amplifier attenuators were used, at no time did the post-amplifier signal level fall below minimum PAL spec.
I have used analogue TV to show this because the quality can be seen as it falls, and the results can be shown using a single still frame. The principles still apply with DTT - the loss of carrier to noise simply gives increasing freezes and block loss on DTT where on analogue it gives rise to more snow in the picture.
The results show that, despite the boosters being configured to keep the levels at the receiver input virtually identical (bar the last test) -
signal quality is a function of the minimum signal level in the system, not the booster output level
and therefore the loss should always follow the amplifier for best results. In any well designed system, the minimum signal level is the output of the aerial. This is why you cannot use booster to make a bad aerial good. A bad aerial means a lower minimum system signal level, which means poorer overall performance, regardless of subsequent amplification. Assuming, that is, you do not have extremely strong aerial signals that would push the booster beyond its limits, in which case you have to ask yourself why you are using a booster in the first place!
|Min signal level (dBuV)||attenuation||amp||Picture (click to enlarge)||Picture, 100%, closeup
(click to enlarge)
|Rx input level
Use and abuse of booster amplifiers
Signal quality variation with falling input level
Selecting and siting your TV aerial
Aerial system components
Why DTT is different from an analogue install