Well, I thought jeejeedr answered that too!
You see the TTL input only accepts either ON or OFF state...
hence the reason this logic is used to start with
You cant make a cow fly very well.. by doing something different.
There is not much in-between.. to get to the ON state the voltage only needs to get to the threshold area
of the chip., this is about 2v on most chips... see chart in link
http://www.interfacebus.com/voltage_threshold.html
that pulsed signal you want to provide will be just that a pulsed signal
The blue area is the threshold region..the pink is the area of ON state
Now this area between threshold and ON range very very small..
this area could be used for vary-ing the brightness some but not very much
you would still have pulses in your lines that are supposed to be fully turned ON or anything in-between
I hope that makes sense?