Yeah, it's possible to do exactly as you require for slow moving signals, by sending a [0( to the right [phasor~] inlet and forcing its phase to 0 on a transition. Problem is that this operation is block-accurate not sample-accurate so for fast signals comparable with the audio block size some dirty clicking will happen. [vline~] on the other hand can construct nice accurate time domain lines that aren't block quantised.

The lowpass used with [sig~] is a little trick to slew the control signal and make it more like an analog synthesiser. You could also use [line~] here with a 5-20ms follow time.

A signal that moves fast contains higher frequencies than a signal that moves slowly.

Let's say we have a signal that moves from 0 to 3 in 4 samples.

A perfectly linear function like a wire outputs the same number that

goes in, so for an input that goes {0, 1, 2, 3 } the output

would also go {0, 1, 2, 3}. Easy.

Now if we replace the wire with a function that makes the output be the average of the current value and the last two values, what happens? A low pass filter works by averaging together previous inputs, so it has to have some kind of memory. If the signal was at 0 to begin with then let's assume previous memory locations will contain 0.

out[0] = (0 + 0 + 0 )/3 = 0 same as before

out[1] = (1 + 0 + 0 )/3 = 0.33 now our output moves more slowly

out[2] = (2 + 1 + 0 )/3 = 1 trying to catch up

out[3] = (3 + 2 + 1 )/3 = 2 the averaging acts like an 'inertia'

out[4] = (3 + 3 + 2 )/3 = 2.66

out[5] = (3 + 3 + 3 )/3 = 3 finally we get there

It took us 6 steps to get to where we would have got in 4 steps.

This way of thinking about filters makes easy sense, but to really to understand you can read about how filters work below, but you need to sit down and scratch your chin over the maths. Millers explanation is in Argand/pole form which I find more difficult, Smiths examples include more block diagrams and C code that are more instructive to programmers imo.

http://crca.ucsd.edu/~msp/techniques/latest/book-html/node140.html

http://ccrma.stanford.edu/~jos/filters/Simplest_Lowpass_Filter_I.html