ERROR: first input is not control rate

Filters
Many Filter UGens have the constraint that the first input (the thing they are filtering) has to be the same rate as them. For example, if we try this:

s.boot x = {LPF.kr(PinkNoise.ar, 100)}.play

we are trying to take an audio-rate noise signal and filter it using a low-pass filter, but we've asked for a control-rate low-pass filter (by using .kr). So how should the system interpret that? Do we want to downsample the noise to control-rate then filter it, or do we want to filter it then downsample the result? The system could perhaps choose one of those automatically, but that might hide a user error that is important to know about - so the system doesn't let you do this.

If you really wanted to filter control-rate noise, you could do:

s.boot x = {LPF.kr(PinkNoise.kr, 100)}.play

whereas if you wanted to filter audio-rate noise and downsample the result, you could do:

s.boot x = {A2K.kr(LPF.ar(PinkNoise.ar, 100))}.play

For more info see the helpfiles: "A2K", "K2A", "Filter".