Thanks, but I've already seen that somewhere else (it's also similar to the wording in the Help text), and it doesn't properly explain what I was seeing (or hearing, rather).
However, I think I've worked it out, at least in principle. If the values are treated as if they were frequencies - so 0 equates to a low frequency and 10 to a high one - then the behaviour does actually make sense: In the HP Filter parameter, a zero value will let through most frequencies, whereas in the LP Filter it will attenuate them. Conversely, a 10 in the HP Filter will attenuate most frequencies (they have to be higher than whatever 10 equates to in order to pass through it), whereas in the LP Filter it will allow most frequencies through (pretty much everything is going to be lower than that high value).
So in my tests, HP Filter 0 and LP Filter 10 had the least (if any) effect on the output, whereas HP Filter 10 and LP Filter 0 silenced the output even before the last step.
This then explains what I described in my original post:
- With both values set to 0, the HP Filter is effectively neutral but the LP Filter is at its most severe, so the output loses more and more treble until there is nothing left.
- With both values set to 10, the situation is reversed, and the LP Filter does nothing, but the HP Filter gradually squeezes out the bass until there is nothing left.
- With both set to 5, both filters are in play, but neither is so extreme as to attenuate everything, and to some extent they cancel each other out.
Essentially then, it seems as if the values work in opposite directions for the two parameters, and I guess the simplest way to use them would be to have one set to its 'neutral' value (so 0 for HP Filter and 10 for LP Filter) and the other set to whatever you want to create the sweep you are looking for.
It would still be good to get some confirmation of this, and maybe an explanation as to why we have values 0 to 10 rather than actual frequencies in Hz.