I have a couple of questions on this hence the rather general subject for this thread
Channel Selection
am I right in thinking that the channels that should be selected depend on whether the PEQ stage is applied before or after room correction? e.g. given a 5.1 setup and a desire to cut a modal peak well beneath the XO; if room correction comes first then apply the filter to all main channels + SW, if PEQ comes first then apply it to the SW only
Yes, but just be aware that the crossovers aren't brickwalls, so if it's close to the XO frequency it might make sense to EQ all channels regardless of placement. For example if you're trying to take down a 65Hz resonance and your XO is at 80Hz, unless you're using a pretty steep crossover slope your mains are still going to have meaningful amounts of program content at 65Hz.
Filter modelling
Is there any mechanism to see what the filters selected should be doing to the response? For example, does jriver's approach to PEQ equate to any of the eq device types that REW supports?
There's no in-program way to graphically see what JRiver's PEQ module is doing. The "adjust a frequency filter" is the same as REW's "PK" type filter, and the REW's low/high shelf and low/high pass can also be modelled in PEQ by using the filters with those names. JRiver is generally more flexible than most of the equalizer types in REW, so I usually use "Generic Equalizer," which tends to just spit out a large number of PK type filters which are easy to feed into PEQ.
How do linkwitz transform & shelving filters work in jriver?
I was playing around with these earlier. I understand how they work in general but I was having no success in applying these in jriver. Specifically neither filter seemed to be having any effect below about 40Hz. .
The Linkwitz transform requires you to know your system Q and f3 (like the original linkwitz spreadsheet) and your target Q and f3. Qz and Fz are the system Q and F3, Qp and Fp are the target. Make sure you have the right channels selected to apply the transform (it's easy to miss the "channels" box at the bottom). If you've done that and are still getting no effect below 40 Hz, could you post your settings and/or a screencap?
The shelving filters work as expected for me, even at low frequencies, but it's possible they may be interacting with another filter. One thing to know is that the shelving filters don't currently support a Q steeper than 1 (even though it will let you put in a higher Q). If you need a much steeper rise or cut, you might need to do it using the "adjust a frequency" filters instead.
My setup was REW to default device routed through jriver via wasapi loopback measuring via an EMM-6 mic. Mixer (saffire mixcontrol) configured to output to L only. Room correction applied to "move bass to sub" at 80Hz. I then added a low shelf filter with Q=1, frequency = 30Hz, gain = +5dB and remeasured.
The resulting trace was exactly the same. To check PEQ was working I added a peq filter (-5dB cut with q=5 at 40Hz) to trim a room mode and could see the cut in the resulting trace. I then shifted the LS filter to 60Hz and could see that the boost had been applied.
Therefore it really looks like there is some setting somewhere that was preventing me applying that sort of boost to the low end but I have no idea what it could be. I am comfortable that my setup can take it btw so I don't have any concern that I'm going to blow something up (excluding any fat finger errors!!).
The set frequency on a shelf is not the frequency where the rise is finished,
the frequency at which the rise begins in earnest (like the corner of a crossover filter). So a 1Q LS with 30Hz frequency wouldn't reach +5 dB until 10 Hz or so (well below the audio band). EDIT: the frequency is actually the center of the shelf transition band [see below]. I was going to suggest you try setting it at 60Hz, but it looks like you did that and that seems to have worked? Just remember that a 1Q filter can take more than an octave to reach its full boost or cut depending on the scope of the rise or fall.
Also which channels did you have the shelf set to affect? Just the sub?
so my Qs here are around shelving filters;
* what does Q mean in the context of a shelf filter? the order of the slope perhaps?
* what does the frequency correspond to? the +/- 3dB point of the filter?
* is there a setting that prevents what it perceives to be overboosting the low end?
Thanks
Matt
I think I got the bulk of this above, but to be clear, Q means roughly the same thing it means in a normal parametric EQ filter. It's a measure of how long it takes the shelf to finish rising or falling. Higher Q's reach their maximum rise faster, lower Q's reach their maximum rise slower. Shelving filters can have orders, but in this case the Q is finely adjustable so that doesn't really make sense here. For your reference, I think a .5Q is equivalent to a "1st order" shelving filter, and a 1Q is a "2nd order" shelving filter
The frequency isn't a fixed +/-dB point because the total rise isn't fixed. It marks the
start of the rise or fall. center of the transition band [see below]
I'm not aware of any overboost protection on the low end.