xover with LRSub mixing?

[tk2k]

Hey all,

I'm mixing LRSub, and we're looking to purchase a system processor (mostly so I can set a hard limiter for novice users) but I'm wondering if there's anything special I need to do because of my setup.

I'm mixing with a dedicated sub send for more control over the low end. Our mains are full-range (down to 55hz) so traditionally I would employ a crossover, but seeing as they are different sources i would probably need a LPF and HPF on each mix respectively, set to the same frequency? I'm worried about phase alignment seeing as the two signals may be at different levels?
i.e. if I set it for 80hz, the bass guitar feed in the sub vs mains might be at a different volume. I know that's one of the downsides of LRSub, but I'm not sure if i'm thinking myself into a problem, or trying to solve a real problem

 

[museav]

How are you deriving the sub send and what control are you hoping to gain? There can be differences between an approach like aux fed subs where only the sources that require low frequency reinforcement are included in the subwoofer mix versus having a more traditional mix but with separate user level control of the subwoofer signal versus dedicated subwoofer control and processing within the DSP.

One aspect with having separate subwoofer level control is that if you raise the subwoofer level relative to the main level then the actual crossover point goes up while if you reduce the subwoofer level relative to the mains level the crossover frequency decreases. Say you have a 100Hz low pass on the subwoofers and a 100Hz high pass on the mains, if the subs and mains are relatively the same level then the crossover is around 100Hz. But raise the subwoofer level and you are not only raising the overall levels below 100Hz, you are also raising the resulting actual crossover frequency.

As far as relative phase, there are two factors. One is that the relative levels of the subs and mains will affect any relative phase effects between them. When they are both at the same level any associated relative phase effects will be maximized while as the difference in relative amplitude (level) increases the effects will become less pronounced. The other relates to the above as not only the amplitude but also the frequencies involved in any relative phase relationships between the subs and mains can vary as the relative levels vary. Put those together and as you vary the relative sub and mains levels any relative phase effects and the frequencies at which they are apparent can vary.

Related to all of this are the low pass and high pass filter slopes. Shallower filter slopes (lower dB/octave values) mean greater overlap and thus more potential relative phase effects while steeper slopes mean less interaction and thus less relative phase effects and reduced variation with relative level changes.

Getting this all back to practical considerations, it is the infamous "it depends". Many people find that in their situation the advantages more than offset any potential disadvantages. Depending on the control envisioned one possible factor might be that you mentioned novices and aux fed subs or separate subwoofer level control may be one more challenge for novice operators. In fact if you go with a system processor that supports it you may even want to consider one processor preset for a more traditional full range mixer output with crossover in the DSP and a second processor preset with a dedicated subwoofer feed from the mixer. There would obviously be much greater programming and system tuning involved but that could also allow greater flexibility.