Hi,
Slowly moving from a prepro + external crossover setup to a full JRiver > pro soundcard > power amps setup I'd like to make sure on the bass management part I will be able to set things up the way I'd like to.
First some background to make sure everyone speaks the same language.
.1 Channel : it contains the separate LFE channel recorded at the same level as one unique standard channel in the 5.1 mix
Sub Channel : references the redirected bass from the other channels
If only one subwoofer is used (my case) both have to be mixed, but what about the levels...
The usual +10db on LFE is because it has the same level as one unique channel (let's say center channel), NOT because it is recorded 10db lower than the oters.
So where do these 10db come from ? Considering coupling at low frequencies (lower than around 80-100Hz) to have the same energy as 2 channels the boost will have to be +6db (twice the surface, and twice the power).
We are considering here that all five channels are only partly correlated (due to distance between them and the room itself) we will be somewhere in-between the standard log 10 (for uncorrelated sources) and log 20 (for correlated sources) as a result for the boost to be applied.
+10db was retained as an overall average answer.
That means it only applies to LFE channel and the redirected bass from other channels has nothing to do with it.
That means that the +10db one LFE are not set in stone, but are function of the number of speaker channels actually used at the output, and the room/acoustics itself.
If you play a 5.1 movie downmixed to a stereo + sub setup, the LFE boost applied and sent to subwoofer will not be +10db, but more like +5db. That's why some AVR have a LFE channel level adjustment, from 0 down to -10 (after the +10 boost every avr applies automatically)
Now there are 2 ways of dealing with this : the AVR way, and the right way
The AVRs will mix redirected bass and LFE into one single output and then you will have to choose between 2 evils :
- loose the upper part of the LFE (up do 120Hz) when your speakers are crossed over with the sub part at lower frequencies to ensure phase match between LFE and redirected bass potentially correlated signals
- filter sub and LFE separately (some AVRs can do it), but then you will have cancellation and loss of power in the summed signals itself due to phase cancellations from different crossovers summed in one channel, because slopes of crossovers cannot be set
The first one is the lesser evil I think, and what is usually done in AVRs
About the right way to do it requires external crossover, and the right prepro (Denon AVP, Trinnov Altitude, Lexicon MC12, that's about it):
- sum all 5 channels into one, unfiltered channel
- output LFE as is, unfiltered
- got those signals into an external crossover, and process them separately, one crossover on the sub part to match the fronts (say 60Hz 24db), double order filter 1 octave higher on LFE to maintain overall summed phase in the end (say 120Hz 48db)
- then mix them and output them to amplifier and subwoofer
From what I've checked so far it seems JRiver will be able to let me do it the right way using its internal DSP, I will use a Motu 1248 as a pro soundcard for output, so I think this should work, but I'd like to have some confirmation before I jump into this
