Outlaw Audio ICBM-1 crossover question(s), bass management

[john61ct]

All six outputs do not allow an explicit LPF setting, can only set HPF points

There is an "aggregated / derived" LPF for the content sent to sub(s) output (which can variably get mixed with the LFE input)

So, my question is, is that LPF derived from

A. the Front L/R HPF setting

B. whichever output setting is at the highest frequency

C. Some sort of "algorithm" acting on all six settings ?

Eventually I will have all the gear set up so I can actually test this with measurements

so feel free to ignore this if you don't have a ready answer.

I've delved pretty deeply into the manual and past discussions, so if anyone is curious about this unit I'll try to answer any questions I can.

The only flaw I've found so far, is the first factoid above, which just means an external LPF is needed for bandpass use cases, e.g. midbass couplers between Front L/R and trueSub boxen

 

[PatentLawyer]

I had one of these, and my understanding is that all material below the HPF for each channel is summed and output to the subs.

 

[john61ct]

(wow that was fast, thanks!)

OK but that refines the question, to the interpretation of "summed"...

So just hypothetically, if outputs 1&2 are set at 120Hz, and the rest 3-6 are set at 60Hz

would there be relatively little content-SPL from the 60-120Hz band in the sub output, compared to from below 60Hz

Or would it be the same output signal if all outputs are set to 120Hz ?

 

[kyuu]

So just hypothetically, if outputs 1&2 are set at 120Hz, and the rest 3-6 are set at 60Hz

would there be relatively little content-SPL from the 60-120Hz band in the sub output, compared to from below 60Hz

Or would it be the same output signal if all outputs are set to 120Hz ?

That would depend entirely on the content. I'm not sure what you're trying to get at here. In your described scenario, the sub would be getting the content below 120Hz from whatever is connected to inputs 1 and 2 (and whatever is connected to the outputs of 1 and 2 get the rest), and the content below 60Hz from everything connected to inputs 3 through 6 (and whatever is connected to the outputs of 3 through 6 get the rest). "Summed" means exactly that, they are simply added together.

 

[john61ct]

OK so I understand what you are saying.

If that is indeed how it works, I'll need to think about feeding both the midbass coupler pair, and the mono trueSubs, from the output of separate bandpass filtering devices, in turn fed from the OG FR Front L/R signals. In effect using the ICBM for HP filtering only.

Just thinking about the fact the ICBM subs output is fed by Center + Surrounds signals in addition to Front L/R, seems off to me anyway.

Maybe I'm just giving all this too much weight, it would be good to actually measure when I am set up to use REW.

 

[john61ct]

That would depend entirely on the content.

Sorry I was not clear, I tried to word my hypothetical so that no, content would not be relevant.

Even if just using pink noise or tone sweeps, if the under 60Hz content is being summed from six channels, vs the 100Hz content from only two, would mean the sub channels' output would be greatly unbalanced in favour of the former, requiring EQ to get back to flat.

Personally I am skeptical that is how Outlaw's designers would've gone, better to only feed off Front L/R, or use the pair with the highest xover point.

 

[TurtlePaul]

Sorry I was not clear, I tried to word my hypothetical so that no, content would not be relevant.

Even if just using pink noise or tone sweeps, if the under 60Hz content is being summed from six channels, vs the 100Hz content from only two, would mean the sub channels' output would be greatly unbalanced in favour of the former, requiring EQ to get back to flat.

Personally I am skeptical that is how Outlaw's designers would've gone, better to only feed off Front L/R, or use the pair with the highest xover point.

You are not thinking about this correctly. The device sounds reasonably designed.

The six channels and the sub channels will sum correctly to flat. If they set the EQ for the sub to be either the higher or lower freqencies for the high-pass channels, then it would clearly not sum correctly. Outlaw is in fact doing it the correct way - removing the bass from all channels with the crossover frequency adjustable per channel.

For the subwoofer the under 60 hz is summed from six channels and under 100 hz from two channels, but for the mains there is nothing below 60 hz, but several channels of 60-100 hz and six channels of 100 hz+. This will sum to flat.

 

[john61ct]

You are not thinking about this correctly. The device sounds reasonably designed.

Thanks for your patience in helping me learn. I am not talking about any EQ, the ICBM does not include any such functionality

> Outlaw is in fact doing it the correct way - removing the bass from all channels with the crossover frequency adjustable per channel

Yes that HPF function is not in question

It's the LPF'd content from the six inputs (not crossovers) that I'm questioning. If there were six sub channels each getting the LF content from the respective input channels, there would be no question.

But in fact there is just one Sub output pair

...

So, are you saying that (in this context) "sum" is **not additive wrt signal strength**, whatever factor conveys SPL at line level?

afaik that is voltage, but for discussion I'll just use a hypothetical unit Y

and again, suppose pink noise with each channel, measuring at 2x Y SPL uniformly across the spectrum

The four channels will be sending a subtotal of 8Y of signal below 60Hz, but nothing above.

The two channels will be sending 4Y both above and below, so SPL will be summed at a grand total SPL of 12Y below 60Hz

but between 60-100Hz no speaker will get higher than 4Y SPL

I do not understand how that could be called flat?

 

[TurtlePaul]

It's the LPF'd content from the six inputs (not crossovers) that I'm questioning. If there were six sub channels each getting the LF content from the respective input channels, there would be no question.

The device takes the low-passed signal from each sub channel and the LFE channel and sums them all together. This is easy to sum the signals with opamps. And yes, the opamps act as simple voltage adders.

 

[john61ct]

OK so my previous skepticism stands, at least I think in my case IRL testing with REW is required before deciding to use it for anything but high-passing.

Unless it turns out all my HPFs are to be similarly set.

Note also that the mixing in of #7 mono LFE channel's input is optional and variable

 

[TurtlePaul]

OK so my previous skepticism stands, at least I think in my case IRL testing with REW is required before deciding to use it for anything but high-passing.

Unless it turns out all my HPFs are to be similarly set.

Note also that the mixing in of #7 mono LFE channel's input is optional and variable

You don't need your high-pass filters to be at the same frequency. If each of the channels individually sums to flat, then the total will also sum to flat.

 

[john61ct]

I really don't understand how we are misunderstanding each other.

"Each channel" cannot sum, that is only something that happens across multiple channels.

When different channels have different frequency ranges LPF'd, to the extent the upper ones are missing in the lower-passed channels compared to the higher-passed ones,

for those frequencies passed less often, their voltage / SPL will be lower in the total mix, by definition of how (afaik) "summing" works.

 

[TurtlePaul]

I really don't understand how we are misunderstanding each other.

"Each channel" cannot sum, that is only something that happens across multiple channels.

Hook up a sub and the five channels. Set the centers and surround to 100 hz and set the mains to 60 hz for the crossover.

Play a sine sweep on the front left speaker only. The sub and the speaker will sum to flat with a 60 hz crossover frequency.

Play a sine sweep on the surround left speaker only. The sub and the speaker will sum to flat with a 100 hz crossover frequency.

Play a sine sweep on all channels at the same time. Each speaker will play the high passed content on its channel. The sub will play the sum of the subwoofer parts of the individual crossovers. This will all sum to flat.

If all channels are playing the same material, the sub will be playing the sum total of two 60 hz low passed channels and three 100 hz low passed channels. This will be a complex signal - not a simple low-pass filter. Fortunately it is easy to implement by using one opamp channel to do a subtraction to create the low pass and then a handful of opamps to accumulate each of the low passed signals. Look inside the ICBM device - a bunch of dual opamps.

 

[john61ct]

Aha, I was mentally "summing" across the LPF'd content only, whereas you are talking about between the HPF & LPF portions on each respective channel, so the "grand total" comes out back to unity as it were.

Once looking at it that way I have a hard time reconstructing my previous PoV

I look forward to testing / tuning with REW when I am sure things will get more clear. Thanks for taking the time!