# Software Crossover Design (3 or 4 way)



## NHTdude (May 2, 2011)

_(crossposted from REW forum on the suggestion this might be a better place)_

Howdy,

I've spent the last few weeks surfing/lurking trying to figure out how to use the 35 woofers/drivers I have had sitting in a box for about 6 years ... 

To start I want to make one 3-way and one 4-way speaker (just adding in a sub)

So far I've figured out/settled on the following:

- Hackintosh on Gigabyte X-58 with built-in 7.1 capability. I figure I have 8 channel output here, so I could complete a stereo 4-way setup. Long term want to have a movie room, so I suppose I'll need another soundcard or something. Definitely would prefer to develop on this system. Could be operational system too.

- Dell Mini 9 with OSX, but could install Windows or Linux if necessary. I was thinking this could be the operational system.

- REW, Audio Hijack, JackOSX and qjackctl, Soundflower(bed), Play (can use AU plugins), LAConvolver

I believe I've figured out how to route audio from a player (Play, iTunes, etc) using Soundflower 16ch to the 8 output channels. I might also have finally figured it out in Jack using qjackctl. 

I am not so sure on how to take the output from the player and apply the 4 required filters for the 4 drivers (let's just assume 4-way) and send the outputs to the correct output channels.

Yeah, the previous two lines seem contradictory, but I just can't quite seem to make the connection mentally. I think this is only my 2nd biggest problem though.

Both Play and Audio Hijack can implement LAConvolver. That part wasn't that hard once I figured out I needed a Filtered Impulse Respone .wav file. On a side note, I keep hearing about FIR filters and can't figure out if REW is doing that or if I need something else to generate those. I am also a Matlab user and have played around a little there with the signal toolbox, but I can't figure out how to "export a filter to a .wav file". 

The main questions right now boil down to the following:

- Do I need to measure each driver independently, look at the SPL curves and eyeball an appropriate XO freq? 

- How do I put it all together? I need 4 separate filters for 4 channels, but then I'm a little stuck on how to proceed to use REW to make sure the overall response of the speaker is correct.

Any help is greatly appreciated.

*update after John's comment from REW forum: 
* - Bottom line is I want to build a 3 or 4 way speaker with an active software crossover using my desktop or netbook computer. I think I'm on the right track, but am missing a couple of pieces or "connections" to put it all together. I've got two SURE 4x100w class D amps ready to go and am itching to make this work.


----------



## GranteedEV (Aug 8, 2010)

NHTdude said:


> To start I want to make one 3-way and one 4-way speaker (just adding in a sub)


Oh boy. As if making a decent 2-way sound coherent isn't hard enough...



> - Hackintosh on Gigabyte X-58 with built-in 7.1 capability. I figure I have 8 channel output here, so I could complete a stereo 4-way setup. Long term want to have a movie room, so I suppose I'll need another soundcard or something. Definitely would prefer to develop on this system. Could be operational system too.


-is Preout voltage sufficient to drive your amps to full power?
-is noise floor and channel crosstalk low enough in a computer?

I highly recommend a USB based outbound 8 channel sound card of some sort. Nothing fancy but definitely something with its own power supply and high voltage/low noise. Look into pro audio gear... consumer gear is likely too lacking for your purposes.



> I am not so sure on how to take the output from the player and apply the 4 required filters for the 4 drivers (let's just assume 4-way) and send the outputs to the correct output channels.


Crossover software does exist. The main one I know of is BruteFIR for Linux.

http://www.ludd.luth.se/~torger/brutefir.html

A lot of diyAudio members use this program so you might be able to get additional support there.

Another one i've seen is Bodzio for Windows:

http://www.musicanddesign.com/Ultimate_Equalizer_promo.html



> On a side note, I keep hearing about FIR filters and can't figure out if REW is doing that


IIR Filter - Infinite Impulse Response - IE what 99.99% of speakers use, are prone to ringing or otherwise adversely affecting SQ. That is why many try to keep the order of the crossover as low as possible (the other being added cost) with as few other filters IE notch or response shaping. They're a good reason why crossover design can be a headache. Opamps, inductors, capacitors(analog) and biquads(digital) are all examples of IIR filters. They have their drawbacks, but they are what most of us are familiar with when it comes to speaker design.

FIR - Finite Impulse Response - in speaker terms are excellent for crossovers because of their transient/phase response. You can basically do a 300db/octave filter without all the "lag" you'd expect of a 30th order or whatever crossover. The drawback to going FIR is that you do need a ton of processing power... analog filters as far as I know will not operate in FIR - you must do something like this digitally. Another great thing you can do with FIR filters is micromanage on-axis frequency response to get extremely tight driver tolerance... if I recall correctly, NHT's XD speakers did this to have something like +/- 1db in-room response tolerance. Trying to brute-force such tight frequency response with IIR EQ filters would just ruin time domain response. Again, the tradeoff is that you need a ton of processing power. I would say that they are in fact "better" with the caveat that they aren't necessarily "easier" to get right.



> - Do I need to measure each driver independently, look at the SPL curves and eyeball an appropriate XO freq?


Drivers must be measured in the indended baffle. Tweeters and small midranges should be appropriately offset before measuring and sharp corners should be rounded. Crossover frequencies must be determined by more than just on-axis frequency response and certainly can't be eyeballed... you need foremost to get a match in directivity index, and also consider power handling, driver spacing, phase, and in the case of a 3 or 4 way, you also need to consider the effect of a given filter in the passband of the third or fourth driver. At least with active you don't need to worry about zobels or L-pads, and with digital active you have some freedom to screw up.

What you can sometimes learn from the on-axis response of individual drivers however, is what sort of filter is needed to get a resulting acoustic crossover order. Of course appropriatre gating must be used for in-room measurements. Outdoor measurements are preferable, and of course anechoic measurements would be ideal, if unrealistic.

99% of the time when people try to design their first speaker based on some drivers they've already got, they end up trying to fit a square peg into a round hole, but... ignorance is bliss. 



> - How do I put it all together? I need 4 separate filters for 4 channels, but then I'm a little stuck on how to proceed to use REW to make sure the overall response of the speaker is correct.


Dunno about REW, but all I can really say is start small. Learn how to integrate two drivers together before you get to the nightmare that is three or four.


----------



## fusseli (May 1, 2007)

GranteedEV said:


> IIR Filter - Infinite Impulse Response - IE what 99.99% of speakers use, are prone to ringing or otherwise adversely affecting SQ. That is why many try to keep the order of the crossover as low as possible (the other being added cost) with as few other filters IE notch or response shaping. They're a good reason why crossover design can be a headache. Opamps, inductors, capacitors(analog) and biquads(digital) are all examples of IIR filters. They have their drawbacks, but they are what most of us are familiar with when it comes to speaker design.
> 
> FIR - Finite Impulse Response - in speaker terms are excellent for crossovers because of their transient/phase response. You can basically do a 300db/octave filter without all the "lag" you'd expect of a 30th order or whatever crossover. The drawback to going FIR is that you do need a ton of processing power... analog filters as far as I know will not operate in FIR - you must do something like this digitally. Another great thing you can do with FIR filters is micromanage on-axis frequency response to get extremely tight driver tolerance... if I recall correctly, NHT's XD speakers did this to have something like +/- 1db in-room response tolerance. Trying to brute-force such tight frequency response with IIR EQ filters would just ruin time domain response. Again, the tradeoff is that you need a ton of processing power. I would say that they are in fact "better" with the caveat that they aren't necessarily "easier" to get right.
> 
> ...


Good post, and ya your IIR/FIR knowledge is sound. FIRs are capable of linear phase, though it may require a 100th order digital FIR filter to acheive the same slope as maybe a 6th or 7th order digital IIR, for example. That's where processing power comes in. Doing it all in software as you're hoping should work with a halfway decent CPU... Hard to say how the free-software implements the filtering. There's a chance that free stuff will use a basic (and unoptimal) filter implementation. Running 8 simulataneous channels isn't going to help either... You'll just have to try it and see. DSP in software run on a computer's CPU is nothing like how a hardware DSP would run. You probably won't have any problems, I'm just speculating. Definitely get your software situation figured out and filtering verified before you actually build anything.

As far as design complexity goes, using active filtering is a far more simplistic approach than passive for a 3 or 4-way because you will dodge having to worry about system impedance. 3-way passive filters can be marvelous fun, and 4-ways I'm guessing would be a nightmare. You can easily simulate your active filters on your raw driver responses in PCD. I think designing this way would be easy!


----------



## Theresa (Aug 23, 2010)

I would use miniDSPs instead of a sound card. I would also stick with a 2 way with sub(s). Use two subs and you basically have three ways. I just don't see the need for another driver sitting between the midwoofer and sub or between the midwoofer and the tweeter in a system using conventional drivers. There are so many fine 6", 7", and 5" drivers that can be used as midwoofers. And many tweeters these days can go down to 1700Hz or even lower. I currently have Exodus drivers for subs, ScanSpeak 7" Revelators for midwoofers, and Eton Mag/Ceramic tweeters. I don't see how I could improve it with an additional driver. Now, with ribbon/planar drivers, I've thought of B&G tweeters and midranges and a woofer for the range from 80Hz to 500Hz and subs to handle the real lows. But that would be too expensive for me right now after putting together my current system which is wonderfully neutral.


----------



## Binary (Nov 23, 2009)

I've been looking into this.

You are by far cheaper/easier to pick up a couple of miniDSP units and be done with it. Plus its a standalone system that doesn't need a special PC to work, and draws less power from the wall. The other side is that you will give up your ability to deal with surround sound properly on a PC using software crossovers. So unless you want to go specifically 2 channel, i wouldn't recommend using software filters.

You will still need an amp per channel in this kind of setup.

Furthermore, you will ALWAYs need the PC as your pre-amp which is a major pain, and increases turn-on time for the stereo as well. The miniDSP will let you use your PC to set up its filters. as well as theres a way to output your response from REW into the miniDSP to create a "perfect" EQ setup.

Sometimes, Its about spending a few extra dollars for all the freedom in the world.

Don't forget futureproof. the DCX 2496 is pushing 10 years on the market soon, and its still the go-to for most people. The miniDSP is much newer than the DCX, but sound equipment and computers go obsolete in different timelines. It would be hell to have to figure out that every future pc purchase would need to "work" with your stereo. Too much hassle IMO.


----------

