A few folks have asked me to make a review on the minidsp 2x4hd.
So lets take a look and see what it is
So we have a very compact yet powerful DSP. This little 2in 4 out DSP can do quite a bit. It's design is discrete enough to make it small and very affordable,yet it will do things most DSPs in its price range cannot.
What makes it different about its 2x4 counterpart is a few small but much needed improvements. It has a 96k internal sample rate, has optical toslink input and it's a usb dac. It also has a small but effective FIR (finite impulse response) bank. FIR gives you the ability to make custom filters that are flexible in ways a standard IIR filter can not do.
The difference between FIR and IIR
A few other convolution engines
It takes a minimum of 400mhz processing to be able to convolute impulse responses. So this DSP uses a 400mhz sharc chipset.
It does 10 band parametric equalizer on the input and each has 10 bands of parametric parametric equalizer on the input and each output has 10 bands of parametric Equalizer
With the parametric equalizer or PEQ you can make any shape peak or shelving filter you want just like with the traditional 2 x 4 . You can link and re-name channels . 80ms delay on each channel on board. This DSP can also play any sample rate media you throw at it 24/96 and higher with no problems. It's optical input will read any speed that Sony Phillips Digital interface format (S/PDIF) can distribute. It carries on board AKM a/d - d/a 24bit converters and 32bit floating DSP. Has 2v/4v input jumpers and has a 2v output with 12db added boost for a measured 2.81v max output.
What really is most appealing about this besides it does all that's is its FIR ability. It has the capability of 4096 Sample taps. Which traditionally in the FIR world that really isn't a whole lot and screams to be justified. If someone wanted a processor just to make some exotic F I R filters , this may not be the one you want to use . But if you want a system with a flat phase response by means of linear phase crossovers this little thing will get the job done just fine and sound very good doing it . It's not powerful enough to do full room correction on top of crossover linearization it's just powerful enough to do one filter per channel . So if you're looking to do linear phase crossovers and not linearized crossovers as well as room correction on top of that per channel you will need a DSP capable Of at least 4096 taps per channel. This one does 4096 taps divided between four channels . You can divide the 4096 anyway you want and allocate taps over the DSP to fit your filter making needs. If room correction is your thing, for a Car the APL-1 is popular and looks like a great solution.
For a 4way you will need either the 4x10hd or a pair of the 2x4HDs
The 4x10hd will cost slightly more than a pair of 2x4hds so this seems like a better option however some like the packaging of a 2x4hd better or want to do it in stages. Personally I have 4-2x4hds running from a dexp99rs. I like having 2048 filter taps per channel and being able to adjust the speakers separately in volume from the deck. However 2-2x4hds will do the job just fine.
On to the meat .
So getting it all set up, you will need a miniDC isolated dc/dc converter Power supply for cars it has 12v/rem/ground inputs and will turn on and off the DSP from your Car deck. That Power supply goes for 12$ from minidsp.
Once you download the 2x4hd plug-in to your computer and install Adobe Air to run the plug-in your ready to start tuning and making filters.
On the configuration side of things, with a pair of 2x4hds I would recommend using the sub and tweet on one and the midbass and midrange on the other. That way will allow you to allocate more taps to the sub because it will only take a few hundred filter taps to make a excellent performing FIR filter for a tweeter.
I would discourage the use of one 2x4hd for left and one for right. Reason is it has the ability to link channels , so you can link Speaker pairs and tune them separately from each other Speaker in the system. It's better practice to tune speakers as pairs before you break them into separate left/right tuning. Having one for left or right separately will make that challenging because you will always be switching back-and-forth between configurations. There's just not enough taps per channel to make filters to do entire bandwidths. It was meant to stretch every last bit of processing. So a small fir filter is all you can make. So that makes it pointless because you won't be able to make those types fir filters.
I'm going to assume that you already know how to tune your system with a standard EQ and time alignment. They built this thing very efficiently between the combination of the regular 2 x 4 abilities along with FIR together you should be able to dial-in about anything you want. It will just take some planning and A sober approach to configuring all of your channels and FIR banks to make things efficient as possible. There's not enough filter taps to wastefully start building FIR filters when some of the filtering can be done in the onboard crossovers, PEQ, or compression/limiter that is already built in. Once you have your system dialed-in the best it can be before you start Making FIR filters.
You can use any convolution software you want as long as it is able to output a I-EEE.bin file for the DSP to import. Re-Phase is a third party software freeware by pos from DIY audio. You can download re-phase from surge force for free. I would recommend donating to pos for such a solid awesome easy to use software for us to make fir filters on. For 200$ you can also get filter hose , it has much more flexaility and can make very good low-tap filters from double convolution . Where you basically convolute a convoluted file . Sounds strange but it works. There are a few other free or inexpensive convolution engines in this review I will be focusing on re-phase.
Once you have re-phase downloaded you will be able to start making FIR filters.
I found optimally the sub should have about 1700 taps and the tweeter can use as low as 128 taps. So allocation of at least 1700 taps to the sub was the only way I could get a real good FIR filter that had the crossover done in FIR. You can linearize a crossover you already have and use less taps for your sub. I liked the way the sub sounded with a filter of a unique shape that I arbitrary made up that seemed to match the responce and phase response in my car.
Let's talk about phase for a minute.
When your measuring phase with your REW and looking at it on the screen you will see a few things.
On the REW screen you will see a "wrapped" phase or a un-wrapped phase. Wrapping phase essentially means that you will see a graph that when it gets to 360° +/- It stops. So your phase response will look like a "sawtooth" if your looking at it on the screen wrapped. Basically where it falls off the screen it comes back on the screen at either top/bottom of screen. If you un-wrap phase you just see a line either going straight , up or down.
The area from 20hz to about 250hz should look smooth and shouldn't need adjustments by means of phase correction as much. Applying PEQ is a better way to adress phase and amplitude together in lower frequencies. 250hz to 500hz is a transition zone and is very modal in a car. Small adjustments of phase from side to side can make that band come into good acoustic polarity from the seating positions. That is an area that often there's a dip and can be corrected with the right adjustment. Above 500hz to 1k is an area that you can make small phase adjustments to to fix a comb filter problem or a reflection problem. If you look at the phase response this area will start to have a lot of peaks and dips. Just focus on fixing the areas where phase is 180° out between left and right. I wouldn't try to make it zero just use the paragraphic phase Eq and small 11.5° increments. As long as the two speakers aren't 180° out of phase you will get mostly in phase this is really nice when you want to fix that one area that forces you to flip the polarity on your mid range to get a good center image you can actually go in and fix that one area that causes that polarity flip, get your nice center image but now you're not sacrificing all the other frequencies or midbass around it. Under 1k is the "minimum phase" region and above 1khz is the linear phase region . A car is somewhat considered min phase because the reflections and speaker interact together.
You can try to get your frequency response as flat as you possibly can between drivers or have your levels matched as perfectly as you can when displaying phase responses between multiple drivers. It's always best to examine speakers phase independently. Looking at a single speakers response with no EQ added is the best way to see what's actually taking place with phase in your car vs. the drivers own phase responce with no added phase change made by use of standard EQ. You Eq will shift phase along with magnitude so is important to take measurements of each one of your speakers with no passive crossovers no active crossovers and a measurement of your speaker in free space with no reflection . Out in the backyard works great for that right on the grass . Mount the speaker up on a stand and take some measurements. A good set of measurements maybe take a few try's, I would mainly focus on frequencies above the Schroeder.
That is where your car is neither wide enough or tall or long enough to equal the wavelength coming out of the speakers so it turns to a "modal" region . It's usually around 315hz and below in most cars , could be either slightly higher or lower. For frequencies below the Schroeder I just leave the speakers in there position and make very very quiet and close mic measurements. It doesn't seem like it makes much of a difference . I just want to capture what the speaker is doing not what is happening at my seat.
Now that you have some good measurements, it's time to take more measurements, this time in car at seating position. Again I take a set with crossovers on and a set with crossovers off.
After I have a whole bunch of measurements taken at least 3 per section. You should be able to see what's happening inside your car with your drivers. Your drivers by them self , and the interaction of the driver with the crossover.
Export the measurements of free space no crossover into a text file. Import to rephase.
At that point you'll see a blue line that is your measurement, you can go to the paragraphic phase Eq and make small changes as well as add Lin phase crossovers and see the prediction combined with the actual measured response of your driver. You can do that with many of its features you can also linearize a passive or active crossover.
So with the minidsp 2x4hd there just isn't enough taps to do some exotic filters. You will be able to linearize crossovers which seems to use less taps than making a linear phase crossover. Or if you are running multiple units you can make linear phase crossovers along with some phase Eq. Linearizing crossovers makes A really big noticeable improvement in a car at lower frequencies. You can offset group delay patterns with rephase box tool as well.
In coming days I will post some screen shots.
For a 205$ price tag it is a solid performer. Again if your looking for complete room correction this is not your piece. This is a great unit if your system already sounds good just needs a few small phase issues worked out.
The APL-1 is a much more robust FIR processing for room correction, the 2x4hd will get you your Lin phase crossovers (which is huge btw) and some small room mode tweaks that you self define.
I have been able to get a flat phase response with a set of the 2x4hds. +/- 90° However if you want a flat phase response with 0° tolerance the apl-1 or some other room correction device with time domain Analysis might be what you want to do. For me , I really like this one for the price , and I'm running 4 of them using them as a 2x2 so I have over 16,000 taps . I can now do anything an APL-1 can do with this much processing, and I'm still able to use my Dex-p99 4 way out and control each speaker from the deck on the fly.
Again, in coming days I'll go through some screen shots and go into actually making a FIR filter to be used with it. It's easy to do, tedious a bit but that's the nature of FIR. If you system needs that one extra tweek on top of you standard TA/EQ this is a fantastic unit.