MiniDSP crossover development

[neildavis]

Re: MiniDSP crossover development

It's only like PCD in the fact that the input is FRD files and you can view the driver, filter, and system responses while you are developing the crossover. The actual look and feel is completely different, however.
-Charlie

That's what I was doing about 5 years ago, but nobody seemed too interested :rolleyes:. Take a look at the old PE post at the link below. It points to an Excel spreadsheet that reads FRD files, allows you to specify filter configurations for up to 7 biquads in a TAS3004 chip, and then it generates the biquad coefficients from the filter specs. Then, it converts the values to HEX strings that can be sent directly to the EEPROM on a board with a TAS3004 audio processor chip. Hit the reset button on the spreadsheet and it will load the EEPROM into the TAS3004, so you can change the DSP programming directly from the spreadsheet, in "real-time".

I've got a later version that programs the chip via a USB connection, but you need to have a microcontroller on the board to interact with the program (same as what miniDSP does). I've got code for directly programming the STA308a DSP and other ST/Apogee chips, but I don't yet have code for the Analog Devices chip used on the minDSP. That, plus code to directly control the DCX2496, are on my to-do list.

Here is the link (note the date):
http://techtalk.parts-express.com/sh....php?p=1401986

This code is very rough and not well documented, but you are welcome to use it. The spreadsheet is not protected in any way. This code went through a translation to .NET and got cleaned up and well documented for my active loudspeaker design suite, so it is probably not too useful as-is.

BTW, at that time there were a number of people having some problems with PCD importing FRD files, so I made the file import code "bullet-proof". It doesn't care if there are blank lines, extra lines or extra characters, and the delimiters can be just about whitespace character. It also has a good interpolation routine for resampling the data to 500 sample points. This version is very hard to read, but the .NET version is nice and modular and easy to follow.

[charlielaub]

more speaker crossover developement to report...

I'm continuing to develop and refine my Excel tools. While I am at it, I am also trying to build some speakers! So here is an example of my latest efforts.

For this speaker, I am using the IKEA "baffleXchange" concept that I developed a while back. The cabinet is about 50 liters total volume. I am using a Peerless 850137 8" CSX woofer and the Dayton RS28F tweeter. The cabinet is ported with Fb=34Hz.

I did some modeling of the driver positions using Jeff Bagby's diffraction modeling tool, and I found that this offset tweeter position help to boost the tweeter response around 1k - 1.5k Hz, and results in the off axis response being smooth and independent of angle. The best setup for listening is with the tweeter offset towards the "inside" of the stereo pair.

I played around with various versions of the crossover, including delay, slope, etc. I ended up using a 4th order/4th order crossover plus a 1k Hz notch on the tweeter. There is also a first order shelving filter to lift the bass response back up to the level of the mid and high frequencies, and a second order high-pass filter on the woofer to reduce cone excursion below tuning. The notch steepens the rolloff below 2k Hz considerably. I could definitely tell the difference during auditioning at higher SPLs, with the notch sounding much less strained (if at all), while a 4th order 1.5k Hz crossover sounded a bit strained. Another benefit of the notch is that is helps with the phase alignment with the woofer so that, after juggling the crossover points a bit, I didn't need to use delay on the tweeter. This will simplify the final analog crossover implementation. The crossover point ended up being at about 1775 Hz, which should be fine for the RS tweeter and is still low enough that the woofer is not yet beaming too much.

In the frequency response plot below, the white line is the response measured with ARTA on axis with the tweeter at 1m, the blue line is the response predicted by my tools, and the other lines are as shown in the figure legend.

-Charlie

[DDF]

Re: MiniDSP crossover development

Hi Charlie,

Looks good. How's it sounding?

Dave

[charlielaub]

Re: MiniDSP crossover development

Hi Charlie,

Looks good. How's it sounding?

Dave

Hi Dave,

Although I only have one speaker put together, it sounds pretty good. There are a few things I want to tweak: I need to add some damping material to the walls, but the cabinet is nicely bolted together and seems to not have any bad internal resonances. I could cut a slightly longer port tube to reduce the box tuning frequency a bit. I will probably increase the baffle step about 1 dB and play with the frequency a little - this should influence the mid to upper bass balance. Unfortunately, I could not measure to a low enough frequency to capture the full baffle step, so I had to model it. Once that is dialed in, I can remove the baffle, remove the drivers, and spray on some paint to both baffles.

I've always wanted to build a 2-way with an 8" woofer. I had a pair of Snell K IIs back in the '90s and I really liked them, so this is my effort to capture the memory of my speakers.

-Charlie

[frangus]

Re: MiniDSP crossover development

Hi Charlie,

Did you develop this any further? It's exactly the kind of software i'm looking for..

[johnk...]

Re: MiniDSP crossover development

Comparison of analog active to the miniDSP 2x8 is something I might demonstrate at the next DIY New England. I have my NaO II RS designed with options for a fully active analog crossover and using the miniDSP 2x8 digital. The transfer functions of both are identical in terms of the specification (Q, corner or notch frequency, gain or cut). They measure within a fraction of a dB of each other. I place both between the preamp and power amps. Both sound very good but I find the analog to be a little better though without direct comparison I probably could not tell you which crossover was in the system.

I used Charlies spread sheet to generate the byquad coefficients.