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A tour of the miniDSP 2×8

Shortly after I completed my last article on my 4-way open-baffle prototype using the miniDSP 2×4, the folks at miniDSP dropped a surprise into their online store: a two-input, eight-output DSP card called the “miniDSP 2×8.” I have to admit that this stopped me in my tracks. I had to think for a while about where I was going with my open-baffle project… should I make a move to the new 2×8 card, or continue with the existing pair of 2×4 boxes? It didn’t take too long to tally the advantages of the new card:

• All inputs and outputs can be used balanced or single-ended
• The internal processing rate is 96 kHz (up from 48 kHz)
• No need to swap the USB cable to adjust settings on the other channel
• Capable of up to 4 input and 10 output channels (more on that below)

All in all, it added up to a compelling proposition. So, I ordered one, and sat down with my laptop and the downloaded software to learn its ins-and-outs. I will use my 4-way open baffle prototype as the working example, with my first task being to “port” my current system across to this new hardware.

The Software

I started by transferring across my settings from the dual 2×4 setup to the “4×10 Xover” plugin for the 2×8 board. For those not familiar with the miniDSP nomenclature, a “plug-in” is the software that runs on your computer and provides the user interface that you use to configure the miniDSP. It communicates with the miniDSP hardware over USB. Once the miniDSP hardware is configured, you can disconnect the USB – you don’t need to keep the computer connected to play audio.

The next thing to clear up is the name of the plug-in. It is called “4×10” because – when the digital inputs and outputs are counted – a total of four input channels and ten output channels are supported. The digital input and output channels are provided by S/PDIF or TOSLINK connections on the miniDIGI daughter board, which is an optional purchase with the 2×8 board.

Why would you want ten output channels? Well, you don’t have to use them all, but having them does provide some interesting flexibility. In my main system, I will be configuring six output channels for the main open baffle panels (woofer, midrange, tweeter), and three or four as output channels for subwoofers.

The “architecture” of the 4×10 plug-in is different from the plug-ins for the 2×4 board. It is more complex, but easy enough to navigate once you understand it. I’ve drawn the overview block diagram in Figure 1 below, and will cover each of the major blocks in the following sections.

Figure 1. miniDSP 4x10 block diagram

Input channels

Let’s start by setting up the input channels. Clicking on the Input button displays the screen shown in Figure 2 below. Each channel has a label that can be edited so that you can give each channel a name that is meaningful to you. This is a very handy feature, although you are limited to eight characters – I have labeled mine “Analog L,” “Analog R,” “Digi L,” and “Digi R.”

The controls on each channel, from top to bottom, are:

• A Monitor button that brings up a live meter for that channel (you can monitor the signal level on only one input channel at a time)
• An input level adjustment (attenuation only)
• A button to bring up the parametric equalizers for that input channel
• A Mute button

(Click on this or any screenshot in this article to bring up a larger version.)

Figure 2. mini DSP 2x8 Input Screen

Clicking on the PEQ button brings up the equalization screen shown in Figure 3, as an overlay in the main window. There are five parametric equalization filters for each input channel, each of which can be set to a boost or notch filter, a low-shelf filter, or a high-shelf filter. If you are familiar with the miniDSP lineup, you’ll notice this is one less than available with the plug-ins for the smaller board – this is simply a matter of how the finite processing resources of the DSP are allocated. I’m happy to trade off one EQ filter per input in return for the extra channels and the 96 kHz processing.

There is also a “Link Enabled” check box at the top left. In Figure 3, I have linked channel 1 to channel 3 – that is, the left analog and left digital channels. This means that identical equalization settings will be applied to both left channels. I have also linked channels 2 and 4, the right analog and digital channels. Each channel can be linked to only one other channel, so I still have to set up the left and right sets of input channels separately.

Figure 3. miniDSP 2x8 input channel parametric equalizer screen

Output channels

Next, let’s set up the output channels. Clicking on the Output button displays the screen shown in Figure 4 below. It looks a bit like a mixing console, with one “strip” for each output channel. A custom label can be set for each channel – I named mine “Sub L,” “Woofer L,” “Mid L,” and “Tweet L” for the left channel, and similarly for the right channel. With so many channels, this is a very handy feature! I have also labeled one of the digital outputs “Mono Sub” – for use as a very-low-frequency mono sub channel via a cheap DAC – and the other “Unused.”

The controls are fairly self-explanatory, with buttons to bring up the monitoring meter, the crossover settings, and the equalization settings. Each output channel has an attenuation slider, and buttons to invert the phase and to mute that channel. Finally, there is an entry box to set to the delay on that output channel, with up to 9 ms of delay available (per channel).

Note that the two digital output channels are not quite “equal” to the analog output channels, as they have no adjustable delay on them. This may limit their application a little. I plan to use one for a mono sub, so I will have to see whether placement, phase inversion, and EQ will be enough to get a good response.

Before moving onto the crossover and parametric screens, one more control to note is the “Master Volume” enable switch at the lower right. This is used to enable volume control of all channels with an external 10 k-ohm potentiometer.

Figure 4. miniDSP 2x8 output screen

Clicking on the Xover button brings up the crossover settings for that channel, shown in Figure 5. The current channel is shown in orange, with three other channels shown in grey. (The selection of greyed-out channels seems to be based on the assumption that the 2×8 will be used for 4-way crossovers.) The crossover controls are the same ones that we are familiar with from previous miniDSP articles, with independent lowpass and highpass filters, each with a slope from 6 to 48 dB/octave and a choice of filter types (Butterworth, Linkwitz-Riley, and Bessel).

The crossover filters of each output channel can be linked to one other output channel. In my case, I have linked each set of left and right channels (for subwoofer, woofer, midrange, and tweeter).

Figure 5. miniDSP 2x8 crossover settings screen

Next up is the PEQ screen, shown in Figure 6. As for the input channels, there are five parametric filters on each output channel. As I copied my settings over, in the cases where I had used six filters with the miniDSP 2x4s, I simply dropped the “least important” one for now.

The PEQ screen also allows channel linking. I have linked the left and right channels for the tweeter, and the left and right channels for the mid. I have not linked the left and right channels for the woofer and the subs, as – due to room effects – I use slightly different EQ settings on those.

Figure 6. miniDSP 2x8 output channel parametric equalizer settings

Routing

With the input and output channels set up, we can define which inputs are sent to which outputs. Clicking on the Routing button displays the screen shown in Figure 7. Although it may seem complicated at first, the way it works is actually simple. You just need to remember two things:

1. Each input channel can be sent to any number of output channels
2. If more than one input channel is sent to an output channel, they are summed (added together)

This makes it easy to (for example) generate a mono subwoofer signal – something that required a special plug-in with the 2×4 series. You can send the analog and digital inputs to the same outputs, or to separate outputs.

In my setup in Figure 7, I have sent the analog and digital left channel signals to the four outputs for the left subwoofer, woofer, midrange, and tweeter; and likewise for the right channels. In addition, all input channels are sent to the digital output labelled “Mono Sub,” for use as a mono sub feed for very low frequencies. The final digital output is unused, so no input channels are sent to it.

Figure 7. miniDSP 2x8 routing screen

Saving the settings

At this point, it’s a good idea to save all of the settings to a file. This is accomplished easily enough, by accessing the drop-down File menu at the top left.

Getting up and running

Now let’s take a look at the hardware. The miniDSP 2×8, with a circuit board size of 20 x 25 cm (8 x 10 inches), is not so “mini” any more. Unlike the miniDSP 2×4 modules I have been using until now, the 2×8 is a bare board, so some DIY work will be needed to install it into a suitable chassis. (Note that the original 2-in 4-out miniDSP is also a bare board – the nomenclature has become a bit confusing, with the “2×4” being the original 2-in 4-out board enclosed in a small chassis, and the “2×8” being a bare board.)

Powering up

The first task is to get the board powered up. Power must be supplied by a 12V DC supply – the 2×8 cannot be powered from USB. Any 12V supply that provides at least 1.5 A of current should work. For the moment, I am using a small 12V switch-mode power supply that I got on eBay for a few dollars, but there are lots of options. You could even power it from a 12V battery – while not stated in the datasheet, the miniDSP folks responded that supply voltages in the range of 9.5 to 14 V are acceptable.

Power is connected by wiring a pair of leads into the supplied Phoenix connector. Use a multi-meter to measure the voltage on the Phoenix connector before plugging it into the board, and check that the polarity matches that shown in the manual and also on the bottom of the board. Be warned that connecting the power backwards can damage your board! Then turn the power supply off, plug in the connector, and turn the power supply back on. You should see a blinking blue LED on the board.

USB connection

Now plug in the supplied USB cable and connect the other end to your computer. Pressing on the Sync button (green button with arrows) should bring up the dialog shown below. Pressing the “Synchronize Config” button downloads all of the settings into the 2×8 board. From then on, the board stays “synced” with the user interface – any changes made in the user interface are sent immediately to the board, so changes to the audio processing happen immediately.

miniDSP 2x8 synchronization dialog

One interesting point to note is that up to four lots of settings (called a “configuration” in miniDSP-speak) can be loaded into the plug-in. Once they are all synchronized to the 2×8 board, you can switch between them with the buttons “Config 1,” “Config 2,” and so on, near the top of the main screen. This makes it easy to set up (for example) different sets of crossover points, slopes, and EQ for quick comparison. Or, if you don’t mind leaving a computer connected to the miniDSP, you could also use this for different subjective settings – for example, a setting for normal listening, a setting for late-night listening, and a setting for bass-shy recordings.

Audio connections

Turn the power off again to make the audio connections. The two analog input channels and eight analog output channels can all be used with single-ended or balanced connections. Connections are made via the on-board RCA connectors for single-ended signals, and the supplied Phoenix connectors (with screw-down terminals) for balanced signals.

I found it helpful to refer to the Input and Output screens, to make sure that I was connecting the inputs the right way round, and the outputs to the correct amplifier. The channel numbers are not printed on the board, so you will need to refer to the User Manual to know which is which.

With all connections made and double-checked, turn the power back on. As usual, I prefer to take the cautious route: play some music at a low level, and turn the amps on one at a time – subwoofer, then woofer, then mid and finally tweeter. While doing so, check that each amp and driver are getting the expected signals. Then, with all amps turned on, use the Input screen to mute the left and right channels one at a time and verify that the left and right signals are going to the correct amplifier channels.

miniDSP 2x8 with miniDIGI

With that all done, I was able to sit back and play some music!

Going digital

With the analog inputs set up already, I decided to press on and get the digital input going. My 2×8 board came with the (optional) miniDIGI daughter board already mounted, as shown at lower left in the photograph above. Checking the jumpers on the miniDIGI board against the miniDIGI User Manual showed that the miniDIGI came configured to accept input on TOSLINK input #1. So I moved the jumper to select S/PDIF input #1, connected the output of a USB to S/PDIF converter to that input on the miniDIGI, and slowly ramped up the volume in iTunes. Et voila!

For USB playback (via an S/PDIF converter), volume can be controlled in the player on the computer. If connecting a CD player, though, a caution is warranted: simply plugging a CD player into the S/PDIF input will result in playback at FULL VOLUME! Not good. The levels can be adjusted using the input channel sliders on the Input screen. A more convenient solution would be to use the “Master Volume” digital volume control function, which requires connecting an external 10k potentiometer to three pins on the board. (I’ll report on this in a later issue.)

Note: Make sure all power is off when changing jumpers and making connections. The USB cable can be connected and disconnected while power is on, but I strongly recommend turning off all the power for any other connections.

Note also: I recommend turning your power amps off before turning off the miniDSP, as the DSP generates a transient noise when it loses power. (A solution to this may be available using the REM IN muting input connector on the board. For now, though, I prefer to play it safe.)

Closing remarks

Well, that was fun! Although it’s early days with the new board, it has definitely found a home in my main system. In the next article in this series, I’ll get back to the dipole system in earnest and get the promised dipole subs running. In the meantime, if you have specific questions about the 2×8 board or the 4×10 plug-in, please feel free to ask below and I’ll try to answer them. And you can, of course, also ask questions on the miniDSP user forum.

Acknowledgements

I’d like to thank the following people who helped improve this article: Rod Mell, Paul Chapkin, Norman Tracy of Audio Crafter’s Guild, and Tony Rouget of miniDSP.