This is a brief review and measurements of miniDSP 2x4 HD Processor and USB DAC. I purchased this online and seems like the price as of this writing is USD $207 as of this writing. This is a USB DAC with Toslink optical input and analog/digital in, four channel digital signal processor (DSP).The DSP features can be used to implement everything from active speaker crossovers to full room correction/equalization. While the former functionality is easy enough to implement, the latter is up to you to program. This is a "raw" platform with no intelligence of its own. If you want room equalization, you have to figure out how to program its individual parametric filters or FIR parameters.The unit comes in a shiny aluminum case that feels decent and has just enough weight to not get dragged too badly by the myriad of connections you would have to hang from it:Above, I have the USB cable on the right, then Toslink and a couple of "monster cable" RCA interconnects. As you can see, my fat Toslink cable and USB barely fit next to each other.There is an IR input which I did not test (no remote came with it). But it is super handy to have to switch between settings (e.g. overall room/target curve for your room EQ). So kudos for thinking about implementing this.What I don't like one bit is that the hardware does not come with software that is essential and mandatory for its operation. Instead you have to go to their website and navigate to redeem a coupon to download that software/USB drivers (if needed). I am assuming they have done this to keep their brother Asian manufacturers from cloning the hardware and benefiting from software that is harder for them to implement. Be that as it may, they turned their problem into ours.To wit, when I plugged the unit and installed the drivers, even though Windows would detect it as a sound card, I could not get any output from it. Turns out you have to install the "plug-in" package and then select the input to be USB. By default the input is the analog input! This cost me many hours of frustration when I first got the unit.And why on earth do they call standard desktop app that controls it, a "plug-in?" That too threw me off thinking it is a plug-in to some other software so I kept search for the desktop software. Their website hides that software anyway unless you go through the coupon redemption process even after you logged in and used that.All in all, made me quite grumpy to get it working. But once there, this thing is not bad!Lots and lots of flexibility with two analog channels in, and four out. The software itself uses Adobe AIR cross platform solution so should run wherever AIR runs. I remember vaguely though that AIR is discontinued and if so, wonder what the future holds for using this device with that app.In this review I am only focused on the device's performance as a DAC and ADC-DAC combo. I did not attempt to use or measure the performance of its myriad of filters and functionality. I will do so in the future. So let's get into measurements and see how it did. If you are not familiar with my tests, I suggest reading my tutorial on audio measurements here: https://www.audiosciencereview.com/forum/index.php?threads/understanding-audio-measurements.2351/ For this testing, I tested three inputs: USB, Toslink and Analog. Let's start with frequency response test of Toslink using 44.1 kHz sampling:Channel matching is not perfect although this is a highly amplified graph. The roll off for high frequencies is quite early at 18.7 kHz. There is also some roll off below 20 Hz but not significant.Here is the response using analog input:We get slightly more bandwidth to 20.7 kHz and that is it. I am unclear why there is extra bandwidth here since the same DAC is used in both cases. Maybe they both run at 48 kHz and again, the bandwidth is cut off too early. I have not found a way to change or monitor the internal sampling rate. If someone knows how, please comment.Let's now look at jitter and noise spectrum:I have overlapped both USB and Toslink on top of each other (red/cyan). Performance is identical in both and a step from high-performance DACs like Topping D50. It fits in the class of $30 USB DACs. Nothing horrific though. Just not as clean with 10 dB higher noise and visible jitter components.Let's compare DAC linearity (ability to reproduce the output voltage representing the input digital PCM samples). Ideal graph would be a line. I tested this using both Toslink digital input and analog input (note, the test conditions vary some):Numerically using my 0.1 dB deviation criteria, the analog input seems to do better by one bit. But its output goes pretty nuts after that, going off the chart at the extreme of -120 dB. Either way, it is average to below average performance and certainly not competitive with well executed desktop DACs.-90 dB sine wave shows similar situation:Sine wave is recognizable but there is fair amount of noise.Digging into the spectrum of a 1 kHz tone (itself filtered) with respect to noise and distortion using Toslink input we get:We see quite a bit more noise and distortion than our class leading dedicated DACs. Running the same test where my analyzer generates the analog output to drive the analog input of the miniDSP 2x4 HD we get:Not surprisingly, the analog input underperforms digital, causing higher noise floor and more harmonic distortion. This confirms yet again that you want to use the USB input, not analog.Let's look at intermodulation distortion using Toslink input:As expected, the miniDSP significantly underperforms our reference DACs such as the Topping D50.Similar story exists for THD+N versus level:As a core audio platform, the miniDSP 2x4HD has OK performance. It has no glaring faults but also doesn't match the best-in-class products either. Seeing how this functionality is part of the larger DSP platform and that component can add noise of its own, this is to be expected. Personally I wish there was a plus version that had better execution on DAC side.As it is, you would be compromising some performance to gain the benefits of its equalization. That technology if well implement, can make significant improvements to sound reproduction in your room so in balance you will come out way ahead.