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QuantAsylum’s QA400 24-bit 48K/192KSa/s Stereo Audio Analyser Review by JIM ROWE Do you need to perform serious testing of audio equipment on a shoestring budget? The QA400 Stereo Audio Analyser offers 24-bit sampling at either 48KSa/s or 192KSa/s and achieves a level of performance which closely approaches that of high-end analysers with prices starting at more than $6000 – despite its own price tag of only US$247 including shipping and handling! There are a few catches though. I F YOU’RE INVOLVED in designing and/or testing audio gear, you may have looked longingly at high-end audio analysers like those made by US firm Audio Precision. But you probably lost interest soon after discovering that their price tags start at about US$6200 and rise rapidly from there. Is there a lower cost solution? Well, there might be. As you probably realise, nowadays even these high-end analysers are basically a “USB virtual instrument box” with a set of audio 44  Silicon Chip CODECs (DACs and ADCs) controlled by software applications running in the PC they’re hooked up to. The same software is also used to analyse the audio measurement data collected using the analyser hardware box. Basically, the main difference between the high-end analysers and much lower cost set-ups like those based on PC sound cards or USB-linked audio interfaces (like the Sound Blaster Extigy, etc) has been in the quality of their hardware, the CODECs and associated circuitry. However, solidstate technology has been racing ever forward, upping the performance of CODEC chips while lowering their cost. And software programmers have been coming up with increasingly powerful, low-priced audio analysis software capable of running on today’s PCs. One result of these is the QA400, from QuantAsylum, which has development offices in the USA (in Snoqualmie, Washington) and China (in Shenzhen). siliconchip.com.au Fig.1: this grab shows the crosstalk into the right channel input (open circuited) when a 1kHz -10dBV signal was being fed into the left channel input from the QA400’s generator 1 (via the left channel output), using 48kHz sampling. As you can see the fundamental is at -115dBV, giving a crosstalk of -105dB. The QA400 comes in a compact aluminium box measuring 175 x 86 x 44mm and weighing a mere 348g. There are no controls on the front panel – just four BNC sockets for the unbalanced or (single-ended) stereo audio inputs and outputs, plus nine LEDs to display the QA400’s operating status. There’s even less on the rear panel – just a USB type B socket that’s used to connect the QA400 to a USB 2.0 port on your PC. The QA400 is powered from the PC via the USB cable, like a dongle. Inside the QA400 there are a pair of low-noise input channels, each driving an ADC able to perform 24-bit sampling at either 48KSa/s or 192KSa/s (software selectable), plus a matching pair of 24bit DACs (also 48KSa/s or 192KSa/s selectable) driving low-distortion output amplifiers as software driven audio generators. All of the CODEC functions are apparently provided within a single siliconchip.com.au Cirrus Logic CS4272 high-performance chip. The QA400 also contains USB data interfacing and power derivation circuitry. The DC input resistance is 100kΩ and the AC input impedance is 10kΩ, with the input clipping level specified as +3dBV/1.41Vrms/4Vpp. A label on the QA400 warns that the signal level at the inputs should not exceed +6dBV, while the maximum DC input level should not exceed ±5V (the label also reminds the user that the QA400 shares its grounding with the PC). The output channels have a low but unspecified output impedance and a rated maximum output level of +3dBV/1.41Vrms/4Vpp – the same as the clipping level of the input channels. The QA400’s rated current consumption from the PC USB port is approximately 300mA – well within the maximum level of 500mA. The unit comes with a 2m-long USB cable to connect it to the PC, plus a couple of 1m-long BNC-to-BNC cables which can be used for making “loopback” connections between the outputs and inputs. The whole QA400 hardware package costs the modest sum of US$199.00 plus US$48.00 for shipping and handling to overseas countries like Australia and New Zealand. Once you’ve purchased and received the hardware package, you can register on the QuantAsylum website (www. quantasylum.com) to download the software. The software installs on virtually any PC running Windows (32- bit or 64-bit) and comes with its own USB drivers because QuantAsylum wanted to achieve a higher level of performance than can be achieved using the Microsoft audio drivers. It also has its own GUI, which you can see in one of the screen grabs (Fig.2). This has a “control panel” on the right and the display window on the left. Along the top of the control panel there’s a row of buttons to select the display options. You can select either the Left, Right or both channels and display either the inputs or the generator outputs. You can also choose to show the display plotted against either time like an oscilloscope or frequency, ie, as an FFT spectrum display. Below these are six further buttons and two mouse controlled rotary knobs, to allow you to select the X and Y axis scaling for the display. The three upper buttons allow selection of dBFS, dBV or dBr for the Y axis, with the two knobs allowing you to set the minimum and maximum values (so you can examine small variations more easily). Two of the lower three buttons in this group allow you to choose either a linear or logarithmic scaling for the X axis (Frequency or Time). The third and slightly smaller button is labelled “Default”, and is used to reset both axes to their default scaling. Just below the Axis controls are two knobs on the left to adjust the Acquisition settings – Resolution (2048 - 131,072 points) and Averaging (0 - 50). To their right are two buttons March 2015  45 The QuantAsylum QA400 stereo audio analyser comes with a 2m-long USB cable to connect it to a PC, plus a couple of 1m-long BNC-to-BNC cables which can be used for making ‘loopback’ connections between the outputs and inputs. under the “Weighting” label, which can be used to either turn off or select an “A” weighting filter. Next down on the control panel are five buttons, used to select an FFT Windowing function with a choice of Rectangular (Dirichlet), Hanning, Bartlett (triangular), Hamming or Flat Top. Further down again are the Measurement selection buttons to select Pwr (Power), THD (Total Harmonic Distortion), THD+N (Total Harmonic Distortion + Noise), SNR (Signal to Noise Ratio) and FR (Frequency Response). Immediately below these are the controls for the QA400’s two audio signal generators. There are two buttons to turn each generator on or off plus four knobs which are used to adjust the amplitude and frequency for each generator. And right at the bottom is the Press to Run/Press To Stop button. Incidentally, QuantAsylum has built some nice features into many of these on-screen controls. For example, if you click on any of the buttons displaying a black or white dot just to the lower left of its function label, while holding down the Control key of the PC’s keyboard, you get a context relevant dialog box which lets you set various key parameters. Similarly the “knobs” are easily controlled by left-clicking them and then moving the mouse wheel with your forefinger to vary their setting. Another nice feature is that as well as the top Settings menu allowing you to select either 48KSa/s or 192KSa/s sampling, it also allows you to change the graphical display from light traces and text on a black background to dark traces and text on a white background. The former is probably easier to read on a PC’s screen but the latter is better for Fig.2: this screen grab of the QA400 Analyser software in operation shows the control panel at right and the display window at left. The traces can be displayed as either light on a dark background as seen here, or dark on a white background to save ink/toner when you print it out. 46  Silicon Chip print-outs and also saves ink or toner. You also have the ability to add, edit or delete a title at the top of the display, the ability to pan and/or zoom the display horizontally in order to examine an area more closely, and add markers to the display traces. You can also copy just the current display window to the Windows clipboard as a bitmap image, for pasting into an image processing application. This is an alternative to the Windows PrtScn option, which lets you save the entire screen to the clipboard. Claimed performance You don’t have to study the QA400’s performance specs for very long to realise how close its basic performance comes to that of high-end audio analysers. For example, those 24bit ADCs and their low noise input amplifiers are claimed to provide a noise performance over the audio range 20Hz-20kHz of -104dBV with high source impedances at the inputs, or -102dBV with low source impedances. The claimed overall (loopback) performance is also very impressive. With Hann windowing, 32K points, no averaging, no weighting and a 20Hz -20kHz measurement bandwidth, the THD for a 1kHz/0dBV signal is specified as less than -102dB/0.0012%, while that for a 1kHz/-10dBV signal is below -108dB/0.00055%. Similarly the THD+N for a 1kHz/0dBV signal is less than -98dB/0.0014%, and that for a 1kHz/-10dBV signal is below -89dB/0.0038%. The loopback frequency response over the same 20Hz-20kHz range is listed as ±0.07dB. After noting these specifications in siliconchip.com.au particular, I ordered a QA400 from QuantAsylum via the web and then waited impatiently for it to arrive. When it did, I lost no time in putting it through its paces, hooked up to an Asus P550L laptop with an Intel Core i7 processor running Windows 7 Pro (64-bit) at 2.0GHz. What we found There were no problems installing QuantAsylum’s Real Time Audio Analyser application (V1.0696) and when I fired up the application, I found that the 57-page QA400 User Manual had also been installed as a PDF file. It turned out to be well-written and easy to follow, although not quite as comprehensive as I’d like (more about this later). I then plugged in the USB cable from the QA400 and noted that the application recognised it had been connected. Then when I clicked on the “Press to Run” button at lower right on the screen, the green “Run” LED began to glow on the front panel of the QA400 and away it went. Incidentally, its measured current drain turned out to be 252mA when the software was running. After going through the recommended calibration procedure (which only requires a known-accurate RMS AC voltmeter and a couple of BNC-BNC cables), I then began checking out its basic functions and performance. And the results were quite impressive, comparing very well with the claimed specs. For example, the overall (loopback) frequency response of both the right and left channels at a level of -20dBV measured +0.02dB/-0.1dB from 20Hz20kHz, drooping to -0.4dB at 12Hz and 25kHz and reaching -1.5dB at approximately 6Hz and 35kHz (192KSa/s sampling). Note that this test is done using an impulse, chirp or white noise stimulus. Similarly, the loopback THD (32K points, Hann windowing, no averaging or weighting and bandwidth 20Hz20kHz) for a 1kHz 0dBV signal measured -101.9dB/0.0008% for the left channel and -99.8dB/0.00102% for the right channel. Lowering the signal level to -10dBV gave figures that were even more impressive: -105.5dB/0.00053% for the left channel and -109.2dB/0.00035% for the right channel. The corresponding THD+N figures for the same two signal levels were siliconchip.com.au Fig.3: this 20Hz-20kHz spectrum plot was taken when a 1kHz -10dBV signal from the QA400’s right channel generator output was being fed into its left input channel. As you can see, the noise and distortion products are below -128dBV for most of the range, rising to -120dBV below 50Hz. also very good: -98.6dB/0.00118% (L) and -97.5dB/0.00133% (R) for 0dBV; and -91.1dB/0.00277% (L) and -91.4dB/0.00269% (R) for -10dBV. These are all very close to the specs but when I checked the noise performance of the two input channels I found what seemed to be a discrepancy between the traces on the screen display itself and the measurements shown at the top of the display window. As you can see from the full screen grab (Fig.2), the traces (yellow = left, red = right) show noise peaks which are generally below -140dBV, only occasionally rising to -136dBV at the very top end (>10kHz). Yet the measurements at the top of the display show “Peak L” figures of -86.72dBV/ 49.1µVrms and “Peak R” figures of -91.91dBV/25.3µVrms. These figures did vary up and down a bit but at no stage did I see them fall anywhere near the levels suggested by the traces below. I find this rather puzzling; perhaps the “Peak” figures represent an integrated figure over the full bandwidth? My next test was for channel crosstalk, and here I used the QA400’s Generator 1 output (1kHz -10dBV) looped back to the input of either the L or R channel, with the other channel’s input left open circuit. Here again the results were impressive when looking at the traces but not as impressive when looking at the measurement readings at the top of the display. The L-to-R crosstalk worked out at around -105dB using the traces (-115dBV vs -10dBV), yet the “Peak R” figure at the top showed -85.19dBV – corresponding to a crosstalk of only -75.19dB. Similarly, the R-to-L crosstalk appeared to be around -103dB using the traces, but only -74.04dB from the “Peak L” figure. Although not mentioned in the current version of the User Manual, I noticed that in the application’s top menu there’s a heading called “Test Plugins”. When you click on this you get a choice of two options: Frequency Response or THD Versus Output Level and Frequency. These seem to be sweptfrequency test options (in the form of bolt-on DLLs), so that the QA400 Analyser app can be programmed to perform these additional tests. When I tried these tests, I discovered that their results can’t be displayed graphically on screen like the other tests. Instead, they can only be exported in the form of a CSV (comma-separatedvariable) text file. To plot and/or print the results, you have to import this file into a spreadsheet like Microsoft Excel or some other graphics application able to accept data in this form. A further comment about features of the QA400 Audio Analyser application: when you move the cursor around the display window it gives you a real-time readout (at the bottom of the display) of the frequency corresponding to its current X position. This is a useful March 2015  47 downloading, dealing with topics such as: making IMD Measurements with the QA400; Extending the QA400 Noise Floor with an external preamp; and Connecting to the QA400 and other QuantAsylum devices from software written in C++/Managed C++. Concluding comments Fig.4: this plot shows the “loopback” frequency response of the QA400 (both channels – red is R, blue is L) over the range from 1Hz to 40kHz, using 192kHz sampling. The -0.5dB points are at 12Hz and 30kHz which is excellent. Fig.5: this screen grab shows the noise floor of both input channels of the QA400, taken over the range 20Hz - 43kHz and using 192kHz sampling, with both inputs open circuited. The noise peaks are below -136dBV over the full range. feature but it would be even more useful if it also gave you a real-time indication of the level corresponding to the cursor’s current Y position. That way, you could make on-screen measurements without even placing markers, simply by placing the cursor on any trace position of interest. Hopefully, this feature will be added in a future version of the application. Now although the QA400 User Manual makes no mention of things like those Test Plugin DLLs and their use, it does have an appendix near the end which gives details of a dot.NET 48  Silicon Chip API (Remoting Test App) which is installed as a zipped file along with the main application, and which can be used to achieve software communication with it. On the QuantAsylum website there’s also a note advising that users can make their own Test Plugin DLLs for use with the QA400 Analyser app, writing them in C#, C++ or Visual Basic. You can also download another API called “QA Connection Manager”, which makes communicating with the QA Analyser app even easier. There are also a number of ‘white papers’ on the website available for So what’s our verdict on the Quant­ Asylum QA400 hardware and software package? Well, in terms of basic performance, it seems to deliver capabilities which really do compare closely with those of high-end realtime stereo audio analysers. This makes the package excellent value for money for those who only want to perform tests on analog stereo equipment and systems. At the same time, the QA400 does lack some of the features that you’d find on high-end analysers. For example, its inputs can only cope with signal levels up to +3.0dBV (1.41Vrms/4.00Vp-p), so for testing the output of audio amplifiers at power levels of more than 250mW (= 1.414Vrms across 8Ω), you’ll need to use external input attenuators. Another feature you don’t get with the QA400 is balanced inputs, which would allow measurements to at the outputs of bridge-mode and ClassD digital amplifiers, as well as balanced line-level preamp or mixer outputs. Just coincidentally we are currently working on an analog “front-end” project which should overcome these input limitations of the QA400 and other low-cost USB instruments. It will provide balanced/differential and unbalanced inputs, combined with three measurement ranges: 1:1, 10:1 and 100:1. Another shortcoming of the QA400 Analyser is that it doesn’t allow you to display the input channel signals, the residual distortion waveform when you’re doing THD or THD+N measurements, or the noise waveform when you are doing SNR tests. Most high-end analysers do provides these facilities, by providing buffered signals which can be displayed on a scope. Finally, the QA400 doesn’t have the ability to cope with digital audio streams either. So if you need to test digital audio systems, you may need to consider one of the newer highend analysers that do offer these capabilities. Just be prepared to pay a lot more than the QA400’s price tag, because the cost of these analysers SC starts at about US$20,000. siliconchip.com.au