Or how to manipulate measurements for fun and profit. This visual tutorial is Microsoft Windows oriented. Let’s examine two hypothetical reviews from two separate hypothetical websites professing to participate in audio equipment reviews and discussion. Reviewer A: It’s a giant killer, the best thing since the invention of the transistor! The inexpensive objective odac outperforms a costly studio grade Violectric V800. Just look how much better the odac performs than the V800 on our expensive analyzer: As you can see we provide the truth other sites can’t or won’t because we have an expensive analyzer. Did I mention we have an expensive analyzer which makes us better than everyone else? Reviewer B: Huzzah, a DAC of quality! The Violectric V800 is definitely a cut above the ordinary. Just look how much better it performs than the common objective odac on our expensive analyzer: As you can see we provide technical insights other sites can’t or won’t because we have an expensive analyzer. Did I mention we have an expensive analyzer which makes us better than everyone else? Ok, so which of those graphs above are real? Give up? They both are real measurements. Whaaat??!! Ok AB, now I know you’re messing with me. How can that be? They are not fair comparisons. Let me explain. Notice the graphs and associated text are devoid of measurement conditions other than both appear to have been made with a 48KHz sample rate by the analyzer analog input given the 24KHz x-axis. Oh, but you have seen text overlays on graphs with a few conditions stated you say? If those reviewers have a history of meticulous attention to detail, then they might deserve the benefit of the doubt. However text entered by hand has a potential for typos, mistakes or worse. I agonize over my measurement posts for this very issue. Let’s look at the following in detail: 1. Odac measurement s with pertinent setup parameters included 2. V800 measurements with pertinent setup parameters included 3. RME ADI-2 pro as DDC measurements with pertinent setup parameters included 4. Shiit Eitr DDC measurements with pertinent setup parameters included Hopefully it will be appreciated that this is a rather complex tutorial and has taken significant time to create. While every effort has been made to assure accuracy, there still may linger a mistake or two. If a legitimate mistake(s) is found, please call to my attention and it will be corrected. There are many analyzer parameters to adjust in making a USB DAC audio measurement, not all of them inside the analyzer control program. Orientation, a few specific parameters to watch as we progress through these examples: A. Soundcard audio driver selection – USB ODAC in this example B. Residual noise floor and spikes C. DAC analog output measured in dBu (reference 0.775 Vrms) D. FFT size (number of points used) E. FFT averages – number of FFTs averaged, lowers residual bin noise F. Not used G. Generator output level First up, odac measurement. let’s adjust the audio analyzer with the following interesting parameters: 1. Analyzer generator set for sine, 12 KHz, 0 dBFS 2. Generator audio goes through Windows Audio Architecture (WAA) WDM driver 3. Odac analog output measured at 8.119 dBu 4. FFT size set for 8K 5. FFT averages set for 8 Note all the spikes on the residual noise floor. Let’s make just one adjustment, the route by which the audio is sent to the odac. Using an ASIO driver we observe two changes: 1. The number of residual spikes have diminished substantially, implying lower jitter 2. Odac analog output has increased slightly, from 8.119 to 8.250 dBu. Now returning to the WDM audio path, let’s make a different adjustment, change the generator from 0.0 dBFS to -0.2 dBFS. Not a large change in amplitude but watch the results 1. Similar reduction in residual spikes, though not as large as ASIO driver change 2. Odac output has again changed slightly from 8.119 to 8.050 dBu, but not as much as expected. Those amplitude changes may not seem like much but they are indicative of a problem. Changing -0.2 dBFS should have resulted in 8.119 dropping to 7.934 dBu in the WDM path. Also note that 8.050 dBu is -0.213 dBFS from 8.250 dBu from the ASIO path indicating that we should be measuring 8.250 dBu in the WDM path. Note that 8.119 is -0.139 dBFS lower than 8.250 dBu. These results will be important in the DDC section below. Combine the unexpected levels with the rise of spikes when the generator is set for 0 dBFS through the WDM path and we can be reasonably certain there is a limiter in the WDM path. So when we see spikes at 0 dBFS that do not occur at a slightly lower level or with a different audio path through the WAA, we should question the validity of our 0 dBFS WDM path measurement. More on that in the DDC section below. V800 measurement: let’s adjust the audio analyzer with the following interesting parameters: 1. Analyzer generator set for sine, 12 KHz, 0 dBFS 2. Generator audio goes through Windows Audio Architecture (WAA) WDM driver 3. V800 analog output measured at 5.664 dBu 4. FFT size set for 8K 5. FFT averages set for 8 Similar profusion of spikes on the residual noise floor as the odac through WDM at 0 dBFS. Again, making just one adjustment, the route by which the audio is sent to the V800. Using an ASIO driver we again observe two changes: 1. The number of residual spikes have diminished substantially, implying lower jitter 2. V800 analog output has increased slightly, from 5.664 to 5.792 dBu. Now returning again to the WDM audio path the generator amplitude is changed from 0.0 dBFS to -0.2 dBFS. 1. Similar reduction in residual spikes, though not as good as ASIO driver change 2. V800 output has again changed slightly from 5.664 to 5.590 dBu but not as large a drop as expected. In our hypothetical example: 1. Reviewer A chose to measure odac using ASIO at 0 dBFS 2. Reviewer A chose to measure V800 using WDM at 0 dBFS 3. Reviewer B chose to measure odac using WDM at 0 dBFS 4. Reviewer B chose to measure V800 using ASIO at 0 dBFS Additionally: 1. Reviewer A chose to measure odac using FFT size 32K, FFT averages 32 2. Reviewer A chose to measure V800 using FFT size 8K, FFT averages 8 3. Reviewer B chose to measure odac using FFT size 8K, FFT averages 8 4. Reviewer B chose to measure V800 using FFT size 32K, FFT averages 32 Observe how ragged the poor performing DAC traces are in the first two reviewer comparison graphs and how much smoother the better performing DAC traces appear. Also note the use of color with poor performing DAC traces in Red and the better performer in Yellow with nice contrast to the Black background. Color choice is intentional. The comparisons weren’t fair. A fair comparison: Both odac and V800 have been measured as follows: 1. Analyzer generator set for sine, 12 KHz, 0 dBFS 2. Generator audio goes through ASIO drivers 3. Analog output measured for 0dBFS: Odac 8.250 dBu, V800 5.792 dBu 4. FFT size set for 32K 5. FFT averages set for 32 In this fair comparison odac is not the giant killer claimed by Reviewer A, but doesn’t perform as badly as indicated by Reviewer B. V800 measurement by Reviewer A was also clearly deficient and the validity should have been questioned before publishing for thousands, perhaps millions of readers to see. The same applies for odac measurement of Reviewer B. Both reviewers have demonstrated their measurement skills are either at what Real Engineers™ call IAO level or are deliberately misleading. I leave it to the reader to interpret the acronym with the hint that O stands for Operator. To those using my graphs in an attempt to establish their measurement legitimacy, please note that my screen name is atomicbob, not AtomicBomb or other variant. It does not instill confidence in ability to correctly use a complex measurement system when one can’t get the simple details correct, though there is possibly an attempt at a subtle depreciative implication.