I'm sure many of us may already familiar with the linearity measurements of DACs from Stereophile and Audio Science Review. Basically it's a graph that describes the difference for DAC's actual output level to an intended output level for a set of output levels from loudest (0dbFS) to an arbitrary soft level (say -120dbFS). Somewhere along the way to the softest sounds, a DAC starts to deviate away too much from the ideal. Usually a 1kHz tone is used for this test. For 16-bit content, we want to see a straight line down to about -96dbFS. As far what how much deviation is good or bad, that's a tough call. There are various blind test resources on the Internet to see how much of a difference you can hear. At moderate volumes, I can discern 0.2db with ease. At softer volumes, I'm not so sure. And even then, when it comes to music with its ebbs and flows, it's debatable if even relatively large differences such as 2db would be discernable. Anyway, psychoacoustics is a tough subject that requires human testing - which could also be subject to individuals' training and abilities. This topic here is about the linearity measurement. The proper way to do a precise linearity measurement for DACs on the APx555 is the Bandpass Level Sweep. A standard linearity measurement will not work because of how the analyzer gathers data. The output level is taken as the RMS on what's on the analyzer. Therefore we need to apply a bandpass filter on the generator signal, otherwise at lower levels, the output level will register as too high because of noise and distortion. Now let's take a look a typical 1kHz linearity measurement from 0dbFS to -120dbFS with the Bandpass Level Sweep where a 1/24 octave bandpass is applied. This is with the Geshelli JNOG DAC's balanced outputs using USB as the input. Things look pretty good until about -110db where things start to deviate a bit. Even then, at -120dbFS, only the right channel is outputting about 0.5db less than it's supposed to be. I really don't know where to draw the line to what is good or what is bad - that is how much deviation from ideal. Personally, I'd say the result is excellent to -120dbFS. So we are done with this measurement right? Hold on folks, I ran this a second time (up to about -75dbFS). Wait, what's going on? Why did the behavior change so much? Then I ran it two, three, four, eight more times for a total of 10. Here are the results overlaid on top of each other. So what's the truth? The fact is, they all are the truth. With certain types of measurements, the limitations of the methods we employ to calculate the results, and the randomness of the real world, it is more or less impossible to get a consistent number (or sets of numbers of a graph). The reason I wanted to bring this up is since I will be using the amazing capabilities of the APx555, I did not want to mislead people on any specific measurement presentation, in this case, the linearity measurement, in the wrong way. Measurements can "lie". With great power comes great responsibility. I'm still not certain how I want to present this measurement. Perhaps taking several sets and overlaying them over each other? There are also custom settings in the APx555 with respect to data collection that I can play with to maybe get more consistent results.