Help me put this guy back in his right place
Reply #82 – 2003-10-17 09:51:15
Guys, help me here you guys are discussing linearity again without looking at those linearity graphs that require interpretation earlier on the thread. thanks in advance. That's because I haven't found an explanation of how the audio precision linearity test is carried out. We had an AP at uni, with all the manuals, but I don't have access to that now. I think they can be downloaded from their website, but you can do that as easily as me I suspect they're using a decreasing amplitude sine wave, and looking at the amplitude of the output. I believe the plots you linked to show input level on the x axis, and output-input level on the y axis. If the output drops below the level of the input, then the system is undithered, and the input has dropped below the LSB and been lost. If it rises above the level of the input (as shown at the left hand side of the plots you linked to), then either it's being becoming swamped by noise (if the noise isn't averaged out - that's what I want to check), or there are extra harmonic distortion components present. As I haven't tried this kind of test (just looking at the amplitude of the output) I can't say what the two results you linked to actually mean. I usually examine the noise and harmonic distortion directly. When I talk about linearity, I mean "does the ADC or DAC have a linear transfer function?". A 17-bit system has twice as many levels as a 16 bit system. An 18-bit system has twice as many levels as a 17-bit system. And so on... The extra levels in a 17 bits system will lie exactly half way between the levels on a 16-bit system. An 18-bit system will have 3 extra levels between each level of a 16-bit system. A diagram would help. I've taken the region between two adjacent levels in a 16-bit system, and shown where the levels in a 17-bit, 18-bit, 19-bit and 20-bit system would fall:16 17 18 19 20 __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ These "levels" are the analogue voltage levels above which the digital output will be the next highest value. The above diagram assumes we have a perfect system. Let's assume the 16-bit system isn't perfect, and the levels don't quite fall where they should. Imagine that the top level shown is pushed about 1/3rd of a bit too low - i.e. the top left-hand level on that diagram is moved about 1/3rd of the way down. It would still fall within the same level on a 17-bit system, but it's actually jumped into a different (wrong!) level on an 18-bit system. so it's linear to 17-bits, but no further. Conceptually, this explains linearity. In reality, most DACs aren't made using 2^16 or 2^20 discrete levels (that would be pure multi-bit technology with no noise shaping), and are swamped by noise anyway. However, in a typical oversampled DAC, it's still possible to average away the noise, measure the distortion, equate this to a bit-level, and conclude that it is linear to so many bits. Cheers, David.