Actually, at low levels, the analog nonlinearities are minimized. They're most important at high levels.
I don't disagree in principle, but it would be interesting to perform a sanity check every now and then.
Actually, at low levels, the analog nonlinearities are minimized. They're most important at high levels.
I think the assumption is made that at -20dB the DAC performs in an ideal manner and the output voltage is recorded, the rest of the level measurements are normalized based on that point.
Normalised how? 0dB f first step equals -20db, -10dB of 2nd step equals -30dB and so on? Or.. ?
I don't think I'll get involved in 'where does the midrange start?", last time I looked it up there was no consensus .Well it is by most definitions the start of the midrange. But I'm not sure what @Soniclife defined as midrange in his measurements.
It is worth noting though that bass noise floor will tend to mask the midrange too, although not to the extent it masks bass.
Essentially all noise is filtered with linearity tests with very narrowband filters. Without that, you would be right.Ok, that sounds reasonable. But noise floor at -120dB seems one thing and amplitude error of +/- 0.5dB of a signal at -120dB seems another, and that last one doesn't seem like a hearable issue at all to me. Btw, wouldn't even a very solid amp have a SNR of 100-110 dB so its noise will anyhow mask those little errors?
You are correct.Linearity deviation is assumed to be 0dB at -20dB output, you record the output voltage at that level, then you calculate expected voltages at other levels and compare them to actual and plot deviations on the linearity graph, something like that. Or I may all be wrong
You are correct.
Essentially all noise is filtered with linearity tests with very narrowband filters. Without that, you would be right.
I only allow 20 Hz worth of bandwidth for the filter. So take your total spectrum of noise and divide it down by 20 Hz and it becomes a very small number. I have done it with 1 Hz too and the results are the same.But noise is broadband, so noise (and other garbage) caught between 200Hz filter limits is still there, isn't it?
I only allow 20 Hz worth of bandwidth for the filter. So take your total spectrum of noise and divide it down by 20 Hz and it becomes a very small number. I have done it with 1 Hz too and the results are the same.
Well, that is the same what I meant: 2V of max output corresponds to 0dB. Based on that you take voltage of -20dB as a new 0dB level and from there you go down in -10dB increments, right? That makes -100dB level of your linearity measurement actually pretty close to the noise floor, doesn't it?
You misunderstood, he measures the voltage at -20dB, say it is 199mV, this is assumed to be 0dB *deviation* from linearity, under this assumption the expected voltage at 0dB *output* is 1990mV, 1990mV is then compared to the actual voltage at 0dB output which gives a point for the 0dB level on the linearity plot, and so on for all levels.
Ok, and how does he measure the level at -10dB, -20dB and so forth..? Does he measure the voltage at -30dB and assume that to be the level at -10dB?
-20dB is the first point, it's always 0dB deviation, note it is 0dB *deviation*, not level. Knowing the voltage at -20dB it's easy to arrive at expected voltages at all possible levels using log arithmetic. He then measures the voltages at -10, -20 and so forth and calculates the deviation from those calculated values. You must establish a reference point first. You can do it at 0dB too but chances are -20dB is in a more linear area of the device so you get a more correct picture of +/- deviation overall. Maybe someone has a better way to explain this.
Ok, got it. So, according to this, when does the measured THD+noise floor level start to interfere with this measurement?
When the noise floor in the band being measured gets within about 25 db of the signal level. With a 20 hz wide filter Amir mentions that means with a device having around a 105 db SNR, you will start seeing some effect at about the -100 dbFS point of the linearity test.