Hello good AS-people,
I've recently been, somewhat belatedly, trying to fill in some of the void of my technical understanding of sampling technology and the gear I'm currently using.
I've read through a load of stuff - much here on ASR now I've discovered it - and currently have indigestion, so seeking some sanity-checking and guidance.
Brief background:
I recently started to use Room EQ Wizard to make some measurements of the frequency response of some outboard gear (a mic preamp) but this led to questions about the behaviour of the ADC and DACs I've been using in my home studio.
I wasn't trying to make noise or distortion measurements of the standalone preamp but rather to check up to as high a frequency as possible for any signs of ringing and to get a general look at the frequency response of the unit, hence I'm coming from that angle.
I wasn't sure at first if I could even realistically use the 18i20 for measurements above 20kHz as not a requirement for an audio interface, and not in the spec, but happily (and slightly surprisedly) found it was not particularly constrained and 192kHz gave a useful measurement range.
However I saw many puzzling things which made me question what was going on, so I ended up measuring the 18i20 interface at a range of sample rates in loopback from an 18i20 line output to a line input, and then using a Lavry DA10 as the output device looped back into the 18i20.
Edit: I mean all of these measurements below were without any additional equipment (mic pre or whatever) between DAC and ADC, just direct connection.
So here's where it started, checking out the FR of the 18i20 itself at 192kHz as a baseline:
The initial low sweep level test (green) at -40dBFS looked pretty horrible at the upper end of the frequency range, then I realised this was a stupid way of generating the low level input for the mic pre I was going to measure...so I changed the sweep level up to -2dBFS and put a hardware pad inline on the output instead to bring it down to the required level. The red 'hi level' above is equivalent to this but using the 18i20 input gain controls to match to the higher output level rather than an external pad. This looked a bit more promising so I made a measurement using a longer sweep length to reduce the noise impact on the FR plot, at high signal level, and used this to generate the REW 'cal' baseline of the interface (to be offset in the further tests of the standalone preamp).
This prompted a few questions for a start:
- The green 'lo' sweep level 'noisyness' looks roughly commensurate with 'hi' red+40dB gain which is consistent at least. But is this amount of mess at the top of the (available/Nyquist) frequency range typical?
- Is this 'noise' (mostly) sampling noise? Thankfully it doesn't appear lower down the spectrum... I'm assuming this is a feature of the converters. I've read a few things now about how they work but so far there's always some point where I get thrown off on a fast bend, or a sudden chasm appears. I'm working on it.
Wondering if it was a random noise effect or a more deterministic phenomenon I ran the maximum (single) sweep length with a low level sweep. This is the blue plot above. IIRC this should reduce noise in the plot by 3dB per doubling, the green is a 1M sweep and the blue is a 4M sweep, so comparing the green with the blue the 'noise' should be about 6dB less.
- The width of the blue mess indeed looks to be about half of the green at first but actually increases disproportionately at higher frequencies - why is it not consistent ? Is there some other factor increasing towards the top of the range?
I keep putting 'noise' in 'quotes' because this is a frequency response plot so we are seeing an effect of noise on the FR measurement rather than noise itself, presumably!? Perhaps not a useful distinction here.
I also wondered a little about the test approach of looping back from an output to an input on the same device. I speculated that on the one hand it should take jitter out of the picture as it is all on the same clock but on the other hand it might be more likely to generate deterministic patterns in the response, or is this nonsense ?
cheers/Henry
I've recently been, somewhat belatedly, trying to fill in some of the void of my technical understanding of sampling technology and the gear I'm currently using.
I've read through a load of stuff - much here on ASR now I've discovered it - and currently have indigestion, so seeking some sanity-checking and guidance.
Brief background:
I recently started to use Room EQ Wizard to make some measurements of the frequency response of some outboard gear (a mic preamp) but this led to questions about the behaviour of the ADC and DACs I've been using in my home studio.
I wasn't trying to make noise or distortion measurements of the standalone preamp but rather to check up to as high a frequency as possible for any signs of ringing and to get a general look at the frequency response of the unit, hence I'm coming from that angle.
I wasn't sure at first if I could even realistically use the 18i20 for measurements above 20kHz as not a requirement for an audio interface, and not in the spec, but happily (and slightly surprisedly) found it was not particularly constrained and 192kHz gave a useful measurement range.
However I saw many puzzling things which made me question what was going on, so I ended up measuring the 18i20 interface at a range of sample rates in loopback from an 18i20 line output to a line input, and then using a Lavry DA10 as the output device looped back into the 18i20.
Edit: I mean all of these measurements below were without any additional equipment (mic pre or whatever) between DAC and ADC, just direct connection.
So here's where it started, checking out the FR of the 18i20 itself at 192kHz as a baseline:
The initial low sweep level test (green) at -40dBFS looked pretty horrible at the upper end of the frequency range, then I realised this was a stupid way of generating the low level input for the mic pre I was going to measure...so I changed the sweep level up to -2dBFS and put a hardware pad inline on the output instead to bring it down to the required level. The red 'hi level' above is equivalent to this but using the 18i20 input gain controls to match to the higher output level rather than an external pad. This looked a bit more promising so I made a measurement using a longer sweep length to reduce the noise impact on the FR plot, at high signal level, and used this to generate the REW 'cal' baseline of the interface (to be offset in the further tests of the standalone preamp).
This prompted a few questions for a start:
- The green 'lo' sweep level 'noisyness' looks roughly commensurate with 'hi' red+40dB gain which is consistent at least. But is this amount of mess at the top of the (available/Nyquist) frequency range typical?
- Is this 'noise' (mostly) sampling noise? Thankfully it doesn't appear lower down the spectrum... I'm assuming this is a feature of the converters. I've read a few things now about how they work but so far there's always some point where I get thrown off on a fast bend, or a sudden chasm appears. I'm working on it.
Wondering if it was a random noise effect or a more deterministic phenomenon I ran the maximum (single) sweep length with a low level sweep. This is the blue plot above. IIRC this should reduce noise in the plot by 3dB per doubling, the green is a 1M sweep and the blue is a 4M sweep, so comparing the green with the blue the 'noise' should be about 6dB less.
- The width of the blue mess indeed looks to be about half of the green at first but actually increases disproportionately at higher frequencies - why is it not consistent ? Is there some other factor increasing towards the top of the range?
I keep putting 'noise' in 'quotes' because this is a frequency response plot so we are seeing an effect of noise on the FR measurement rather than noise itself, presumably!? Perhaps not a useful distinction here.
I also wondered a little about the test approach of looping back from an output to an input on the same device. I speculated that on the one hand it should take jitter out of the picture as it is all on the same clock but on the other hand it might be more likely to generate deterministic patterns in the response, or is this nonsense ?
cheers/Henry
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