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Thought it would be interesting to see how much aliasing there is in some ADCs. The three ADCs compared were a Focusrite 18i20 recording interface, Focusrite Forte recording interface, and Antelope Audio Zen Tour recording interface. The source of the signals used is a TC IMpact Twin running at 192 khz.
The first signal is -4db white noise covering 0-96khz. I used a brickwall high pass filter starting at 27 khz, -96db at 24 khz and -120 db at 23 khz. I wanted to see what would show up in these three ADCs running at 48 khz sample rates and covering 24 khz of sound. Below are spectrograms of that for each ADC. The spectrogram uses 2k FFT and goes to a light gray background at a signal level of -120 dbFS. Anything below -120 db will not show up. The 4th spectrogram at the bottom is the signal being recorded.
The top two show an elevated noise floor in the presence of the ultrasonic noise. Along with stronger aliasing in the upper two khz of the band. The noise floor is raised about 3 to 4 db which isn't a lot. The noise floor ignoring the highest frequencies is still several decibels below -100 db. More interesting is the Zen Tour which has its noise floor raised about 8 db on average, but also that noise floor is undulating with frequency by about 6 db between low and high points. The Zen Tour is very good with strong signals in the 24 khz band not modulating the noise floor at all. Yet ultrasonics apparently can via aliasing. Even then it fortunately has a low noise floor and this elevated level is still a few db below -100 dbFS at worst.
Next I used a -1dbFS single tone sweep from 2 hz to 96 khz while the ADCs recorded at a 48 khz sample rate.
Here you see the distortion harmonics of the main signal until you reach 24 khz. You see the stronger aliasing at the top of the 24 khz band, and then aliasing folding over every 24 khz above the 24 khz band of the ADC. The Zen Tour shows the highest levels of the aliasing products reaching -88db FS in a few places. Though mostly it skirts around -100 dbFS. The Forte in the second spectrogram shows some additional aliasing. It is centered around 31.6 khz. This device has the same effect even if used sampling at 96 khz or 192 khz. It will act like there is aliasing around 31.6 khz which is rather strange behavior. It mostly also meanders near the -100 dbFS level.
Next I used a twin tone sweep with tones 1 khz apart for the test signal.
The main thing worth noting is even though the signal is ultrasonic it still shows intermodulation difference products at the 1 khz difference down in the audible band. I believe this is in the analog stage which has 100 khz of bandwidth in these devices and the intermodulation is showing up which gets recorded by the ADC. In a few places this IMD difference signal reaches - 88 dbFS though mostly it lies in the mid 90s to -100 dbFS.
Now one final spectrogram where I extended the range to -144 dbFS and used a 32k FFT.
You don't see anything new show up for the Zen Tour or Forte ADC, but there some curious things in the 18i20 ADC. Firstly there are low level constant tones spaced exactly 1 khz apart. These are around -126 to 128 dbFS so not really a problem, but strange nonetheless. These are present with signal or silence at all times.
Even more odd is in the presence of a strong signal in the audible band, but not with ultrasonics are tones from 125 hz and spaced 125 hz apart up to the point of 625 hz. These tones come and go related to the strength of the strongest signal. All are the same strength. At worst they reach - 106 dbFS so not a big concern, but again strange how that must be happening.
All of these ADCs are good about not modulating the noise floor or having too many artifacts when there is no ultrasonic content. However, we do see them perturbed a little when ultrasonics are present. How important is this? Many condenser microphones will have some considerable response into the mid 30 khz range. Some instruments do put out considerable levels at these frequencies especially important if they are close miked. There are microphones that have extended bandwidth like Earthworks for one example. If you were doing a line level recording of an SACD or DSD the ultrasonic rise in noise could show up as slightly elevated noise at lower frequencies in these ADCs. Sometimes during recording lighting or switching supplies or other devices can leak high levels of ultrasonics into the microphone path. That might show up at low levels. Of course one could record at 96 khz reducing all of these aliasing related issues quite a lot nearly to the point of being a complete non-issue. My test signals were something of a worst case scenario and the resulting audible band spurious signals are never high enough in level to be audible unless heard at very high gain level against a silent background. Even then not likely to be heard.
The first signal is -4db white noise covering 0-96khz. I used a brickwall high pass filter starting at 27 khz, -96db at 24 khz and -120 db at 23 khz. I wanted to see what would show up in these three ADCs running at 48 khz sample rates and covering 24 khz of sound. Below are spectrograms of that for each ADC. The spectrogram uses 2k FFT and goes to a light gray background at a signal level of -120 dbFS. Anything below -120 db will not show up. The 4th spectrogram at the bottom is the signal being recorded.
The top two show an elevated noise floor in the presence of the ultrasonic noise. Along with stronger aliasing in the upper two khz of the band. The noise floor is raised about 3 to 4 db which isn't a lot. The noise floor ignoring the highest frequencies is still several decibels below -100 db. More interesting is the Zen Tour which has its noise floor raised about 8 db on average, but also that noise floor is undulating with frequency by about 6 db between low and high points. The Zen Tour is very good with strong signals in the 24 khz band not modulating the noise floor at all. Yet ultrasonics apparently can via aliasing. Even then it fortunately has a low noise floor and this elevated level is still a few db below -100 dbFS at worst.
Next I used a -1dbFS single tone sweep from 2 hz to 96 khz while the ADCs recorded at a 48 khz sample rate.
Here you see the distortion harmonics of the main signal until you reach 24 khz. You see the stronger aliasing at the top of the 24 khz band, and then aliasing folding over every 24 khz above the 24 khz band of the ADC. The Zen Tour shows the highest levels of the aliasing products reaching -88db FS in a few places. Though mostly it skirts around -100 dbFS. The Forte in the second spectrogram shows some additional aliasing. It is centered around 31.6 khz. This device has the same effect even if used sampling at 96 khz or 192 khz. It will act like there is aliasing around 31.6 khz which is rather strange behavior. It mostly also meanders near the -100 dbFS level.
Next I used a twin tone sweep with tones 1 khz apart for the test signal.
The main thing worth noting is even though the signal is ultrasonic it still shows intermodulation difference products at the 1 khz difference down in the audible band. I believe this is in the analog stage which has 100 khz of bandwidth in these devices and the intermodulation is showing up which gets recorded by the ADC. In a few places this IMD difference signal reaches - 88 dbFS though mostly it lies in the mid 90s to -100 dbFS.
Now one final spectrogram where I extended the range to -144 dbFS and used a 32k FFT.
You don't see anything new show up for the Zen Tour or Forte ADC, but there some curious things in the 18i20 ADC. Firstly there are low level constant tones spaced exactly 1 khz apart. These are around -126 to 128 dbFS so not really a problem, but strange nonetheless. These are present with signal or silence at all times.
Even more odd is in the presence of a strong signal in the audible band, but not with ultrasonics are tones from 125 hz and spaced 125 hz apart up to the point of 625 hz. These tones come and go related to the strength of the strongest signal. All are the same strength. At worst they reach - 106 dbFS so not a big concern, but again strange how that must be happening.
All of these ADCs are good about not modulating the noise floor or having too many artifacts when there is no ultrasonic content. However, we do see them perturbed a little when ultrasonics are present. How important is this? Many condenser microphones will have some considerable response into the mid 30 khz range. Some instruments do put out considerable levels at these frequencies especially important if they are close miked. There are microphones that have extended bandwidth like Earthworks for one example. If you were doing a line level recording of an SACD or DSD the ultrasonic rise in noise could show up as slightly elevated noise at lower frequencies in these ADCs. Sometimes during recording lighting or switching supplies or other devices can leak high levels of ultrasonics into the microphone path. That might show up at low levels. Of course one could record at 96 khz reducing all of these aliasing related issues quite a lot nearly to the point of being a complete non-issue. My test signals were something of a worst case scenario and the resulting audible band spurious signals are never high enough in level to be audible unless heard at very high gain level against a silent background. Even then not likely to be heard.