I plan to do audio performance measurement of the audio equipment that I design using REW and a sound card. My ESI Audio U24 XL I have at home doesn't perform well and this is why I was looking for better options. I heard that the Focusrite sound cards are good and wanted to test this myself. In this article I present my findings evaluating the fitness of the Focusrite Scarlett 2i2 Gen3 sound card for audio measurements. Of course, the measurements are also a general indicator of the audio quality of this interface.
For the measurement, the sound card was connected to my notebook running Debian buster using the provided USB cable.
Each line output was wired to the corresponding analog input using a short, but cheap cable. Audio interconnections were unbalanced.
Noise floor was measured in 192kHz mode and 96kHz mode.
FFT was done in 96kHz mode.
Noise Floor in 192kHz mode
With all controls set to minimum, the noise floor is as good as it gets. What strikes me is the increasing noise floor in the higher kHz range. However, this should not matter too much for normal audio use.
With the monitor set to maximum position and gain is set to lowest level, the high frequency noise floor now increases earlier in frequency and higher in level. This is still outside the normal audio spectrum.
With the gain set to maximum position and monitor set to lowest level, now the noise floor is around -100dB and falls off at higher frequency with some spikes in the higher kHz range. I guess that noise from the analog circuitry now dominates the noise floor.
This is the noise floor with gain and monitor set to 50%. At -130dB, this acceptable, but far from exciting.
Noise Floor in 96kHz mode
Since I experienced issues in 192kHz mode, I repeated the measurements with the sound card set to 96kHz operation:
Above plot shows the noise floor with all controls set to minimum.
This is the noise floor with the monitor set to maximum position and gain set to lowest level.
This is the noise floor with the gain set to maximum position and monitor set to lowest level.
Above is the noise floor with gain and monitor set to 50%. At -135 to -130dB, this acceptable, but again far from exciting. Note the spikes at 8kHz and multiples of 8kHz. This is likely USB packet noise.
Long after doing the measurements I learned that the increase of the noise floor with higher frequency is having to do with noise shaping of Delta-Sigma DACs. Noise is pushed from the audio spectrum into higher frequency spectrum. So this is normal and to be expected with any DAC that uses Delta-Sigma architecture. I dislike the artifacts at 8kHz and beyond. The root cause of the USB noise is likely an inadequate power supply.
FFT was done with the sound card set to 96kHz operation.
Above screenshot shows THD with gain and monitor set to 50% level. This is short of the level where the clipping warning of the device turns orange (-6dBFS).
Since the sound card shows USB packet noise at 8kHz, the spectrum is misleading because the 8th harmonic is exactly at this freequency as well. Above is a plot at 1200Hz for direct comparison with the ESI U24 XL that I tested earlier. The ESI sound card has packet noise at 1kHz and this is why I used a 1200Hz signal instead as well.
Low frequency behaviour is okay.
The noise floor of the Scarlett sound card can be lower than the one of the ESI U24 XL, but this is only true in case the gain is set to impractical levels. With the gain set to more practical levels, the advantage is gone. This leads to the conclusion that the analog part is the weak link. While the traget customer are likely musicians and find volume controls practical, I dislike volume controls because it is difficult to repeat the same level for doing any measurements.
Selling for roughly 150€, the sound card offers somewhat reasonable value. I was disappointed by the high frequency noise floor and decided to return the sound card because it does not add any additional value to me over the old ESI sound card I have at home already.
Here is some background information about the device:
- This is how it identifies on the USB:
- The device works plug and play with Linux. It also registers a a mass storage device. Accessing the mass storage reveals some files.
Unfortunately I wasn't able to have a look under the hood since this would have left marks and I wanted to return the device in mint condition.
I found out that Audio Science Review has reviewed the device as well and, unlike myself, has access to proper measurement equipment and knows how to use it well. I highly recommend to read the review of the Focusrite Scarlett 2i2 gen3 on Audio Science Review.