Signal To Noise Ratio In Ds- / Qs-Operation - RME Audio Micstasy User Manual

18.8 Signal to Noise Ratio in DS- / QS-Operation

The outstanding signal to noise ratio of the Micstasy's AD-converters can be verified even with-
out expensive test equipment, by using record level meters of various software. But when acti-
vating the DS and QS mode, the displayed noise level will rise from -115 dBFS to -112 dBFS at
96 kHz, and –79 dBFS at 192 kHz. This is not a failure. The software measures the noise of the
whole frequency range, at 96 kHz from 0 Hz to 48 kHz (RMS unweighted), at 192 kHz from 0
Hz to 96 kHz.
When limiting the measurement's frequency range to 22 kHz (audio bandpass, weighted) the
value would be -115 dB again. This can be verified even with RME's Windows tool DIGICheck.
Although a dBA weighted value does not include such a strong bandwidth limitation as audio
bandpass does, the displayed value of –107 dBFS is nearly identical to the one at 48 kHz.
The reason for this behaviour is the noise shaping technology of the analog to digital convert-
ers. They move all noise and distortion to the in-audible higher frequency range, above 45 kHz.
Therefore the noise is slightly increased in the ultrasound area. High-frequent noise has a high
energy. Add the quadrupled bandwidth, and a wideband measurement will show a significant
drop in SNR, while the human ear will notice absolutely no change in the audible noise floor.
As can be seen in the above picture, the noise floor stays fully unchanged up to 45 kHz. With
sample rates up to 96 kHz the noise shaping completely happens outside of the transmission
range.
User's Guide Micstasy © RME
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