Signal To Noise Ratio In Ds / Qs Operation - RME Audio ADI-8 QS User Manual

16.7 Signal to Noise Ratio in DS- / QS-Operation
The outstanding signal to noise ratio of the ADI-8 QS 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 -114 dBFS to -110 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 range from 20 Hz to 20 kHz (so called audio bandpass) the
value would be -114 dB again. This can be verified with RME's DIGICheck. The function Bit
Statistic & Noise measures the noise floor by Limited Bandwidth, ignoring DC and ultrasound.
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 ADI-8 QS © RME
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