Every acquisition step in a sequence produces a Memory List value named “Max FSD In” (FSD=Full Scale Deflection). When this value is 0 dB re 1 FSD, the input is using all of the available bits (dynamic range) of the audio interface input channel. In practice, this value should not fall below -30dB and should be as close to 0 dB as possible. Anything lower than -30dB potentially has too low of a signal to noise ratio for accurate harmonic analysis. Adjusting your input signal path gain can help to optimize the Max FSD In value of your acquisition.
Traditionally, the Max FSD In value is monitored by the user and the input signal path gain is manually adjusted based upon the output level produced by the device under test (DUT). If you always test at the same reference level (e.g. 94 dB SPL) then setting input gain is a relatively painless process and once set, no further attention is required. However, if you are trying to characterize the performance of your DUT at various output levels, maintaining an optimized Max FSD In value can be a challenge.
SoundCheck (versions 14 and higher) includes features that can automatically set Listen Hardware to the appropriate input gain level. Supported Listen Hardware includes AmpConnect, AudioConnect and SoundConnect 2.
To take advantage of this feature, we enable the Auto Read function in Calibration and the Auto-Range function in the Acquisition Step. Here's an example using AmpConnect hardware.
Auto Read Enabled on Reference Mic Input Signal Path for AmpConnect Reference Input:
Auto-Range Enabled on Reference Mic Input in Acquisition Step:
0 dB of Input Gain on AmpConnect Reference Input Results in Max FSD In of -39.5 dB re 1 FSD, which is low.
Auto-Range automatically increased the AmpConnect Reference Input Gain to +30 dB and re-ran the acquisition step. This resulted in a Max FSD In of -9.64 dB re 1 FSD which is within the recommended range.