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Meter Measurements - Manual Output Calibration
The basic strategy is to select each attenuator step with the Left Step control, read the output voltage from the meter, and enter it in the Volts box. (Left Step controls only the Left output; the Right calibration is not measured, but assumed to be identical.)
You must work from the loudest to softest, starting with Left Step set to 0 and working down to more-negative values. The 0 step is the full-scale reference, and each step below that is stored as a dB change relative to the prior step. That value will be shown in the dB / Step box for reference.
If Init Table is active, any voltage you enter is copied to all softer volume steps, and their dB / Step values are set to 0 dB (no change from the prior step). Since sound cards typically have many dummy "no change" steps, you only have to enter the values that are different.
Once you have completed the initial calibration, you can switch from Init Table to Edit mode if you decide you need to change any individual step voltage. Any changes will affect only that step. You can do this even in a future Daqarta session if you have previously saved the calibration to a file. (Whenever you perform any calibration or make changes, you will automatically be prompted to save the file when you exit Daqarta.)
For best results, you should use a digital voltmeter that can display the AC output voltage from the sound card. Note, however, that the maximum sound card output will be on the order of 1 volt RMS, and many inexpensive meters don't have AC voltage ranges with enough sensitivity for this purpose; their ranges are more for checking mains voltage, often 199.9 V full-scale.
Inexpensive meters usually do have sensitive DC voltage ranges that would be perfect for this job, if you can first convert the AC of the sound card output to DC. If you are handy with electronics, you may want to build an AC-to-DC converter for your meter. This basically consists of an active rectifier that uses an op-amp plus a few resistors and diodes to full-wave rectify the AC input, followed by a filter stage to remove ripple and give fairly smooth DC. (You need an active rectifier because the voltage drops across the diodes in a passive circuit would give intolerable nonlinearities in low-level measurements.)
Note that expensive meters feature "True RMS" conversion circuits, but you don't need that here. The reason is that we are not particularly concerned with an accurate measurement of absolute voltage, but rather with relative changes in voltage at each attenuator step. The meter reading may be off by any arbitrary factor, as long as it changes in proportion to changes in the signal.
Similarly, expensive meters give accurate AC readings over a wide range of frequencies, but again that doesn't matter here since you can hold the signal frequency constant.
Be sure that you set the signal to a sine wave. The frequency should be compatible with your measurement system. Meters with AC ranges can always handle 50-60 hertz, and those with sensitive ranges can probably go up to several hundred hertz with no problems. If you build your own active rectifier, you may want to try several hundred Hz to get better smoothing from the DC filter.
Note that Daqarta provides a built-in Voltmeter option, or alternatively allows you to read the sine wave peak voltage from Spectrum mode. But to use these your card must be capable of full-duplex operation so you can monitor the output signal via the inputs... in which case Auto-Calibration is a better choice.
However, if you are using manual calibration because your card doesn't support full-duplex, you can still run Daqarta on two separate systems, with one in Input mode to monitor the outputs of the other. No input calibration is needed here, since the inputs are used only for relative measurements, not for setting the Full-Scale Range. Just enter the Voltmeter or Spectrum peak values in the Volts calibration box.
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