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Science with your Sound Card!

Sound Card Auto-Calibration Features

Windows does not support calibrated volume controls: When you move a slider up or down, you don't know how much the level really changes. (Or even if it changes at all... many slider positions are simply filler!)

Daqarta's Auto-Calibration option determines the actual change at each volume step, allowing Daqarta to show the true volume, in dB relative to full scale. This procedure only needs to be done once for each line you intend to use, and Daqarta will use that calibration in future sessions.

After Auto-Calibration, you can use the Full-Scale Range calibration procedure to get absolute calibration, allowing readings in true Volts, SPL, or custom User Units.

Just as important as volume calibration, Auto-Calibration can also measure the relative timing between your sound card's input and output streams ( Duplex Delay), so that input and output waveforms can be displayed with the correct time alignment.

Auto-Calibration works via a "loopback" connection from the sound card's output to input. The Daqarta Generator produces a calibration tone which is monitored via the input line while the volume is changed step-by-step over the entire volume range. This arrangement allows calibration of both input and output controls, simply by holding one constant and varying the one being calibrated.

Below is an Auto-Calibration in progress:

[Auto-Calibration (39K image)]

The Auto-Calbration dialog is shown on the right in the above image. To use it, you select one Input line from the top panel, and (initially) Master Out, Wave Out, and Duplex Delay from the bottom, as shown here. Connect the selected input to the sound card's output (Line Out, etc), typically using a standard mini-plug cable, and hit Begin at the bottom of the dialog.

Daqarta will load a special AutoCal.GEN setup file and verify that the specified connection is present, then it will report the number of volume steps to be calibrated on the selected Input line... typically 192 steps.

The time to calibrate each step depends upon the Frames control near the bottom of the dialog. Calibration uses spectral averaging to reduce variability, and this control sets the number of frames to be averaged for each calibration step. The control defaults to 8 frames, which takes about one second per step and is adequate for most purposes, but here it has been set to 32 (about 4 seconds per step) for better accuracy.

The calibration procedure begins by adjusting the initial volumes as needed to avoid excessive distortion when the line being calibrated is at full scale. This may take a few seconds. The "Dist Limit" value at the bottom of the dialog is set at the default of 2.00 percent. This value has been found suitable for most sound cards, but it could introduce small errors (less than 0.17 dB) on the top steps of some cards, so you can use a lower limit for critical work.

Then the actual line calibration proceeds, showing the status on a message box at the center of the screen. In this example, the calibration is working on step -6, where the final step is -191. (Steps are numbered downward starting from 0 at full scale.) The Averager Frame count near the upper left of the screen shows that at the instant this image was captured, 15 out of 32 frames for this step has been completed. (This value normally changes very rapidly.)

Note that although the sound card driver may report 192 steps, the actual hardware on the card typically has far fewer functional steps (typically 32 to 64), scattered over the total range with arbitrary spacing and magnitude. There is no way to know which steps actually produce volume changes other than by testing, so the calibration procedure must plow through all the dummy steps to find the real ones.

When each line has been calibrated, the label next to it in the dialog changes from "Uncal" to "Cal". Notice that the selected lines in this example have all been calibrated previously; this is a re-calibration just for the purpose of obtaining this image.

After all the requested lines are calibrated for volume, Duplex Delay is measured. This uses the same basic setup but replaces the test tone with a series of pulses, whose exact rising and falling edge samples can be easily found. This allows determination of the time between the start of an output pulse and when it appears in the input stream. Daqarta does this for all combinations of inputs and outputs, and will use the proper value in the future when you have, say, a mono output with stereo inputs.

The Duplex Delay time may only be stable at a single sample rate, such as 48000 Hz. Daqarta measures the drift at 44100 and 48000 Hz, plus at whatever the current sample rate was before you started Auto-Calibration, and reports those values when it is done. Later, you can view all delay and drift values via the Duplex Delay option in the Calibration menu, where you can also manually adjust specific delay settings for Daqarta to apply.



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