Data AcQuisition And Real-Time AnalysisScope - Spectrum - Spectrogram - Signal Generator
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Sound Card Calibration Overview
The Calibration menu (ALT+C) includes the following:
Calibration is necessary to allow Daqarta to use your sound card as a precision measurement system. Windows has never considered the card for anything but entertainment, so it is very casual about issues like volume and timing. Windows provides no way to determine many critical details about the sound card via software, so you have to do it yourself.
Calibration is traditionally a rather tedious process, so Daqarta automates it as much as possible. Still, it is something that should not be rushed. It only needs to be done once for your system, and it is worth doing it properly so you can trust your results in the future.
Important: In Windows Vista and later (7/8/10) you should make sure that the Volume Mixer (found by right-clicking the speaker icon in the system tray) has the "Speakers" slider at maximum, and all other sliders except Daqarta's set to Mute (click the little speaker icon below each slider). In general it is always best to fully close all other applications that use the sound card, to avoid interference. In that case, the Volume Mixer should show only Speakers, Windows Sounds, and Daqarta sliders.
There are several distinct features of the sound card that require calibration. The most important is calibration of the mixer attenuators, which is done with the Auto-Calibration dialog. This allows Daqarta to keep track of changes in sensitivity with changes to Input Line and Master ranges, and changes in output level as you change Generator volume.
The mixer calibration only determines the relative sizes of the attenuator steps, not absolute sensitivities. For example, reducing the Generator volume one step down from maximum may, on your particular card, have the effect of cutting the output voltage to 84.14% of maximum, or -1.5 dB. The next-lower step may cut it another 1.5 dB, down to 70.07% of maximum or -3.0 dB overall. Still-lower steps may be greater than 1.5 dB. Some steps may do nothing at all, dummies that the card maker (or Windows) includes to make the normal Windows mixer sliders appear to have finer resolution than the card actually provides.
Note that mixer calibration only applies to normal stereo output channels; Multi-Channel Outputs (4, 6, or 8 channels) are controlled via the custom control panel that comes with your multi-channel card.
Mixer calibration allows the controls to use relative dB values, but that alone doesn't reveal the actual output voltage on any given step. That information is provided by the Full-Scale Range dialog. For example, the above sound card may put out +/-1.23 volts maximum; Daqarta will show that value on the Y axis of the display, and if you reduce the Generator volume by 1.5 dB it will show +/-1.035 volts. Knowing the ranges and the attenuator steps, Daqarta can determine the absolute voltage at any setting.
Another aspect of calibration has to do with timing. Most cards allow "full-duplex" operation, which means Daqarta can simultaneously generate a stimulus and display a response to that stimulus. But if Daqarta is to show the response in the proper time alignment relative to the stimulus, it must know about any internal delays in the sound card. It also must make sure that the input and output streams proceed at identical sample rates, or they will drift apart in time.
Delay and drift are automatically measured as part of the Auto-Calibration operation when the Duplex Delay option is checked, and the results are summarized in the Duplex Delay dialog. You can also manually enter delay values there if you feel the need.
The attenuator, full-scale range, and delay calibration information are stored in file called Daqarta0.ATN that includes identification information for the sound card. If your system has more than one card (which you can select via the Device controls in the Start Preferences dialog), then the files may be called Daqarta1.ATN, etc. Note that the Output Device number is used here. The Input Device number is usually (but not always) the same.
The Daqartan value may also be changed via shortcut command-line parameters for use with multiple desktop icons when managing multiple devices.
The .ATN file information is specific to each sound card and should be the same even if you move it to another system. In principle, you should be able to use an .ATN file that was calibrated on another card of the same make and model, possibly obtained from another user. However, device numbering may be different depending on the system, so you may have to rename it from Daqarta0.ATN to Daqarta1.ATN, etc.
IMPORTANT: Some sound cards identify the outputs as different devices (Speaker or Headphones) depending on whether the headphone jack is in use. Each such output device must be calibrated separately (just like a separate sound card), and will result in a separate .ATN file. You must then choose the proper setup for your later Daqarta sessions.
Besides your sound card itself, you may want to connect external amplifiers or attenuators to your system. Daqarta uses the External Gain dialog to keep track of these, so it can report true input and output values.
For example, suppose your card's Line Input has a sensitivity of +/-1.00 volt at its most-sensitive setting. If you connect a +/-0.01 volt signal, it will span 1% of the full-scale Y axis when no display magnification is present, assuming a proper attenuator and range calibration. You will be able to read the amplitude of the signal using the cursor readouts, or estimate it visually from the Y-axis scale.
Now if you insert an x10 preamplifier on this line, the display will span 10% of the Y axis. If you don't tell Daqarta about the preamp via the External Gain dialog, the signal will appear as a +/-0.10 volt signal... 10 times larger than it really is at the input to the preamp.
The above calibration procedures insure that Daqarta can correctly measure input voltages or supply correct output voltages. But often you may be interested in some other variable than voltage. For example, a microphone converts sound pressure to voltage, but you may prefer to see the signal as sound pressure. And you may want to see that pressure signal without any irregularities caused by the microphone frequency response.
The User Units dialog lets you tell Daqarta how to convert volts to any unit of your choice, either by supplying a unit name and conversion factor, or by loading a calibration (.CAL or .FRD) file that includes that information plus a frequency response.
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