Data AcQuisition And Real-Time AnalysisScope - Spectrum - Spectrogram - Signal Generator
Software for Windows
Science with your Sound Card!
Contact us about
This dialog is accessible from the Calibration menu, or via CTRL+O.
Vista or Windows 7 - 10 users: Before you begin calibration, please see Vista / 7 / 8 / 10 Issues.
Auto-Calibration determines the relative step sizes of the mixer attenuators on your sound card, and also determines the timing delay needed to synchronize simultaneous input and output data streams ("full-duplex" operation). It eliminates the need to use manual Generator Calibration, unless your sound card does not support full-duplex mode.
Note, however, that after this relative calibration you must also perform a Full-Scale Range Calibration to set the absolute calibration for correct voltage measurements.
The Auto-Calibration method involves connecting the output to each of the sound card inputs in turn, via a direct "loopback" cable. Typically, this is just an ordinary audio cable with standard mini-plugs on both ends. That will allow you to connect the output to the Line input, and possibly the Aux input. It may also work for the Mic input. If you plan to use Daqarta with inputs that are only available via special connectors on the sound card, such as the CD input, you will probably want to build a custom connector panel for easy access, and then the loopback will need to mate with whatever style of connectors you choose.
NOTE: Some sound cards like the ASUS Xonar U7 use standard 3.5 mm stereo jacks for inputs and headphone outputs, but use RCA jacks for line outputs. Since the Xonar also identifies itself as a different device (Speaker or Headphones) depending on whether the headphone jack is in use, simply using a 3.5 mm male-male cable for the loopback will result in calibration of the Headphones device only. If you want to later run using the RCA outputs you will need to calibrate using a special RCA-to-3.5 mm cable. Note that the separate device calibrations will result in separate .ATN files... see Managing Multiple Devices.
The Auto-Calibration dialog shows all the sound card input lines at the top, even those that may be irrelevant or inaccessible. You only need to calibrate the lines you are going to use, typically Line, Mic, and maybe Aux and CD. You can ignore input lines that simply monitor the outputs, such as Stereo Mix, Mono Mix, or Synth. You will probably also want to ignore lines such as TV and Phone.
NOTE: Some laptops that only have a Mic input jack may nevertheless auto-detect and support full-performance stereo Line-type inputs on that same jack. The Input dialog and the Auto-Calibrate dialog may continue to show only Mic. See the Input Line Selects topic for a simple test approach.
Some cards have separate connectors for Line Out and Speaker. In this case Speaker is usually just an amplified version of Line Out, and uses the same attenuator. Use the Line Out connector for Auto-Calibration.
To get started, connect the loopback cable between Line Out and Line In. (Some cards may label the input as Aux instead of Line In.) Then in the Auto-Calibrate dialog put check marks next to Line (or Aux) and Master In in the Inputs box, as well as next to Wave Out, Master Out, and Duplex Delay. Leave Frames at the default of 32 and Dist Limit at 2 percent, and hit Begin.
Daqarta will warn you that the output may become very loud, so you should disconnect all headphones and speakers. It will then proceed with the calibration of the selected Input line, then Master In, Wave In, Master Out, and finally Duplex Delay.
The basic strategy for the calibration is to create a test tone on the output and measure it with the input. Then the input level is reduced one step and the measurement is repeated. The ratio of the two measurements gives the attenuation of that step. Since we are using a ratio, it doesn't matter if Daqarta isn't able to translate the measurements into actual voltages at this point. The process is repeated for all steps of the input line, before calibration moves on to Master Input.
A special AutoCal.GEN file is loaded into the Daqarta Generator to produce a test tone of about 2 kHz. Daqarta first measures the distortion of the input signal and adjusts the output level to insure that the input is not overdriven, since a clipped signal doesn't respond proportionally to attenuation changes. The maximum amount of distortion that Daqarta will allow is set by the Dist Limit control, which defaults to 2 percent.
Then the actual level measurement phase begins. Daqarta uses a modest amount of spectral averaging to reduce random variations, which is especially useful for very low-level measurements at high attenuations. The default is 32 frames, which works out to about 0.75 second per attenuation step measured, or under 3 minutes for a typical input or output line with 192 steps.
While the calibration is running a status window appears that shows the current attenuator step and the maximum step number. Steps start at 0 and run negative. There is a Cancel button on this dialog that allows you to abort the calibration at any time.
As the calibration runs, you will see the spectrum peak height change with the attenuation. Don't be alarmed, however, if you see the step number changing while the peak height is unchanged. Windows sound card drivers include dummy steps that make the Windows mixer sliders appear to have finer resolution than the card actually provides. One of the benefits of calibration is that Daqarta will allow you to see and set the actual volume in dB, regardless of dummy steps.
After all the selected mixer attenuators are calibrated, the Auto-Calibration proceeds to calibrate Duplex Delay, if that has been selected. Delay and drift are measured for all combinations of input and output channels, at the current sample rate. (The Duplex Delay summary holds results for up to 4 different sample rates, each requiring a separate Auto-Calibration run using Duplex Delay.)
A status window will show the running results, and these may also be viewed at any time after the calibration via the Duplex Delay dialog.
When you later run in full-duplex mode, be sure to select one of the calibrated sample rates, and make sure that no drift was found at that rate. See Full-Duplex Techniques for approaches to use where drift is unavoidable.
The calibration is done when the status boxes disappear. The Auto-Calibration dialog will change 'Uncal' to 'Cal' next to each of the newly-calibrated lines. If the 'Cal' doesn't appear, then something went wrong with the calibration of that line and it was skipped.
Questions? Comments? Contact us!We respond to ALL inquiries, typically within 24 hrs.
Over 35 Years of Innovative Instrumentation
© Copyright 2007 - 2021 by Interstellar Research
All rights reserved