Audio Calibration

Original Author(s): Lynn Leith

The "Calibration" measures the noise threshold (the background noise and system generated noise) of the recording environment. Record levels should be set with the Windows and/or on-board sound card prior to calibration. When the calibration is complete, the status of the current noise threshold value and the DC offset value are displayed. The text box in the lower left of the Audio Calibrator box will display the results of the calibration indicating 'low', 'medium', 'high', 'very high' or 'unable to calibrate'. If calibration is not successful, adjust the levels in the Windows Mixer and recalibrate. This measurement is very important in the phrase detection process. It may seem like it is silent or quiet, but there is always some level of noise in any recording environment. All three of the DAISY production tools allow you to use phrase detection based on this calibration. Accurate phrase detection creating audio events of a reasonable length will make editing much more efficient. Therefore, calibration is very important for editing.

Step1 Adjust recording level

Select the "Calibration" button from the toolbar (or Options/Audio Calibrate, Cntrl A, from the menu) to open the "Audio calibration dialog" box. Observe the VU meter at the right. If additional adjustments to the recording level are required for the maximum level (peak) to be between -3db to -6db, make adjustments in the Windows Mixer. When the peak exceeds -3db, the signal display on the VU meter will rise and the colour of the bar will go into the yellow area near the top. It is acceptable if the level rises above -3 and into the yellow area occasionally. It should not go into the red zone at the very top at any time.

Adjust recording volumes in the 'Calibration' dialog

Step2 Test recording

When the recording level is correctly set, select the "Start calibration" button. If you are making an original recording with a microphone, please remain quiet for 10 seconds while the system calibrates. If you are inputting audio from cassette tapes, you should play a short selection from the tape that contains as little narration as possible, but still contains a typical sound of the tape (tape hiss, etc.) because the calibration must be based on the noise threshold of the recording environment.

10 seconds test recording

Step3 Verify the result

When the calibration is complete the values of the noise threshold and DC offset will be shown on the bottom left side of the dialogue box. If the recording environment is exceptionally good, the message in the dialogue box may report that "The DC offset value is low. It seems you have not connected the audio line correctly!" If the recording environment is extremely quiet and this message is reported on a regular basis, there is no need to check the audio line or mixer settings. Finish the process by selecting "Exit" or with the "ESC" key. The values are automatically transferred to the "Recording settings".

'Low level' message

If calibration is not successful

When the "It was not possible to find a low noise level" message is displayed, you should make appropriate changes to the recording environment and make a new test recording. For a tape transfer, you can try adjusting the output level or correcting the sound quality with an external device such as the mixer, then repeat calibration. When the tape medium is a cassette, it may be necessary to use an equalizer, as the sound quality of cassette recordings is often poor. Another alternative is to digitize the analog recording and clean it (remove tape noise and hiss) with an external digital audio editing software system.

The message for readjustment of recording level

See also

DAISYpedia Categories: 

This page was last edited by LLeith on Tuesday, August 10, 2010 11:48
Text is available under the terms of the DAISY Consortium Intellectual Property Policy, Licensing, and Working Group Process.