Adjusting for DC to Avoid Delta Recording Overload

M-Audio's tech support department has recently fielded some calls about our Delta audio cards in regard to DC offset. As an example, one customer reported that while his Delta 1010 worked fine most of the time, recording in Cool Edit occasionally pegged the meters and resulted in a recorded signal that was a single electronic tone. After discussing this issue at length with Syntillium's tech support people, he defragmented his hard drive and made a few other adjustments, but the problem remained.

In our experience, this phenomenon has to do with DC offset. Many sound cards record audio with a slight DC offset, which means that the center or zero amplitude point of the waveform being recorded is not at the exact vertical center of the waveform display, but a little above or below it. This can dramatically throw the level meters off since the amount the waveform is displaced could be interpreted as a constant sound that loud.

To compensate, enable ''Adjust for DC'' in Cool Edit. (For those who use applications other than Cool Edit Pro, there are numerous programs that can compensate for DC offset, including Wavelab 4.0.) In the current version of Cool Edit, select Device Properties from the Options menu. Then select the Wave In tab that allows you to select any sound card installed on the PC, and check the Adjust to 0 DC check box.

If you are using an earlier version of Cool Edit such as Cool Edit 2000 or Cool Edit Pro 1.2, the Device window is in a different place. Press F4 to open the Options window and go to the Devices tab, where you will see that same check box to enable DC adjustment for recording.

In either case, the recording meters will dynamically adjust to the DC offset, and will display the true amplitude of the signal in decibels. (The clip indicators will always light up if clipping occurs, but if ''Adjust for DC'' is enabled, the indicators may also light up when the audio has a DC offset.)

You can also adjust DC on an existing file. With the file open, select the Edit view and go into Effects Amplify/Amplitude and check the box for DC.