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Q. Are my finished tracks clipping?

By Sam Inglis

Most digital recorders, including the HHB Burnit, will indicate overs cause by input gain boosts, but few have  meters sophisticated enough to work out whether a peak-level output sample represents a clipped waveform. Most digital recorders, including the HHB Burnit, will indicate overs cause by input gain boosts, but few have meters sophisticated enough to work out whether a peak-level output sample represents a clipped waveform.

I edit completed mixes in Wavelab on my PC and then normalise them to 0dB and send them via digital link to an HHB Burnit CD writer. My problem is that the volume on the finished CD-R is very quiet when compared to commercial CDs.

The Burnit does allow you to increase the digital record level and if you do this it sends the record meter into the red, but not when you play it back! I've found that you can increase the record level by 10dB and this does get the CD-R volume comparable with commercial CDs.

I'm confused. Why don't the meters go into the red on playback when they do on record? I can't hear any audible distortion — is any taking place? Is it a safe practice to increase recordings by 10dB?

John Ablitt

Features Editor Sam Inglis replies: There are two basic issues here. The first is that the music on most modern CDs undergoes heavy processing in order to make it as loud as possible, usually through multi-band compression and limiting — and often to the detriment of the sound quality. If you want to make your music that loud, the place to do it is in your mastering program (Wavelab in this case) using these kinds of tools.

The second, and the reason that the Burnit will report overs on record but not on playback, is that it's impossible for any of the data values coming off a CD to exceed 0dBFS, since that is defined as the highest value that can be represented in a digital medium such as compact disc. A waveform that has been clipped or overloaded on the way in (as you're doing by raising the level 10dB) will be distorted on playback, but it can't and won't contain any individual samples higher than 0dBFS. A simple digital meter will just report the output as peaking at 0dBFS, which is perfectly 'legal'. More sophisticated digital meters can anaylse a signal and work out where a series of 0dBFS peak samples is likely to have been caused by waveform clipping, but I suspect the output meter in the CD burner is of the former kind.

What you're doing by raising the level 10dB in the Burnit is, in effect, 'chopping off' the top 10dB of the dynamic range in your signal, in such a way that any peaks higher than -10dBFS in the original signal will be 'clipped'. How noticeable this is on playback depends on how loud the music was to start with, and also on the nature of those peaks — brief transient peaks such as snare hits can often be clipped without too much in the way of obvious distortion. In fact clipping is often used deliberately in hip-hop records to create a really punchy drum sound, so it's not necessarily a no-no.

If you want to raise the level of your finished tracks, explore the mastering options which are included in Wavelab or offered by third-party plug-ins. Craig Anderton's recent article on mastering using a computer, in SOS August 2004 (www.soundonsound.com/sos/aug04/articles/computermastering.htm) is a good starting point.

Published January 2005