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Fixing Audio CD distortion

Here's the problem - you have some audio files on your hard disk but when you burn them to CD they sound distorted. Ian Waugh has some solutions...

Distortion is the bane of audio life. One of the most common problems we get asked about is the appearance of distortion when burning audio CDs. This can be caused by a range of situations so let's look at a few and see how to prevent it.

The most important message is GIGO - Garbage In = Garbage Out. If the source material is distorted, when you burn it to CD, then that will be distorted, too.

It's very easy to create a distorted recording when transferring material to your hard disk. If you're recording from an analogue source such as a record deck or tape machine, it's tempting to whack up the record levels but this can spell disaster.

When recording to tape you can overdrive the recording level a little. In fact, this can create a warm sound due to a process known as tape saturation. However, you cannot overdrive a digital recording at all as it only has so-many bits (16 or 24 for example) for storing the data and if you exceed this some of the data will be lost resulting in distortion.

So, the first rule is to make sure that the original recording is not distorted. If you see the recording meter flash into the red, lower the level a little.

Killer pitch

If you are recording your own material, it's very easy to distort low-pitched sounds such as a killer bass drum or bass sound. Because they are low-pitched they are also relatively low volume so it's easy to whack up the drum or bass track and push them into distortion.

EQing is also an easy way to add distortion to a signal. If you boost a frequency band, this adds more energy to the signals which results in a higher volume. Potential distortion alert here, too.

When tweaking your material, keep a close eye on the output meter to make sure they don't sneak into the red. You can often see distortion if you examine the waveform in an audio editor. You'll notice that the tops and bottoms have been clipped. This will not sound nice.

EQ myth

Let's clear up a myth - EQ cannot remove distortion. Distortion is caused when part of the original signal is lost and once it's gone, it's gone. No amount of EQing will recover it. EQ may improve the output by attenuating the frequencies in which the distortion lies but you won't get the original signal back.

Here's something else to be aware of. As you probably know, many sound cards now offer 24-bit recording. This gives you a larger dynamic range than 16-bit and ought to produce a better quality recording.

However, you probably also know that audio CDs are 16-bit so before you can burn a 24-bit recording to an audio CD you need to reduce the number of bits to 16. and there's a right way and a wrong way to this.

Usually the downsampling will be handled by the music software or possibly the CD burning software. The quick way to do it is simply to reduce the number of bits (we'll spare you from the gory details) but the result may leave something to be desired.

In a dither

A better way is to use a process known as dithering. This actually adds a special type of low-level noise to the recording and then it reduces the number of bits. The noise increases the level of the quiet sections that are represented by few bits which might otherwise be lost during the process so preventing noise known as quantisation error.

So, if you need to downsample, check that the software doing the job dithers rather than truncates.

Okay, you've done all that. The audio on your hard disk is clear as crystal and more perfect than a perfect thing, but it still distorts when you burn it to CD.

Burn away

Beware of audio CD burning software that lets you tweak the audio. This could include a stereo separation function, compression and our friend EQ. These can increase the volume of the audio so try burning with no effects at all.

It is not unknown for some CD burning software to take a dislike to some CD-Rs so if all else seems hunky dory, try some different software. There is lots on the web, free and shareware, and some commercial programs have demo versions.

It is also possible that your CD-R is not very good at audio CD creation. A remote chance with modern CD-Rs but see if you can borrow another CD-R or try burning part of a problematic file on a friend's system.

Finally, don't automatically think that if you perform a digital audio extraction (DAE) on a CD that the resulting file will be perfect. Much depends on your CD-R and the DAE software. Again, some don't work well together and some CD players are not particularly adept at DAE. You should easily be able to spot any problems here simply by listening to the extracted file.