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Do digital signals degrade at higher levels?

A post by David Mellor
Thursday June 14, 2012
Do your recordings brush up against the full scale mark? If they do, might they be distorted or degraded in some way compared to lower-level signals?
Do digital signals degrade at higher levels?

Do your recordings brush up against the full scale mark? If they do, might they be distorted or degraded in some way compared to lower-level signals?

I have two scenarios in mind. One is where you record a single signal, either from a microphone or a line-level source. Suppose you set the gain so that the meter in your DAW comes very close to the top, but doesn't quite make the red light flash. This will create an audio file on your disk that comes close to the 0 dBFS absolute limit. The other is when you are mixing. Your meters again show the signal coming very close to full-scale, but don't quite hit the red lights. When you bounce the mix, again this will create a file that comes close to 0 dBFS. As I am sure you know, there is no higher level possible in a digital audio file.

If you believe what you read on the Internet, you might consider that this a cause for concern. After all, if you push anything close to its limits, you would expect some kind of strain to show up.

Well I would like to confirm that with digital audio, this is not so. Digital audio works perfectly all the way up to 0 dBFS. That is all the way between 1 and 16,777,216 in a 24-bit recording, or 0 and 16,777,215 if you prefer to start counting from zero. That last tiny less than 16-millionth fragment of audio is just as good as as any other audio midway in the scale. In fact, it's better!

Digital audio gives its best quality right at the top end of the scale, where the signal level is way above the noise and distortion levels. Any lower than that, then noise and distortion make more of a contribution to the overall sound. It has to be said however that you have to go pretty low in level for noise and distortion in a 24-bit signal to be any kind of a problem.

This is in contrast to analog audio. In analog, signal-to-noise ratio improves as the signal level gets higher. But there comes a point where distortion starts to kick in. Go too high in level and the distortion will reach an unacceptable level. But in digital audio you can go as high as you like... all the way until the 'over' light comes on, and then you have serious distortion.

But you might not believe me. There might be a lurking shadow of a doubt that digital audio really does degrade at higher levels. OK, here's a test...

Make an original recording from a microphone that peaks at a safe -12 dBFS or so. Peak as low as -18 dBFS if you like, to be really sure. You now have a file on the disk of your computer where the top two bits (or three bits if you peaked at -18) are completely unused. Now normalize your recording so that it peaks at 0 dBFS. This will create another file on your disk where all of the bits are fully used.

Now import your original recording back into the session onto another track, so you can play the two files in exact synchronization. Invert the audio on one of the tracks. Bring down the fader on the normalized audio gradually. Because the two signals are in opposite polarity, when they are both at exactly the same level they will cancel out. You should quite easily be able to find a point where you can hear hardly anything.

But... if the higher-level file really was degraded, then the cancellation would only affect the clean portions of the signal. You would hear the degradation very clearly because it would not be canceled. It would stand out like a proverbial sore thumb. But it doesn't. The two signals cancel perfectly.

This test clearly demonstrates that you can go as high in level as you like in digital audio, as long as you don't try to go above 0 dBFS. Anything lower in level than that, even by the tiniest fraction, will be completely clean.

By the way, so-called intersample peaks that sum to levels higher than 0 dBFS are only a problem on digital-to-analog conversion, and then only on poorly designed converters. You can find further information on this topic here...

A post by David Mellor
Thursday June 14, 2012 ARCHIVE
David Mellor has been creating music and recording in professional and home studios for more than 30 years. This website is all about learning how to improve and have more fun with music and recording. If you enjoy creating music and recording it, then you're definitely in the right place :-)
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