Dithering
A Crucial Step

Dithering is the last, but most important, step before writing your audio files to your CD master.

(There can be one extremely rare situation where you won't need to do this – but that's for later!)

What is it?

Why is it important?

The simplest explanation is it's the addition of very low level noise to your audio files as you convert from a higher bit depth (usually 24 bits) to the CD format of 16 bits. (You are doing your recording and mixing at 24 bits aren't you?!)

Why add noise? Keep reading...

Any audio file that has processing applied – for instance a simple volume change or fade (which is a continuous volume change) increases the bit depth. All audio needs to be put back to 16 bits for CD. Simply removing the excess bits will introduce noise and harmonics – known as quantization error.

The 'dithering' noise added has some special properties. It's completely random and uncorrelated with the music material. There are no periodic repetitions. It is continuous and free from spikes or drop outs. And it is in stereo.

Different companies have developed different ways to generate this type of noise to be effective with music and other types of audio.

It doesn't end there.

Although this noise is very low in level, further steps are taken to lower its perceived level, and interfere less with our listening enjoyment. This is called noise shaping. It simply means that less noise is applied where human hearing is most sensitive (generally around 3 kHz and 12 kHz), and more where it is not. As frequencies get higher, the noise gets higher as well, since human hearing gets less sensitive at these higher frequencies. Many years of research are involved in creating different suitable noise shapes.

So why not just record in 16 bits and not worry about dithering.

Well, that rare case I mentioned earlier would probably involve a 'perfect' live recording where nothing needs to be changed. Those happen all the time don't they?!!

However you still probably don't want to do that.

Here's why.

CD's have a signal-to-noise (S/N) ratio of 96dB. Signal-to-noise ratio is a scientific measurement of the loudest possible sound, all the way down to where noise begins (the noise floor).

Dynamic range, on the other hand, is the human perception of the loudest possible sound down to the very lowest threshold of hearing.

Science can't measure this exactly.

But it has estimated it to be around 120dB.

Humans don't stop hearing once the noise begins. They can hear into the noise. Remember cassettes? They usually had a S/N of 40 - 60dB at best. But you could hear probably 70 - 80dB of dynamic range. You still heard the quieter passages of music even though you could also hear the noise.

Making our 'perfect' live recording at 24 bits will give it a S/N of 120dB which now matches human perception of dynamic range. You add dithering to reduce the bit length to 16 bits for an audio CD. It, remember, has a S/N of 96dB.

But because of the special properties of the this noise mentioned earlier, the perceived range is now the human perception of 120db - or very, very close to it. You've now just increased the dynamic range of the CD by applying dithering noise to a 24 bit recording, instead of just offering a 'straight' 16 bit recording with only 96dB S/N and dynamic range.

Dithering reduces the scientific S/N from 96dB to about 91dB.

But it increases the perceived dynamic range to almost 120dB.

The trade-offs, I think, are very acceptable!

Don't you?!

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