The topic of CD dithering can seem like much ado about nothing, but it's a crucial step to creating a finished, professional master.
16 Bits is the standard delivery format for all digital audio from CD's all the way down to mp3 files.
So, it's not only crucial, it's necessary! (You
are doing all your recording and mixing at 24 bits? Aren't you?!)
Any audio file where we do the following to it:
...increases its bit depth.
All audio needs to be put back to 16 bits for CD and other delivery methods.
Simply removing the excess bits - known as truncation - will introduce noise and harmonics. This is referred to as quantization error.
Truncation sounds similar to an audio gate when it's hovering at the threshold level. If no noise is added, we will hear this error in the background when a guitar tone fades quietly on its own to silence. Or when a song fades to silence. Or any - audio - goes to silence...
The noise will let us 'hear' right down to the true silence by increasing the perceived dynamic range.
But not just any noise will do...
Here they are:
Different companies have developed different ways to generate this type of noise. Companies like Apogee, iZotope, and POW-R to name a few.
When properly applied, all are very effective with all types of music and other audio:
Although this noise is very low in level, a further step is taken to lower its perceived level to human hearing - to make it interfere less with our music and to increase our listening enjoyment.
It's called noise shaping.
This simply means that less noise level is applied where human hearing is most sensitive (generally around 3 kHz and 12 kHz), and more where it is not.
So generally as frequencies get higher, the noise level gets higher as well, since human hearing gets less sensitive at these higher frequencies.
Many years of research have been involved in creating different noise shapes suitable for different types of applications.
We've all made that perfect recording. Right?!
One where nothing needed to be changed? No editing? No processing?
Even if the perfect take did happen, and even though all delivery mediums are 16 bits, we still want to take advantage of all that extra dynamic range higher bit rates offer.
We can only take that advantage because of the opportunity dither offers.
Delivery mediums like 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).
In other words: when noise starts, music stops. As far as science is concerned.
Dynamic range, on the other hand, is the human perception of the loudest possible sound down to the very lowest threshold of hearing.
This is a subjective measurement.
But it has been estimated to be around 120dB.
We humans don't stop hearing once the noise begins. We can hear into the noise. Remember cassettes?
They usually had a S/N of 40db. Maybe 60dB at their 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.
Let's get back to our 'perfect' recording:
Making it at 24 bits will give it a S/N of 120dB. This now matches the human perception of dynamic range. We then add this noise I've been talking about, when reducing the bit length to 16 bits, for an audio CD. It, remember, has a S/N of 96dB.
But because of the special properties mentioned earlier, the perceived dynamic range now stays at 120db - or very close to it. We've now just increased the dynamic range of the CD by applying this special 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!
Each and every master I create at Mister's Mastering House has dither properly applied.
This ensures proper playback on any and every iDevice.
It insures every CD and digital file has the most dynamic range possible.
This gives your listener the most impact possible!
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