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Part of the mastering process is eventually obtaining a stereo mix of your track in a format that is supported by mainstream playback systems. Now, whether that be a CD player, MP3 or other digital format, it is important to understand how digital audio is stored and ultimately the effect this has on the audio quality at the end of the day. So, in its uncompressed form inside of Pro Tools, digital audio exists as a finite series of discrete samples or frames where each sample is measured with a finite number of bits, ones or zeros, from the continuous analog waveform and this happens at the analog to digital converter stage.
This sampling process is known as Linear Pulse Code Modulation and the data is most commonly stored in the WAV or AIFF file formats. So, an example would be audio recorded at 44,100 times per second or 44.1K at a resolution of 24-bits is measured 44,100 times per second or 44,100 frames with each frame or sample represented by 24-bits of data.
So, the sample rate is going to determine the maximum frequency oscillation that can be recognized by the system according to Nyquist and the bit-depth determines the quality or dynamic range that can be stored in each sample. So, this diagram shows you here on the x-axis. We are looking at samples and then the y-axis would be our bit-depth or the resolution available in each sample. So, generally when we record in Pro Tools, we like to record at higher sample rates in bit-depths to achieve maximum audio fidelity.
But because your average consumer playback system cannot read these formats, we must truncate and downsample our audio, throwing away some of the precision to comply with standards. So, for example, a CD is 16-bit, sampled at 44,100 times per second. So, despite all of this, it's still ideal to record at a higher sampling rate and bit-depth to maintain precision until the very last stage. Now, when this happens, inevitably, we have to truncate and some error occurs during the truncation, because we are going from a higher precision bit-depth to a lower precision, 24 to 16, the engineer is likely going to use dither to help alleviate some of the quantization error that occurs when moving to a lower bit-depth.
So, what happens is at the lower level, at the lower level details, error occurs when the specific amplitude of the waveform either triggers or doesn't trigger that least significant or that 16th bit, and at this low level, that error is actually correlated with the signal, or we get a correlated distortion that kind of sounds nasty on our low level amplitudes within our waveforms. Now, what dither does is it removes this distortion by adding some low level random noise, similar to wide noise and it's added at the truncation point or the least significant bit, the 16th bit in this case.
And this is effectively going to smooth out this quantization error associated with lopping off that lower 8-bits. So, if we look at this diagram, we are going to see that a low level sine wave at 24-bits, when truncated to 16- bits, we get a significant amount of quantization error. So, that is to say the difference between what the analog waveform would be and how it's digitally represented in each sample, there is a difference there and that difference is the quantization error.
By adding dither, we are actually able to smooth out this quantization error, this distortion. Dither is just sort of randomizing essentially what occurs at that least significant bit, so is it on or is it off. It's a randomization of this and humans are really good at actually hearing through this noise and pulling out the signal. So, what this allows us to do is effectively retain much more of our dynamic range, than if we were to just truncate at the 16th bit, without using the dither.
So, what are the rules for using dither in Pro Tools? Now, what I can do is if I'm balancing my mix internally and exporting it from the Region's list, dither is automatically added when I change the bit-depth here. It's just something that Pro Tools does behind the scenes. You don't have to worry about it. However, if you want to use your own special flavor of dither, which a lot of engineers, specifically master in engineers will do, what you need to do is apply it as the last plug-in on your Master Fader, so it needs to be the last thing in the chain.
Now POW-R dither comes with every Pro Tools systems and it actually has some noise shaping selections you could make. Now, noise-shape dither, instead of just being broadband noise, it actually pushes some of that noise into the higher frequencies that are less audible for humans, so it sort of optimizes the noise. You probably aren't going to be able to hear too many differences between the noise shaping types but the real audio file master in engineer types, you know, will swear by one noise shaping over another. Now, this is the last stage of my change and I don't want to change the volume after I hit the dither.
So, I don't want to raise or lower the level of the signal after it goes into the dither plug-in, because that will result in that noise, either being shifted up a bit or shifted down a bit, in a way that's not going to help us at all. So, I would use Bounce to > Disk in this case to perform a truncation down to 16 without Pro Tools adding any dither. So again, if you are going to use your own dither plug-in or if you are going to use the dither built into a brickwall limiter, some maxim has its own built in dither and in this case, I would not use POW-R dither.
So, if you are using the waves L2, L3 or some of these other brickwall limiters that have built in dither, you'll have that as the last plug-in in your chain and activate the dither on the plug- in and then use something like Bounce to > Disk. Now, if you still want to bounce your audio inside of Pro Tools into a new track and use your own flavor of dither, you have a few options. What you can do is you can export a 24 -bit version and then bring that into another program that allows you to add dither, so Bias Peak or Sound Forge or something like that or what you can do is just bounce it internally to a stereo track in Pro Tools and then add the dither to that track and then use the Bounce to > Disk engine to truncate down to 16-bits.
Now, ultimately the difference between bit-depths and sample rates may be very subtle and the differences in dither are even more subtle, but it's important to understand the role they play in mixing and ultimately, the mastering process, as your goal should be to retain every detail from your hard work during the mixing stage when the listener pops it into their CD player.
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