Pro Tools HD and dither

madtheory

Just reading some crap on another forum. Guy says it sounds better to put 24 bit dither on the master fader. Reading the PT reference guide, it explicitly states that you shouldn't do this when destination is 24 bit. But they don't say why this is bad, and they don't state in no uncertain terms that there is 24 bit dither on the master fader already. To confuse things, there is also the optional dithered mixer, which appears to have dither on every bus out as well as the master. It seems they put that there because they claim- and this really sets off alarm bells- you can't hear the truncation from 48 bit to 24 bit because it's at -144dBFS!!!! Problem is, that distortion could cause problems down the line, especially if dynamic range processing is applied.

To be honest, I've never put dither on for a 24 bit bounce, and I never heard a problem. Which doesn't mean much!

I could test this, and see exactly what's going on, where there's dither and where there's truncation. But has anyone here done that already?

kabaal

If you have a session recorded at say 44100 Khz / 24 bit and are bouncing out to a file at that sample rate and bit depth there is no requirement to dither since there is no change in bit depth. That is there is no truncation of the final 8 bits of each sample to 16 from 24 bits. Thus if you add dither to a 24 bit bounce you are in fact merely adding noise for no reason. When you are going from 24 to 16 bit dither serves a number of purposes in which case the addition of low level noise (dither) overcomes larger problems, thus producing better results overall. For a detailed discussion of dither see Ken Pohlmann's book "Principles of Digital Audio" it is all explained well there. Hope this helps

DamagedTrax

the 48bit mixer is only the internal precision and not related to your final mix bit rate.

madtheory

Yes, damagedtrax that's what I always assumed, but the lack of clarity in the PT Reference guide is bothering me. And the bald statement that "it's too quiet to hear".

kabaal, you're not quite right. a "24 bit" session means that the audio is recorded at 24 bit. However, the output of the mixer is 48 bit regardless of the input file resolution, because the resolution is changed simply by adjusting level and pan.

woodsdenis

http://akmedia.digidesign.com/support/docs/48_Bit_Mixer_26688.pdf

More detail on this discussion. I have always used the Stereo Dithered mixer in PT under advice from people who know more than I do about these things:D

PaulBrewer

Who da feck uses dithering anymore ???

DamagedTrax

anyone that needs to go from a higher bit rate to a lower one. so every mastering engineer for a start.

madtheory

1. It's bit DEPTH. Bit rate is to do with data transmission, and compressed audio formats.
2. Anyone who uses a DAW uses dither- but it's bull**** that we have to be concerned about it because Digidesign decided that we would prefer to have 15% more processing power instead of 100% accurate sound!!!!

PaulBrewer

I'm going to invent a De-Dither Plugin ....

woodsdenis

madtheory wrote: »

1. It's bit DEPTH. Bit rate is to do with data transmission, and compressed audio formats.
2. Anyone who uses a DAW uses dither- but it's bull**** that we have to be concerned about it because Digidesign decided that we would prefer to have 15% more processing power instead of 100% accurate sound!!!!

I am sure I am being stupid here, what is your problem. If you feel it is an issue using the "Stereo dithered mixer" in PT then just use the default "Stereo Mixer" instead. Nobody is making you do anything.

The general consensus is that the "Stereo dithered mixer" sounds better but with slightly more DSP required to run it. I am talking about HD .

madtheory

My problem is that:
1. It's buried in the manual, and
2. The default is no dither.

Surely sound quality should take precedence over getting a tiny bit more processing power?

Also, I can't find any definitive statement as to whether or not PT native uses dither on the master.

Paul, no need for a deditherer, Avid have already done that for us...

woodsdenis

madtheory wrote: »

My problem is that:
1. It's buried in the manual, and
2. The default is no dither.

Surely sound quality should take precedence over getting a tiny bit more processing power?

Also, I can't find any definitive statement as to whether or not PT native uses dither on the master.

Paul, no need for a deditherer, Avid have already done that for us...

There is a dithering stage in most double precision plug-ins
and one final dithering stage at the post master output of the
summing mixer. Dither is noise with very specific properties
added to the signal in order to de-correlate the noise floor
from the original signal so that when length reduction
occurs, the resulting waveform does not contain any
harmonic distortion or noise floor modulation artifacts.
This allows the plug-in and the mixer to perform very
precise calculations while maintaining the low level detail
when handing the signal off to the next process. Within
the Pro Tools TDM mixing environment, when the dithered
mixer is used, dither is added to the 48-bit signal before it
is reduced to 24 bits to be placed back on the TDM bus and
sent to the DACs. Dither is not added on a track by track
basis, as this would produce unwanted accumulated noise.
Dither is only added once at the Master Fader output of the
mix bus so the total system dither noise is 3 dB at -144 dB.
“Not so fast!” you say. “You just added noise to my signal and
truncated my word from 48 bits to 24 bits—I’ve been robbed!”
Well, it’s true that within this system topology, the signal must
be 24-bit in order to pass from DSP to DSP via the TDM bus,
but it’s important to understand that the level of the dither
added to the signal is around -144 dB, which is below the
noise floor of the converters. Consider that our ears can deal
with a dynamic range of around 120 dB (from the threshold
of audibility to the threshold of pain). This gives you an idea
of how low “-144 dB” is in terms of audibility—you’d have to
have your monitor system cranked up to extremely high levels
to even hear any sound that might exist in a noise floor this
low. The same could be said about the loss of the lower 24 bits.
The sample values represented by the lower 24 bits in a 48-bit
word are between -144 dB and -288 dB, so the dither only
affects the signal at -144 dB or lower.
THE PRO TOOLS 48-BIT MIXER PLUG-IN
Just as EQs, dynamics, and reverbs are plug-ins, the Pro Tools
Dithered Mixer is a plug-in as well. It differs from most other
plug-ins in that it can grow to span multiple DSPs and as it
grows, it passes signal from DSP chip to DSP chip at a full
48 bits instead of dithering and truncating back to 24 bits.
It does so by using two 24-bit TDM timeslots per connection.
This enables the mixer to maintain an internal dynamic range
of at least 288 dB from beginning to end.
The extra 24 bits in the system are used to provide channel
faders with additional dynamic range above and below the
original 24-bit word, and it guarantees that the same fidelity
is maintained when adding more inputs to the mix bus. In the
Pro Tools +12 mixer, 9 bits are reserved for levels above 0 dBFS,
providing 54 dB of headroom. This is enough headroom to
allow 128 tracks of full code, correlated audio (imagine sample-
aligned sine wave source files) to be summed with all faders at
+12 without clipping the “input side” of the bus. It also provides
enough bits below the 24-bit word to allow channel faders to
be placed at nearly -90 dB before they stop contributing a full
24 bits to the mix.
Since in the real world, audio signals are almost never exactly
correlated in this way, you can mix a far larger number of
inputs within the Pro Tools mixer without clipping the “input”
side of the mix bus at all. You can clip the “output” side of
the mixer, but that’s what Master Faders are for—they allow
you to trim your final output level to avoid clipping the DAC
or 24-bit digital output when your mixed signals leave the
Pro Tools mix environment. This is analogous to an analog
console mix bus, where you trim the master bus with a master
fader to avoid clipping the output circuitry in the console.
Similar to analog mixers, the Pro Tools mixer is comprised of
individual input channels and a summing stage. At the input
stage, each channel’s 24-bit word is multiplied by 24-bit gain
and pan coefficients to create a 48-bit result. The new 48-bit
word contains the original 24 bits “shifted” lower in the 56-bit
register to allow for headroom and “footroom” below unity
gain, enabling channels to be turned down without losing
precision. Specifically, it’s possible to pull any channel fader
down to -90 dB and its signal still retains 24 bits of precision.
As channel faders are pulled down, there is a loss to the lower
bits of the newly extended 48-bit word which represent signals
down to about -240 dB—but a full 24 bits of precision is main-
tained down to -90 dB.
It’s important to understand that while some lower bits
of the 48-bit word are truncated when reducing individual
channel levels, the quantization (or rounding error) that
occurs when they are lost adds about one millionth of a
dB to the total quantization error. This is determined by
adding the quantization noise from the single precision
quantization to the quantization noise of the double preci-
sion quantization. For example, when the mixer represents
-144 dB, the actual value is more like -143.99999 dB and it
is quantized to -144 dB. The difference is about one millionth
of a dB and is astronomically close to the “ideal” mixer—
one that doesn’t ever lose bits internally.
At the summing stage, Master Faders perform in a similar
way to channel faders in terms of resolution. The final gain
stage is in effect a master fader, which is always present;
and, when the Pro Tools UI displays a Master Fader, the user
is given a “handle” to adjust the range of the final gain stage
output. As a built-in part of the mixer, Master Faders don’t
consume any additional DSP resources, so they are the best
way to adjust the final output of an internal bus or external pair
without losing precision.



TBH PT HD has been this way for years and no one has had an issue with this AFAIK

As regards PT Native I think that uses the LE mixer architecture so this scenario does not occur. Do any LE or Native users have the Dithered mixer plug in their systems?

kabaal

Madtheory, I was talking only bouncing out. In other words I was only addressing your question of "why it is bad to add dither" to the master fader (for example manually using the dither option on a waves L2) when bouncing to the same bit depth. It is unnecessary noise since there is no truncation from 24 to 16 bits. I realise now that you were asking about dithering during the transition from 48 to 24. In this case adding 24-bit dither would be again unnecessary since dither is only required at the level of the LSB, which in this case would be at -288dbfs. Hope this helps.

PaulBrewer

madtheory wrote: »

My problem is that:
1. It's buried in the manual, and
2. The default is no dither.

Surely sound quality should take precedence over getting a tiny bit more processing power?

Also, I can't find any definitive statement as to whether or not PT native uses dither on the master.

Paul, no need for a deditherer, Avid have already done that for us...

Fair point Mad. I know a couple of Avid Top Dogs and I'll pass your query on.

madtheory

woodsdenis wrote: »

-144 dB

The problem with this figure is that it does not take post processing into account, such as mastering. That will change the noise floor. And why would you not strive to make the audio as good as possible? IMO it's not a very "pro" tool because of this.

woodsdenis wrote: »

TBH PT HD has been this way for years and no one has had an issue with this AFAIK

Except those people who still use tape, or use an external analogue mixer or summer, or avoid PT and use Logic, Fairlight, Samplitude etc. etc. Seems to be a lot of that about!

woodsdenis wrote: »

As regards PT Native I think that uses the LE mixer architecture so this scenario does not occur. Do any LE or Native users have the Dithered mixer plug in their systems?

The dithered mixer is not supplied with LE or Native because it is designed to solve the problem of sending data between slots on the TDM bus.

Doge

You should'nt be so easily "dithered" from getting on with your musical creations MT!