Dither and Noise Dither and Noise ShapingShaping

When do we use When do we use DITHER?DITHER?

• Most commonly when Most commonly when Converting DIGITAL AUDIO Converting DIGITAL AUDIO BITRATESBITRATES

• Ex: 24bit Ex: 24bit 16bit for CD 16bit for CD

What is DITHER?What is DITHER?

It is actually adding NOISE to an audio It is actually adding NOISE to an audio signal.signal.

We make a trade:We make a trade:

A little low-level hiss is added BUT…A little low-level hiss is added BUT…

There is a big reduction in There is a big reduction in DISTORTION. DISTORTION.

16-Bit CD Audio16-Bit CD Audio

• Remember: 16-bit CD has a 96dB Remember: 16-bit CD has a 96dB MAX dynamic rangMAX dynamic rang

• BUT we don’t ALWAYS listen to our BUT we don’t ALWAYS listen to our music at its full dynamic rangemusic at its full dynamic range

More on BITS…More on BITS…

• We are using less BITS when volume We are using less BITS when volume level is not FULLlevel is not FULL

• QUIETER digital audio has more QUIETER digital audio has more DISTORTION because it is utilizing DISTORTION because it is utilizing less of those BITS!less of those BITS!

Here you can see the low bitrateHere you can see the low bitrateHere you can see the low bitrateHere you can see the low bitrate

TRY THIS ANALOGY:TRY THIS ANALOGY:

Spread your fingers and hold them up a few inches in Spread your fingers and hold them up a few inches in front of one eye, and close the other. Try to read this front of one eye, and close the other. Try to read this text. Your fingers will certainly block portions of the text. Your fingers will certainly block portions of the text (the smaller the text, the more you'll be text (the smaller the text, the more you'll be missing), making reading difficult.missing), making reading difficult.

Wag your hand back and forth (to and fro!) quickly. Wag your hand back and forth (to and fro!) quickly. You'll be able to read all of the text easily. You'll see You'll be able to read all of the text easily. You'll see the blur of your hand in front of the text, but the blur of your hand in front of the text, but definitely an improvement over what we had before.definitely an improvement over what we had before.

The blur is analogous to the noise we add in The blur is analogous to the noise we add in dithering. We trade off a little added noise for a dithering. We trade off a little added noise for a much better picture of what's underneath. much better picture of what's underneath.

The LSB. What is that?The LSB. What is that?

• LSB stands for the LEAST LSB stands for the LEAST SIGNIFICANT BITSIGNIFICANT BIT

• This is where the noise is This is where the noise is addedadded

• It spreads individual ERRORS It spreads individual ERRORS into NOISE into NOISE

In other words…In other words…• Creating NOISE actually helps Creating NOISE actually helps us hear LOW LEVEL SIGNAL us hear LOW LEVEL SIGNAL BETTER!BETTER!

So when does dither So when does dither happen?happen?

• Converting Sample RatesConverting Sample Rates• Mixdown of a digital projectMixdown of a digital project• Normalizing: What is that?Normalizing: What is that?

Your DAWYour DAW

• The good news is that your The good news is that your software usually does this software usually does this automaticallyautomatically

• Bitrate conversion often allows for Bitrate conversion often allows for the optionthe option

The Power of our BRAINThe Power of our BRAIN• Hearing through dither is like picking Hearing through dither is like picking

out a voice in a crowded roomout a voice in a crowded room• Our brain can separate low level Our brain can separate low level

sound from the noise created by sound from the noise created by ditherdither

In Short…In Short…• Dither is INTENTIONAL Low-Level Dither is INTENTIONAL Low-Level

NOISENOISE

• Is used in DIGITAL SYSTEMSIs used in DIGITAL SYSTEMS

• It’s better than DISTORTION It’s better than DISTORTION

Finally… Noise ShapingFinally… Noise Shaping

• This uses DITHER to make it SEEM This uses DITHER to make it SEEM like there is a lower NOISE FLOORlike there is a lower NOISE FLOOR

• This also uses the ‘Fletcher-munson’ This also uses the ‘Fletcher-munson’ curve: It considers how we HEARcurve: It considers how we HEAR

• Again, fooling our brain into hearing Again, fooling our brain into hearing something elsesomething else

Listening examples (bottom of page):Listening examples (bottom of page):

http://www.hydrogenaudio.org/musepack/klemm/www.personal.uni-jena.de/~pfk/mpp/dither.html