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Dither: Compensating for a Digital Problem

But what about volume levels that fall in between two steps? Too bad—the analog to digital converter can't accept "in between" values. It must round off a volume level to one step or another. (This problem doesn't exist in analog recording; analog equipment records subtly changing electric voltages without assigning them to quantization stair steps.) The difference between the wave's actual voltage versus the stair step it must be rounded off to is called quantization error. This quantization error can lead to audible noise, which is more noticeable with low amplitude (low volume) recordings. That's why it's a good idea to record at the maximum level of input that your system can handle.

Hot Link: EQ Magazine (http://www.eqmag.com/)

The good thing about this magazine is that even though it's information-packed, it's still accessible to the beginner (see Figure 3.10).

Figure 3.10 EQ magazine has a lot of articles for the desktop audio designer.

Low amplitude waves cause problems for digital recorders. The difficulties arise when the analog to digital converter rounds off the sound wave's voltage to the closest stair step. That's not a problem with high amplitude sound waves, which use the full quantization range, or most of the 65,536 values. But since low amplitude signals use only the bottom few steps, the recorder doesn't have many stair steps to round off to. This leads to inaccuracies in the digitization process that can cause quantization distortion.

But this is no reason to dump your PC and go back to analog recording, because a good digital recording system compensates for low signal quantization distortion using dither. (Finally, a technical term that has a fun name. Just say it to yourself a few times...dither...it's a lot more pleasant than quantization value, huh?)

Hot Link: Mix Magazine (http://www.mixonline.com)

You've got to be a serious dude or dudette to read Mix magazine (see Figure 3.11). It's the recording industry trade magazine, and it's aimed at professionals. Still, even if you're not mixing big-budget film sound in L.A., it's got a ton of good information. If you want to know how the pros do it, take a look at Mix.

Figure 3.11 Mix magazine is geared for the audio professional.

Dither is a small amount of white noise—yes, that's right, a digital recorder actually adds noise to the sound wave. (This allows the analog aficionados to say, see, analog is better. But not so fast.) This noise is added to the sound wave before it is digitized. This aids the quantization process by allowing samples with insufficient bit depth to be properly quantized. This added noise gives a necessary boost to the low amplitude sound waves, allowing them to retain their original shape. It helps the input signal to come closer to the quantization stair steps much more often. Moral of the story: A little added noise (in the form of dither) is a good thing.

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