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It’s conceivable that you might want to make a 24-bit recording directly into your DAW. In
this case, set your levels carefully and/or use some preprocessing in the analog domain to ensure
that the audio signal fits into the conversion window.
Because our ears are unable to detect short-duration clipping, the AD9624’s meter requires
three consecutively clipped samples in order to indicate clipping. Whatever you do, you should
try to avoid seeing the C (clip) segment on the AD9624’s meter.
Clock and Sync Issues
Any time that you have multiple digital audio signals at one time the issue of sync becomes
important. If you are using multiple AD9624s for multitrack work, it may be necessary to
synchronize the converters in all of the different converter boxes. You do this by using the
Word Clock input on the rear of the unit. You can chain multiple units together by using a Tee
connector. In many cases, the digital multitrack can handle non-synchronized sources, so this
may not be an issue. Experimentation (before that important session) is the key here.
To Dither or not to Dither, That is the question.
What is dither? Dither takes two forms: very low level white noise added to the signal before
conversion, or a similar process performed after conversion via a DSP chip. In some respects,
dither allows 16-bit digital to act more like good old-fashioned analog. No, there’s no tape
compression, but the ability to resolve details that reside in the noise floor (like ambience,
decay, and space) remains.
The second dither method mentioned above takes place in the digital domain. Having a 24-bit
wordlength allows this with a minimum of compromise. We can add white noise at a suffi-
ciently low level to only modulate the last one or two bits of the 16-bit signal. This has the
effect of putting a slight veil on the signal, resulting in reduced soundstage, definition, and
clarity.
An improved method (which of course is what the AD9624 uses) is known as “noise shaping.”
Noise shaping alters the audibility of the dither signal by equalizing or shaping its spectrum.
This minimizes its audibility by moving the noise away from the parts of the spectrum where
our ears are most sensitive (-3kHz) and placing it into an area where our ears are less sensitive
(10-22kHz).
When should you dither? Once, and once only. Dithering should be your last step upon leaving
the 24-bit domain and entering the 16-bit domain.
Summary of Contents for AD9624
Page 1: ...AD9624 DA9624 24 Bit Converters User s Guide...
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