Lucid AD9624, User Manual

Page: 14 / 24

12
S
ECTION
5 U
SING THE
AD9624 &
THE
DA9624
This chapter discusses various aspects of the operation of each converter. For the most part, this
applies to both units, but if it doesn’t, we’ll let you know.
Why 24-bit?
The obvious answer is: “More is better.” If you need more, consider:
· The trouble zone for any converter is the last (lowest) one or two bits of resolution. At 24-
bit resolution, the amount of signal represented gets smaller, which means that if there is
trouble in the lowest bits, this represents less of the overall signal. Remember that the
output signal swing is fixed, so we’re simply dividing the output signal into smaller and
smaller pieces. Yes, in 24-bit converters, the output noise floor is also lower, and so we’re
actually getting two extra benefits for using a higher resolution converter.
· If you’re mastering directly to DAT, using a 24-bit converter and letting the converter
dither the result to 16-bit is superior to using the 16-bit converters in your DAT machine.
· For some material (especially if it has little dynamic range), you may get superior results by
converting to 24-bit and throwing away the last 8 bits (truncation).
· If you’re downloading into a digital audio workstation (DAW) directly from your mixer
(and you have a 24-bit capable DAW), then it makes sense to use a high-resolution con
verter. Perform all workstation tasks at 24-bit wordlength, and do the final dithering to 16-
bit when you record the CD master disk. When you fiddle with signals in DSP, the simple
act of changing the gain can increase your wordlength. Having the extra resolution at the
onset means less error when the increase in wordlength exceeds the internal capabilities of
your DAW software (rounding error).
AD9624 Level Setting
Like any A/D converter, the converter’s digital output represents a fixed window that must
properly frame the audio signal. Set it too low, and lose the signal peaks; set it too high and lose
the low-level detail.
In the 16-bit world, when that was all there was for conversion, the placement of the converter
window was more critical due to the larger step size represented by each bit of the digital
output. Low level details in the signal represent things like ambience and soundstage; thus they
are critical details. Capturing these details faithfully means becoming more adept at either
setting the window properly or fitting the signal into the window itself via dynamic range
compression.
A 24-bit converter eases this task somewhat. Since the steps are smaller, you now have the
option of placing the audio signal either a bit higher (better resolution for low-level signals, but
lower headroom) or lower (less resolution for low levels, but more headroom). Of course, if the
signal is already recorded, then it’s just a matter of getting levels set properly and then making
the transfer into your digital recorder or workstation.