dbx Type IV
TM
White Paper
504X
504X Digital Output Card
®
17
Fig. 2 illustrates the mapping function in a different way. Input levels are
shown on the left of the graph, while converted levels are shown on the
right. Notice the mapping of large signal excursions to the 4 dB “Type IV™
Over Region.”
One might question the validity of such an approach—trying to repre-
sent a lot of signal information within a smaller “space.” The reason why
this is not only valid but makes a whole lot of sense is that the digital codes
in a converter are linear, or evenly-spaced, meaning that each consecutive
code represents the same change in voltage of the input signal. This
implies that half of the digital codes are used to represent input signals
whose voltage level is below 1/2 of the full-scale A/D input voltage, while
the other half of the codes are used to represent signals above 1/2 of the
full-scale A/D input voltage. This seems reasonable until you realize that
1/2 of the full-scale input is only 6 dB below full-scale! So half of the codes
are used to represent only the top 6 dB of signal information, while the
other half are used to represent the remaining 80 to 110 dB of signal infor-
mation, depending on the quality of the converter. It seems not only rea-
sonable, but also desirable, to utilize the increased signal resolution afford-
ed by this density of digital codes to represent more input dynamic range
in this region.
Figure 2 - Input Signal Levels Mapped to Type IV Over Region
-4 dB
-4
0
+4
+8
+12
Input
Signal
Level
0 dB FS
TYPE IV™
Over
Region
Noise Floor
A/D Converter
Linear Region
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