dbx Type IV™ Conversion System
White Paper
Roger Johnsen
Director of Engineering, dbx Professional Products
®
The dbx Type IV™ Conversion System is a proprietary analog-to-digital (A/D)
conversion process that combines the best attributes of digital conversion and analog
recording processes to preserve the essence of the analog signal when it is converted to a
digital format. dbx Type IV™ not only exploits the wide linear dynamic range of today’s
A/D converters, but also enhances it and extends the useable dynamic range beyond the
linear range. By providing a logarithmic "Type IV™ Over Region" above the linear A/D
range, we benefit from the extended high-level headroom that is inherent in analog
recording without compromising the noise performance of the A/D conversion process.
Digital conversion and recording processes proliferated in the 1980’s primarily due
to the "cleaner" sound of digital versus analog, an advantage resulting from the comparatively wider linear dynamic range of digital. Anyone who is familiar with the technical
specifications of digital equipment knows that the typical maximum signal-to-noise
specifications for 16-bit systems is in the neighborhood of 90-something dB. Compare this
to the typical signal-to-noise specifications for professional analog tape of about 55 dB
without the aid of noise reduction and around 75 to 85 dB with noise reduction such as
dbx Type I™ or Type II™ applied.
This seemingly tremendous signal-to-noise advantage of digital over analog would
suggest that digital would become the unanimous choice for recording. For the most part
this has occurred, not totally due to its signal-to-noise advantage, but as much due to the
benefits of digital storage such as random access and the inherent ability to withstand
degradation, unlike that of analog tape or LP’s. In spite of the benefits of digital, no one in
the audio world can refute the rediscovery of analog recording and tube gear that has
occurred in the 90’s, attributable to the quest for that "analog character" that is missing
from digital recordings. This continued use of analog gear with modern digital systems
brings to light a favorable characteristic of analog recording which those who abandoned
analog and jumped on the digital bandwagon were either never aware of or simply took
for granted.
Anyone who has ever used analog tape knows that you can "hit it hard" without
destroying the recording. The printed specifications of analog tape don’t take into
account the practical headroom available. The max signal-to-noise specification of analog
tape is measured by defining the "max" signal as the point where a given signal level and
frequency produces a given percent Total Harmonic Distortion (THD)—typically the level
at which a 1 kHz signal produces 3% THD. In actual use, the signal can easily exceed this
"max" signal level by 5, 10, or even 15 dB on peaks, depending on the type of signal being
recorded, without unacceptable artifacts. High signal levels can be tolerated (i.e. more
headroom) at the expense of increased THD which, incidentally, is often desirable as an
effect, evidenced by the renewed popularity of tube equipment.
The obvious conclusion is that analog recording actually has more useable dynamic
range than the specifications seem to indicate. For example, let’s say we’re recording a
kick drum. If analog tape measures 55 dB from the 3% THD point down to the RMS noise
floor and the peaks of the kick drum exceed the 3% THD level by, say, 15 dB and it still
sounds good, then we have 15 dB of extra useable headroom. Therefore, we end up with
70 dB of useable dynamic range. Throw in noise reduction and we push into the 90something dB dynamic range territory of 16-bit digital. This explains why well-recorded
analog master tapes make good-sounding CD’s with no objectionable noise.
One main drawback of digital is that it inherently lacks this forgiving and beneficial
characteristic of analog recording. Although digital conversion exhibits wide linear
Type IV™ Conversion System White Paper - August 1998
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dbx Type IV™ Conversion System
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dynamic range, when you run out of headroom for high-level signals, hard clipping or even
ugly signal wrap-around occurs, not to mention that A/D converters have their own nasty
side effects such as going unstable when their modulator is overdriven with high-level
signals.
This shortcoming of digital conversion has drastically affected the way users
operate their equipment. Users are paranoid of overdriving the converter input and end
up recording at lower levels to ensure that there is ample headroom to allow for the large
peaks that would ruin an otherwise perfect recording. This, of course, compromises signalto-noise performance since the signal is now closer to the noise floor. Because users of
digital equipment have to be extremely careful not to exceed 0 dB FS (full-scale), they must
use peak-reading headroom meters. On the other hand, the forgiving nature of analog tape
allows users of analog recording equipment the luxury of only needing to monitor the
average level using VU meters, often having no peak indicators whatsoever. If only digital
were more forgiving like analog, we could really exploit its wide dynamic range and more
completely capture the essence of the musical performance.
Enter the dbx Type IV™ Conversion System. Like its related predecessor
technologies—Type I™,Type II™, and Type III™—dbx Type IV™ succeeds in preserving
the wide dynamic range of the original analog signal within a limited dynamic range
medium. Whereas Type I™ and Type II™ expand the dynamic range of analog tape and
other limited dynamic range media, and the simultaneous encode/decode process of Type
III™ similarly expands the limited dynamic range through minimum-delay devices,Type
IV™ breaks new ground by greatly enhancing the useable dynamic range of the analog-todigital conversion process.
The dbx Type IV™ Conversion System combines proprietary analog and digital
processing techniques to capture a much wider dynamic range than the A/D converter
could by itself, preserving the maximum amount of information from the analog signal.
This information is then encoded within the available bits of whichever A/D converter is
used. This means that Type IV™ improves the performance of any A/D converter, from
low-cost 16-bit to high-performance 24-bit! And no decoding is necessary beyond the
conversion process!
As we have previously mentioned,digital systems have a wide linear region
compared to analog tape and the dynamic range of A/D converters has improved significantly in recent years. The dbx Type IV™ Conversion System takes advantage of this and
utilizes the top 4 dB of the A/D converter’s linear dynamic range to create a logarithmic
"overload region". This allows high-level transient signals passing far above the point
where the overload region begins to be adequately represented in just 4 dB of the
converter’s dynamic range, whereas a typical A/D converter would clip. With Type IV™,
you can never clip the A/D converter!
Fig. 1 illustrates this concept showing the level of the converted signal below and
above the start of the overload region. The converted signal level is plotted along the Y-axis
(vertical axis) of the plot vs. the level of the input signal along the X-axis (horizontal axis).
The logarithmic mapping of the overload region begins 4 dB below 0 dB FS (full-scale) of
the A/D converter. What this shows is that below -4 dB FS, in the linear region, the output
signal is the same as the input signal. Above this, in the logarithmic region, high-level input
signals get "mapped" into the top 4 dB of the A/D converter. This mapping is analogous to
the signal compression effect that occurs when recording high-level signals onto analog
tape.
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dbx Type IV™ Conversion System
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".
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dbx Type IV™ Conversion System
Converted
Signal
Level
dB FS
0
-4
A/D Clip Point
Linear Region
n
o
i
g
e
R
c
i
m
h
t
i
r
a
g
o
L
-4
0 +4 +8 +12
Figure 1 - Converted Level vs. Input Level
dB
Input
Signal
Level
Input
Signal
Level
TYPE IV™
Over
+12
+8
+4
0
-4 dB -4
Region
Converted
Signal
Level
0 dB FS
Am
A/D Converter
Linear Region
Noise Floor
Figure 2 - Input Signal Levels Mapped to Type IV Over Region
One might question the validity of such an approach—trying to represent 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 evenlyspaced, 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 fullscale! 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
information, depending on the quality of the converter. It seems not only reasonable, but
also desirable, to utilize the increased signal resolution afforded by this density of digital
codes to represent more input dynamic range in this region.
Another advantage of the logarithmic mapping of our dbx Type IV™ Conversion
System is that it preserves the high-frequency detail of the signal in the overload region.
Figs. 3a through 3d illustrate what happens when you overload an A/D converter without
Type IV™. Fig. 3a shows an input signal having both low-frequency and high-frequency
components. When the signal overloads, or clips, (Fig. 3b) at the A/D converter, a disproportionate amount of high-frequency signal information is lost compared with the lowfrequency information. The low and high-frequency components of the signal are
separated in Fig. 3c to illustrate this more clearly. As you can see, the low frequency signal
simply gets distorted but maintains most of its signal characteristics, while sections of the
high-frequency signal are completely lost! With dbx Type IV™, its mapping preserves
high-frequency signal information, as illustrated in Fig. 3d, since the signal is confined
within the Type IV™ Over Region and never clips. The dashed line indicates the original
input signal level. Below the Over Region no mapping occurs, while above this, mapping
keeps all peaks of the signal below the A/D clip level, thus preserving the high-frequency
content of the signal.
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dbx Type IV™ Conversion System
Amplitude
r
Figure 3a - Signal Having Low and High Frequency Content
Time
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dbx Type IV™ Conversion System
Amplitude
A/D Clip Level
Time
A/D Clip Level
Amplitude
Figure 3b - Signal of Fig. 3a Going Beyond the A/D Clip Level
High-Frequency Information Completely Lost
High-Frequency Content
A/D Clip Level
Low-Frequency Content
Time
A/D Clip Level
High-Frequency Information Completely Lost
Figure 3c - Disproportionate Loss of High-Frequency Information Due to Clipping
Now you’re probably wondering, "What’s the catch? I can’t get something for
nothing so what did I give up?" You may be worried that your A/D noise floor got 4 dB
worse because we borrowed the top 4 dB of your converter. This is certainly a valid
concern. Fortunately, we have the answer! Without going into the confidential technical
details, by using our proprietary analog and digital Type IV™ processing, we reclaim the
original A/D noise level! So what you get is free headroom!
The benefits of the dbx Type IV™ Conversion System can easily be heard by
switching it in and out while listening to signals with high-level peaks captured in the
Type IV™ Over Region. You will notice an obvious audible difference. With Type IV™
bypassed, you can’t help notice the harsh, edgy sound of the A/D converter clipping. With
Type IV™ enabled, those nasty artifacts disappear revealing a more open and natural
sound. With Type IV™ enabled,you will get a more accurate and pure representation of
the original wide-dynamic-range signal. You will absolutely agree that we really do give
you "something for nothing". We give you peace of mind knowing that you never have to
worry about clipping your A/D again! And when you listen to the noise floor of your A/D,
you’ll realize that we never compromise your noise performance with
Type IV™!
The dbx Type IV™ Conversion System succeeds in combining the best of the
analog and digital worlds to capture the truest essence and fullest dynamic range of audio
signals. Who else but dbx would bring you this technology!
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dbx Type IV™ Conversion System
Amplitude
0 dB FS
-4
-4
0 dB FS
Figure 3d - Type IV Mapping Preserves High-Frequency Information
A/D Clip Level
TYPE IV™ Over Region
}
Time
TYPE IV™ Over Region
}
A/D Clip Level
®
8760 South Sandy Pkwy.
Sandy, Utah 84070
Phone: (801) 568-7660
Fax: (801) 568-7662
Int’l Fax: (219) 462-4596
Questions or comments?
E•mail us at: customer@dbxpro.com
or visit our World Wide Web home page at:
www.dbxpro.com
A Harman International Company
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