DBX Type IV Conversion User Guide
dbx Type IV™ Conversion System
White Paper
Roger Johnsen
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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 e xploits the wide linear dynamic r ang e 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 headr oom that is inherent in analog r ecording 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 c haracteristic 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 Har monic 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 w e’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 90-something 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 e xhibits wide linear dynamic
range,when you run out of headroom for high-level signals,hard clipping or even ugly signal
Type IV™ Conversion System White Paper - August 1998
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dbx Type IV™ Conversion System
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wrap-around occurs, not to mention that A/D converter s 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 ensur e that ther e is ample headroom to allow for the large peaks
that would ruin an otherwise perfect recording. This,of course,compromises signal-to-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—T ype I™,Type II™,and Type III™—dbx T ype 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-to-digital conversion
process.
The dbx Type IV™ Conversion System combines proprietary analog and digital
processing techniques to capture a much wider d ynamic rang e 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 rang e to create a logarithmic “overload r egion.” 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 logar ithmic 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
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