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Aphex Model 2020 -vs- Orban Model 8200 "Optimod"
By Donn Werrbach, VP of Engineering, Aphex Systems Ltd
Introduction
This paper discusses the merits and advantages of the new Aphex model 2020 FM Pro in
relation to the Orban model 8200 Optimod.
An Historical Account
Several years ago, Orban introduced their Model 8200 digital processor to a broadcast
industry embarking on a feverish "go all digital" agenda. The fever was fueled
by a general ignorance in the industry about the limitations of digital audio signal
processing. An incredible amount of misinformation was spread throughout the industry
by well meaning but misinformed digital audio proponents. Now, after years of battleground
digital experience in the broadcasting industry, the true advantages and disadvantages
of the digital media are better known. It is recognized that the so-called "all
digital" audio chain considerably degrades the sound quality. Consequently, broadcasters
are starting to make better use of both digital and analog technology to achieve more
ideal results.
The naïve belief that digital processing is inherently better
than analog is finally falling apart. It has become widely acknowledged, within the
broadcast community, that the nonlinear digital signal processing artifacts of the Model
8200 result in audible and undesirable "digital grunge". Nevertheless, Orban’s
Model 8200 supplied, without meaningful competition for many years, a strong market
demand for a true digital audio processor. As a result there are a great many radio
stations running with Model 8200’s on the air. Amid the present day controversy
over digital audio processing, Orban claims that owners actually love the synthetic
sound quality of the Model 8200. The reality is that dissatisfaction is growing rapidly.
An alternative that offers better sound quality is sought by the market place. Whether
that alternative is of digital or analog technology is of less importance than quality
of sound. The fact is that advanced analog signal processing still yields the best sound
according to most listeners.
FM Broadcasters are ready for the next generation audio processor.
One without digital grunge but with all the features and conveniences afforded by digital
control: an analog audio processor that readily connects with digital audio systems
while maintaining high resolution and low distortion. They are ready for the Aphex Model
2020 FM Pro.
The Aphex Model 2020 is a new benchmark for FM audio processing.
It is louder, better sounding, more flexible, and easier to use. The following comparison
shows the many advantages of the Model 2020 when used in digital, analog, and hybrid
audio systems.
The Comparison
Loudness
Subjectively, the 2020 can be tuned louder than 8200 with more appealing results. The
8200 quickly becomes distorted and irritating when it is driven into competitive loudness
settings. The 2020 gathers loudness with greater clarity and punch, avoiding the artifacts
of the 8200.
Some of the 8200’s degrading factors that worsen with high
loudness are: raspy multiband clipping, extreme equalization contouring, pumping, depressing
of the vocals, and frequency band dissection making the spectrum seem "overhyped".
Noise
When set for dynamic processing headroom comparable to that available on the 2020, the
analog-to-digital conversion noise of the 8200 is so high that it is plainly visible
over the pilot tone when displayed on an oscilloscope. Programs processed by the 8200
also contain audible and annoying noise modulation. Voices carry perceptible quantizing
noise that comes and goes with articulation, an artifact sometimes referred to as voice
"hair". During low program levels, when the leveling gain is increased and
compression is also releasing, a large amount of gritty background noise is audible.
In many cases, a breathing background noise is audible behind music selections. Under
similar circumstances the 2020 sounds quiet and clean.
Fatiguing Distortion
One of the common complaints by users of the 8200 is the grunge, brassiness and fatiguing
quality of its sound- even at mild settings. Many hours of investigation suggests this
is caused by dynamic control loop instability and inadequate sampling rate of the various
digital processing algorithms of the 8200.
A host of revealing evidence of the 8200’s defects can be seen
just by sweeping a sinewave through it and observing the output on an oscilloscope.
You will immediately observe the multiband clipping at most frequencies. At higher frequencies,
i.e., above 2KHz, you will observe severe beat frequencies that are digital artifacts
of poor DSP technique. At certain frequencies and amplitudes you will observe the audio
envelope to be ripped and saw-toothed, indicative of unstable limiting or compression
loops, again a particular DSP limitation. These anomalies are indeed audible as you
can prove just by listening. It is ludicrous to claim they are inaudibly masked in program
material or that tone tests are irrelevant.
The 2020 uses clean and stable processing methods that generate
no beat frequencies or other anomalies as described above.
Loudness & Distortion
The 8200 gathers loudness by the liberal use of multiband clipping. This is part of
the 8200’s processing strategy and is a legacy of the prior XT analog processor.
Multiband clipping causes a thick, buried kind of distortion which makes the sound "loud"
but not "big". Rather than creating a large enveloping loudness, this technique
creates a crunched, unflattering, one dimensional effect diminishing the apparent size
of the stereo image. The main advantage (in one way of thinking) of this kind of multiband
clipping is to transform widely varying program sources into a "lowest common denominator"
of average sound quality. Taken to competitive limits, this method creates unlistenable
sound. In contrast, the Model 2020 uses no multiband clipping but depends upon an intelligent
interaction between a post-crossover multiband compressor (patented) and a separate,
independent, phase coherent distortion cancelled multiband limiter (patented). With
the 2020 you can achieve anything from a big enveloping loudness to a flat, thick wall
of sound by proper balancing of the processing. At comparable extreme loudness settings,
the Model 2020 sounds bigger, cleaner, and better.
Vocal Attenuation
Another complaint about the 8200 is the feeling that vocals are pushed back into the
mix, making lyrics hard to hear. The problem gets worse when you need voice focus most:
in dense mixes. This is the result of the 8200’s particular processing strategy.
The problem is endemic and cannot be corrected by any of the 8200’s user adjustments.
By comparison, the Aphex patented "Automatic Limiting Threshold" (patented)
and "Wave Dependent Compressor" (patented) algorithms help the 2020 to balance
vocals more naturally, allowing even the densest mixes to remain fully intelligible.
You never get the impression something is being "pushed back" out of the mix.
Fullness Of Bass
Another complaint is the lack of any real low frequencies coming through the 8200. Bass
sounds like it is restricted and forced up into a narrow band of energy. Music containing
deep bass elements loses feeling. The 2020 handles bass energy in a totally different
manner specifically allowing bass to fully develop dynamically at all bass frequencies.
A specialized bass processor in the 2020 allows you to enhance the bass very musically,
not simply through boosted equalization as with the 8200. The 2020 uses a patent pending
dynamic process that fills the modulation envelope with bass energy to the greatest
possible extent consistent with instantaneous overload distortion constraints.
Bass Continuity
The transition of notes from the high to low bass region is not seamless with the 8200
taking on a "dissected" characteristic. Very low bass notes seem to disappear
while high bass notes take on a different tonal character. In contrast, the 2020’s
bass is very fluid and flows seamlessly throughout the bass frequency range, holding
a more natural or even enhanced balance of loudness as notes progress lower in the range,
compensating for the dynamics restrictions of FM transmission. The model 2020 produces
the best sounding bass of any audio processor yet developed.
Analog Advantage
The model 8200 suffers from typical DSP errors, i.e., truncation and rounding errors,
and further multiplication of those errors as the processing continues. These errors
are manifested as very characteristic digital audio distortions and are especially acute
in the 8200 because of it’s fixed-point DSP architecture, minimal digital audio
sample rate, and economized DSP engine. In comparison, the model 2020 processes in the
analog domain using premium circuitry and is thus inherently more perfect at retaining
the audio quality.
Digital I/O & Sample Rate Problems
Information revealed by Orban indicates the Model 8200 DSP sample rate runs at merely
32KHz and is spot upsampled to only 128KHz at a few critical points. This is not sufficient
sampling to avoid frequency aliasing that is caused by nonlinear digital signal processing
algorithms such as compression and limiting. It is also too slow a sample rate for some
of the agc control loops to remain stable. You can see the instability while observing
sinewaves in limiting. You'll observe a sawtooth shaped amplitude irregularity that
translates as a raspy effect on the sound (more grunge). Using perfectly stable analog
algorithms, the Model 2020 avoids all these grunge mechanisms. Digital I/O of the Model
2020 is handled by accepting any sample rate at the digital input, converting it to
48KHz, then converting the digital to analog for processing. The digital audio input
is fully reconstructed by the comprehensive analog processing chain, maximally de-quantizing
the audio signal. The Model 2020 then converts the finished analog audio back to 20-bit
digital at any selected output sample rate. This makes the model 2020 the best quality
sample rate converter available as well as a better digital audio processor from the
viewpoint of optimum and clean signal processing.
Digital FM Exciters
Digital I/O options of the 2020 not available with the 8200 create greater flexibility
and facilitate much higher audio quality in a digital audio system including interfacing
with digital FM exciters. For example, the Harris "Digit" digital FM exciter
can accept the full 20-bit AES/EBU bitstream at 48KS/s rate from the model 2020. On-air
comparisons have shown there is a very discernible improvement in the sound of the Digit
at each incremental change when switching between 32, 44.1, and 48K sample rates.
A complaint about the 8200 is the large d.c. drift present at the
composite and discrete outputs. The drift is the result of the 8200’s DSP processing
error and can cause alarming carrier frequency offsets. The only workaround currently
available is to turn on the 8200’s 30Hz highpass input filter. This further degrades
the already lacking bass feel of the 8200.
The 2020 suffers no d.c. drift problems. To absolutely guarantee
d.c. stability in the digital domain, we invented a patent pending method specifically
to cancel all d.c. a/d converter digital output drift. The 2020 delivers deep and full
bass, with rock solid digital and analog domain d.c. stability.
Flexibility
The philosophy behind the 2020 is to allow users a great latitude in the setting up
of processing to accommodate a wide range of individual requirements and preferences.
The 8200 much more severely restricts the range of adjustments you may try and omits
many key elements and parameters from user control altogether. For example, the 8200’s
release time can only be set to one of four speeds (not continuously adjustable), and
all bands have to be set equally. The 2020 allows you to set the bands individually
and with continuous range so you can find the "sweet spots".
Metering
The 2020 front panel metering shows the activity of all important processing functions
continuously and in real time. Metering on the 8200 is by way of an LCD panel which
only approximates the activity of the audio processor and requires you to switch between
screens to see all the meters. With the 2020 you get a picture of the audio processing
at a single glance.
Stereo Insert
The 2020 allows you to insert additional audio processing at the ideal location: between
the leveler and multiband compressor. The 8200 has no capability for external processing
to be inserted.
Other 2020 Advantages
Auto Log-Out Feature
Presets Auto Save Feature
Input Auto Source Feature
Hard-wire Bypass Feature
Universal Power Input
Optimal Panel Size of 2RU
end
Trademark notification:
"FM Pro" is a trademark of Aphex Systems Ltd.
"Orban" and "Optimod" are trademarks of Harman International Industries,
Inc.
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