Orban
Optimod-FM 8400 Signature Series

Tech Talk

OPTIMOD-FM 8400 Signature Series Tech Talk:

Signal Processing

By Robert Orban
Chief Engineer

Orban's new OPTIMOD-FM 8400 contains exciting signal processing innovations that clearly advance the state-of-the-art in FM on-air processing.

New Controllability

Many users have requested more controllability—they want to be able to "get under the hood" and custom-tweak the processing at a level of detail unavailable from Orban's OPTIMOD-FM 8200. In the 8400, we provide three levels of control.

The first, BASIC CONTROL, features Orban's popular one-knob LESS/MORE adjustment. The single control knob affects many different processing parameters, letting you optimize the trade-off between loudness, brightness, and distortion.

In the 8400, LESS/MORE affects only the dynamics processing (compression, limiting, and clipping). Unlike the 8200, the 8400 has equalization and stereo enhancement that are decoupled from LESS/MORE. So you can change EQ or stereo enhancement and not lose the ability to use LESS/MORE. When you create a user preset, the 8400 will automatically save your EQ and stereo enhancement settings along with your LESS/MORE setting. When you recall the user preset, you will still be able to edit your LESS/MORE setting if you wish.

The second level of control, INTERMEDIATE, is similar to the level of control available in the 8200's FULL CONTROL screen. It exposes controls that require more experience and sophistication than BASIC CONTROL to use correctly. This level of control allows you to edit the dynamics processing.

The third level of control is ADVANCED and makes available to the user virtually every control available to the 8400's factory programmers and developers. The only ones we left out are a few relating to some new proprietary techniques and algorithms. (Please don't ask…they really are proprietary!) For example, for the first time you can access attack time, release time, and threshold controls for the compressors in each band.

Dual-Mono Architecture

Per user request, the 8400 implements full dual-mono architecture in both the AGC and the multiband compressor sections. You can couple each band in both the AGC and multiband compressors to a variable extent—anywhere from perfect stereo coupling to completely uncoupled operation. The coupling control determines the maximum amount of gain imbalance permitted between the left and right channels in a given band, and therefore the amount of stereo image shift permitted in each frequency band.

Although the processing is dual-mono, you cannot adjust setup controls independently on the left and right channels. We assumed that the 8400 would always process stereo program material.

Signal Flow

The signal flows through the 8400 through the following blocks:

Input Conditioning: The 8400 operates at 32 kHz sample rate and multiples thereof (up to 512 kHz in the stereo encoder). No commercial A/D converters or sample rate converter chips convert to 32 kHz to the standards we demanded for this new product. To ensure high quality A/D and sample rate conversion, we operate both the SRC and A/D chips at 64 kHz output sample rate and then downsample to 32 kHz in DSP. By designing and implementing our own downsampler, we can ensure full frequency response to 15 kHz as well as very low spurious images.

Despite propaganda from competitors who have attempted to confuse the marketplace regarding the supposed superiority of higher sample rates, we continue to believe that 32 kHz is far preferable to 48 kHz or 96 kHz as a basic sample rate for FM audio processors. It allows us to use DSP horsepower much more efficiently, adding features that really improve the sound. It also makes it easier to spectrally protect the stereo pilot tone and RDS subcarriers by strictly limiting our output bandwidth to 16 kHz. Like the 8200, the 8400's output spectral control is immaculate, ensuring maximum stereo and RDS coverage. Moreover, the 8400's digital output will pass through any uncompressed digital STL without added overshoot and without the need for distortion producing overshoot compensation schemes.

A defeatable 30Hz 18dB/octave highpass filter and a defeatable phase rotator complete the input-conditioning block. These have both been features in Orban FM processors for many years. Most users will defeat the 30Hz filter and leave the phase rotator in-circuit, although the choice is always yours.

Stereo Enhancement: The 8400 provides two different stereo enhancement algorithms. Based on Orban's patented analog 222 Stereo Enhancer, the first increases the energy in the stereo difference signal (L–R) whenever a transient is detected in the stereo sum signal (L+R). By operating only on transients, the 222 increases width, brightness, and punch without unnaturally increasing reverb (which is usually predominantly in the L–R channel.)

Gating circuitry detects "mono" material with slight channel or phase imbalances and suppresses enhancement so this built-in imbalance is not exaggerated. It also allows you to set a "width limit" to prevent over-enhancement of material with significant stereo content, and will always limit the ratio of L–R/L+R to unity or less.

The second stereo enhancement algorithm is based on the popular "max" technique. This passes the L–R signal through a delay line and adds this "decorrelated" signal to the unenhanced L–R signal. Gating circuitry similar to that used in the "222-style" algorithm prevents over-enhancement and undesired enhancement on slightly unbalanced mono material.

Two-Band Gated AGC: One of the 8200's secrets was that its AGC is a two-band device, using Orban's patented "master/bass" band coupling. The 8200's LCD showed only the AGC "master" band gain reduction.

In the 8400, we bring out all of the two-band AGC controls so the user can adjust them, including thresholds, attack times, release times, and master/bass coupling. We also add an important feature: target-zone gating. If the input program material's level falls within a user-settable window (typically 3dB), then the release time slows to a user-determined level. It can be slow enough (0.5dB/second) to effectively freeze the operation of the AGC. This prevents the AGC from applying additional, audible gain control to material that is already well controlled. It also lets you run the AGC with fast release times without adding excessive density to material that is already dense.

Another user-requested feature is the ability to operate the AGC in left/right or sum-and-difference modes. The user can preset the maximum amount of gain difference permitted between the sum and difference channels. Sum-and-difference can provide a different style of stereo enhancement than the 8400's purpose-built stereo enhancers, which we nevertheless prefer for stereo enhancement applications because of their more sophisticated gating and decreased tendency to add multipath distortion.

For the first time, the AGC contains a compression ratio control that allows you to vary the ratio between 2:1 and ¥:1. Lower ratios can help make gain riding subtler on critical formats like classical and jazz.

Finally, because of a user suggestion, the AGC now has its own silence-gating detector whose threshold can be set independently of the silence gating applied to the multiband compressor.

Equalization: The 8400 improves on the 8200's steep-slope bass shelving equalizer, and also adds three bands of fully parametric bell-shaped EQ.

In the 8400, you can set the slope of the bass shelving EQ to 6, 12, or 18dB/octave and adjust the shelving frequency. This significantly adds to its versatility.

The 8400's bass, midrange, and high frequency parametric equalizers have curves that were modeled on the curves of Orban's classic analog parametrics (like the 622), using a sophisticated and proprietary optimization program. This means that their sound is very close to the sound of an Orban analog parametric. They also use very high quality filter algorithms to ensure low noise and distortion.

The 8400 HF Enhancer is a program-controlled HF shelving equalizer that was originally introduced on Orban's 2200 OPTIMOD-FM. It intelligently and continuously analyzes the ratio between broadband and HF energy in the input program material, and can equalize excessively dull material without over-enhancing bright material. It interacts synergistically with the five-band compressor to produce a sound that's bright and present without being excessively shrill.

Multiband Compression: The basic sound of the five-band compressor is similar to the 8200's five-band compressor. However, in response to user requests we have increased the number of release time settings from four to seven, doubling the resolution of this audibly important parameter.

The multiband now uses a "look-ahead" topology, which means that the program audio is delayed until the gain control signal has time to attack fully. This significantly reduces compressor overshoot and eases the burden on the following peak limiting stages. Because look-ahead compression sometimes reduces transient punch, we also offer an "auto" mode that activates look-ahead compression on speech (to decrease clipping distortion by reducing the peak level applied to the clippers), and defeats look-ahead on music (to maximize transient punch consistent with low perceived distortion).

High Frequency Control: As in the 8200, we control high frequencies with distortion-canceled clipping. However, the clipper in the 8400 operates at 256 kHz sample rate and is full anti-aliased.

We are well aware of the controversy regarding the audible benefits of anti-aliasing a digital clipper and we continue to believe that it provides no audible benefit with program material in a well-designed system at sufficient sample rate. Nevertheless, we chose to put the issue to rest by adding anti-aliasing to all primary 8400 clippers.

The 8200 determined the gain reduction in band 5 from the gain reduction in band 4; these bands are only independent from the viewpoint of the downward expander and multiband clippers. In the 8400, we have added a high frequency limiter, which causes additional gain reduction in band 5 when band 5 multiband clipping alone would be insufficient to prevent HF distortion. The HF limiter uses a sophisticated analysis of the signal conditions in the 8400's "back end" clipping system to do this, maximizing brightness while still retaining silky, clean-sounding highs.

We also improved on the embedded bass clipper that protects bands 1 and 2 from transient overshoot. In the 8400, you can adjust this clipper for "hard" (equivalent to the 8200), "medium," or "soft" operation. Each step gives a further reduction in audible distortion by means of increasingly sophisticated signal processing. We prefer "medium" as the best compromise between cleanliness and punch, but "soft" can add that final element of cleanliness in softer formats.

"Intelligent" Clipping: There have been major changes and improvements in the 8400's back-end clipper by comparison to the 8200. The clipping system is "what separates the men from the boys" in on-air processing. A good clipping system is the key to being simultaneously loud, clean, and bright.

In general, the improvements come in two areas: (1) more intelligence in preventing audible clipping distortion with difficult program material, and (2) better overshoot compensation. (1) means that the 8400 is much less likely to encounter program material that unexpectedly causes gross clipping distortion—a particular problem with systems that rely on simple composite clipping for peak control. (2) means that the 8400 controls overshoots more tightly than does the 8200 and does so with less audible distortion and HF loss. Unfortunately, we can't go much further in describing the improvements because of their proprietary nature. However, we would submit that the proof is in the listening. The 8400 is capable of dramatically more brightness on-air than is the 8200, and the new clipping system is the reason for this.

DSP-derived Stereo Encoder: The 8400's stereo encoder is derived from algorithms first developed for the high-performance Orban 8218 stand-alone encoder. It operates at 512 kHz sample rate to ease the performance requirements of the D/A converter's reconstruction filter, making it possible to achieve better than 70dB measured and verified stereo separation from 30Hz to 15 kHz.

The 8400 has two independent composite outputs, whose levels are both software-settable. The second output can be configured to provide a 19 kHz-reference output for subcarrier generators that need it. For convenience, two SCA inputs sum into the 8400's analog composite output amplifier.

SCAs are not digitized by the 8400.

(Beware of anyone who claims "85dB" stereo separation in a stereo encoder; it cannot be measured with year 2002 state-of-the-art instruments and cannot be verified, directly or indirectly, by any means known to us. Theory suggests that it is almost impossible to obtain, requiring magnitude flatness in the reconstruction filter and output amplifier in the order of 0.0005dB from 30Hz to 53 kHz!)

Composite Level Control Processor: Orban has traditionally opposed composite clipping because of its tendency to interfere with the stereo pilot tone and with subcarriers, and because it causes inharmonic aliasing distortion, particularly between the stereo main and subchannels. Protecting the pilot tone and subcarrier regions is particularly difficult with a conventional composite clipper because appropriate filters will not only add overshoot but also compromise stereo separation—filtering causes the single-channel composite waveform to "lift off the baseline."

Nevertheless, we are aware that many engineers are fond of composite clipping. We therefore undertook a research project to find a way to peak-control the composite waveform without significantly compromising separation, pilot protection, or subcarrier protection, and without adding the pumping typical of simple gain-control "look-ahead" solutions.

We succeeded in our effort. The 8400 offers a composite processor that provides excellent spectral protection of the pilot tone and SCAs (including RDS), while still providing approximately 60dB separation when a single-channel composite waveform is clipped to 3dB depth. To ensure accurate peak control, it operates at 512 kHz sample rate.

Like conventional composite clipping, the new algorithm can still cause aliasing distortion between the stereo main and subchannels. However, this is the inevitable cost of increasing the power-handling capability beyond 100% modulation above 5 kHz—the characteristic that makes some people like composite clipping. This exploits the fact that the fundamental frequency in a square wave has a higher peak level than the square wave itself. However, any process that makes squared-off waveforms above 5 kHz creates higher harmonics that end up in the stereo subchannel region (23-53 kHz). These harmonics are then decoded in the receiver as if they were L–R information, ending up at new frequencies not harmonically related to the original frequency that generated them.

A patent is pending on the new algorithm.

We still prefer to use the 8400's main clipping system to do the vast majority of the work because of its sophisticated distortion-controlling mechanisms. This means that the 8400, unlike some of its competitors, does not rely on composite processing to get loud. So the benefits of the 8400's processing can be enjoyed equally as well by people using its left/right-domain AES3 digital output—the 8400 gets very loud without composite clipping.

Two-Band Purist Processing

In addition to five-band processing, suitable for pop music and talk formats, the 8400 offers a very high-quality two-band algorithm. This is phase-linear, and features the same AGC as the five-band processor, followed by a two-band processor with look-ahead limiting. Sophisticated multiband high frequency limiting and distortion-cancelled clipping complete the chain.

We believe that this is the ideal processing for classical music because it does not dynamically re-equalize high frequencies; the subtle HF limiter only acts to reduce high frequency energy when it would otherwise cause overload because of the FM pre-emphasis curve. We have heard four-band, allegedly "purist" processing that caused dynamic HF lift. This created a strident, unnatural sound in the strings and brass. In contrast, the 8400's two-band phase-linear structure keeps the musical spectrum coherent and natural.

The look-ahead limiter prevents speech from being audibly clipped, and prevents similar audible problems on instruments with rapidly declining overtone structures like grand piano, classical guitar, and harp.

(To keep his ears honest [and because he loves classical music] this writer attends between 15 and 20 live San Francisco Symphony concerts per year. He also paid part of his college tuition by doing live recordings of classical music events, and studied harmony, counterpoint, and composition at the same university. So he has a good understanding of the unique esthetic requirements of classical music reproduction, and is very well acquainted with the sound of the "real thing.")

Input/Output Delay

The algorithmic improvements in the 8400 have one significant cost—the input/output time delay is approximately 21 milliseconds (with Bass Clip set to "hard" or "medium") and 40 ms (with Bass Clip set to "soft"). To make intelligent decisions about how to process, the 8400 needs to look ahead at the next part of the program waveform. As digital on-air processing moves further and further from its analog roots, this is the inevitable price of progress.

(The delay times stated above assume 8400 version 2.0 software or higher. Earlier versions had substantially larger delays.)

A 21 ms delay is below the psychoacoustic threshold that creates distinct echoes, while 40 ms causes audible echoes that make off-air headphone monitoring uncomfortable (or impossible) for most talent. Therefore, if live headphone monitoring is necessary, we recommend using "hard" or "medium" bass clipping. Because the audible difference between "medium" and "soft" bass clipping is subtle (and much smaller than the difference between "hard" and "medium"), this seldom (if ever) will limit the user's freedom to get a desired bass texture.

Summary

The 8400 provides numerous important advances over the digital signal processing in Orban's popular 8200. By devoting over five times the DSP processing power to the problem, the 8400 reduces bass distortion, opens up high frequency response, and prevents audible clipping distortion on difficult program material. Added stereo enhancement, high frequency enhancement, and composite peak control complement the basic processing improvements, together providing the most powerful and effective on-air FM processing system that Orban has ever created.


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