Orban
Optimod-FM 5300

Signal Flow

The signal flows through the 5300 through the following blocks:

Input Conditioning: The 5300 operates at 32 kHz sample rate and power-of-two multiples thereof (up to 512 kHz in the stereo encoder). Despite myths circulating in the marketplace regarding the alleged superiority of higher sample rates in FM stereo processors, 32 kHz is, in fact, preferable to higher rates as a basic sample rate for these devices unless they are required to produce more than 16 kHz audio bandwidth for simultaneous digital radio processing. 32 kHz allows us to use DSP horsepower more efficiently, adding features that really improve the sound. By strictly limiting the output bandwidth to 16 kHz, it also makes it easier to protect the stereo pilot tone and RDS subcarriers spectrally. Although a 16 kHz bandwidth limitation is more than is strictly needed to protect the pilot tone, the RDS requires protection over a substantially wider bandwidth (±2 kHz), and 16 kHz provides this protection.

The 5300’s output spectral control is immaculate, ensuring maximum stereo and RDS coverage. Moreover, the 5300’s digital output will pass through any uncompressed digital STL (including those operating at 32 kHz sample rate) without added overshoot and without the need for distortion-producing overshoot compensation schemes.

A defeatable 30Hz 18 dB/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 5300 provides a stereo enhancement algorithm based on Orban’s patented analog 222 Stereo Enhancer, which 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.

Two-Band Gated AGC: The AGC is a two-band device, using Orban’s patented “master / bass” band coupling. It has an additional important feature: target-zone gating. If the input program material’s level falls within a user-settable window (typically 3 dB), then the release time slows to a user-determined level. It can be slow enough (0.5 dB/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.

The AGC contains a compression ratio control that allows you to vary to ratio between 2:1 and essentially ∞:1. Lower ratios can make gain riding subtler on critical formats like classical and jazz.

Equalization: The 5300 has steep-slope bass shelving equalizer and three bands of fully parametric bell-shaped EQ. You can set the slope of the bass shelving EQ to 6, 12, or 18 dB/octave and adjust the shelving frequency.

The 5300’s bass, midrange, and high frequency parametric equalizers have curves that were modeled on the curves of Orban’s classic analog parametrics (like the 622B), using a sophisticated, proprietary optimization program. The curves are matched to better than 0.15 dB. 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 5300 HF Enhancer is a program-controlled HF shelving equalizer that was originally introduced in 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 sound that is bright and present without being excessively shrill.

Multiband Compression: The multiband compressor can be operated in five-band or two-band mode. In addition to using a special high-frequency limiter, we control high frequencies with distortion-canceled clipping. The clipper in the 5300 operates at 256 kHz-sample rate and is full anti-aliased.
A clipper, embedded in the crossover, protects bands 1 and 2 from transient overshoot. This clipper has a shape control, allowing you to vary the “knee” of its input/output transfer curve from hard (0) to soft (10).

“Intelligent” Clipping: The 5300 prevents excess clipping distortion by dynamically reducing the drive level to the clippers as required, using an intelligent analysis of the clipping distortion produced in the final clipper and overshoot compensator.

DSP-derived Stereo Encoder: The 5300’s stereo encoder operates at 512 kHz-sample rate to ease the performance requirements of the D/A converter’s reconstruction filter, making it possible to achieve excellent stereo separation that is stable over time and temperature.

Composite Limiter: 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 5300 offers a patented “Half-Cosine Interpolation” composite limiter that provides excellent spectral protection of the pilot tone and SCAs (including RDS), while still providing approximately 60 dB of separation when a single-channel composite waveform is clipped to 3 dB depth. To ensure accurate peak control, the limiter operates at 512 kHz sample rate.

While the processing never clips the pilot tone, the extra spectrum generated by the processing can fall into the 19 kHz region, compromising the ability of receivers to recover the pilot tone cleanly. Therefore, the 5300’s composite processor has a 19 kHz notch filter to protect the pilot tone. This filter does not compromise stereo separation in any way.