Introduced in 1985, the Valley People 440 Limiter/Compressor/Dynamic Sibilance Processor combined compression, expansion, peak limiting, clipping and specialised dynamic sibilance processing within a single 1U professional audio processor. Rather than treating these as entirely separate signal processors, the Valley People 440 integrated its main dynamics functions around a common voltage controlled amplifier, allowing the compressor, expander and limiter control sections to work interactively.
The compressor section of the Valley People 440 provides continuously variable threshold, ratio, attack and release controls. Compression ratio is adjustable from 1:1 to 20:1, while attack time extends from 200μs to 200ms. The compressor incorporates gain recovery computation designed to maintain a preset nominal output level as input level, threshold and ratio settings change. This arrangement means compression can increase the level of signals below the selected compressor threshold as well as reduce signals above it.
An interactive downward expander is integrated with the compressor circuitry. Its purpose is particularly important when substantial compression is applied, as the increase in idle gain produced by compression can raise the apparent noise floor during pauses and low-level passages. The expander reduces the gain of the common voltage controlled amplifier when the signal falls below its threshold, thereby reducing this unwanted noise. The compressor and expander also share release control, with the circuitry arranged to provide a smooth transition between compression and expansion rather than behaving as two completely independent processors.
The limiter section is designed for considerably faster control of transient peaks. It operates at a fixed 60:1 ratio with an attack characteristic typically 1μs/dB or less, together with continuously adjustable threshold and release controls. The Valley People 440 can therefore operate with compression followed by fast peak limiting, allowing the compressor to manage the overall programme dynamics while the limiter controls short-duration peaks.
A separate safety clipper can also be brought into operation. Its threshold automatically tracks the limiter threshold, allowing exceptionally fast transients that escape the limiter to be controlled. The combination was intended for applications where an absolute peak voltage had to be maintained, including recording, broadcast feeds and master disc cutting.
The Valley People 440 also includes an Automatic Gain Control mode for the limiter. In this configuration the limiter release time is multiplied by a factor of ten, allowing it to maintain a relatively constant output level over a much wider range of input levels. This could be combined with the compressor and expander sections for applications requiring substantial overall level control.
For simplified operation, the Valley People 440 provides an Auto mode in which compressor attack time, ratio and release time are preset and the expander threshold is linked to the compressor threshold through the internal control circuitry. The amount of compression can then be altered primarily through the compressor threshold control. This provided an alternative to manually balancing each individual dynamics parameter.
One of the most distinctive sections of the Valley People 440 is its Dynamic Sibilance Processor. Rather than applying broadband high-frequency gain reduction, the circuit analyses sibilant material for relatively narrow-band, high-frequency sinusoidal components described as whistles. When such components are detected, the processor cancels only the narrow region containing the offending energy while leaving the remainder of the broadband signal unaffected. The intention was to control those portions of vocal frication most likely to cause overload or splatter without substantially changing the normal spectral distribution of the sibilant itself.
Two Dynamic Sibilance Processor modes are provided, DS1 for lower-frequency processing and DS2 for higher-frequency processing. The Dynamic Sibilance Processor can be operated independently or in conjunction with the other processing functions, allowing the Valley People 440 to combine vocal de-essing with compression, expansion and limiting within the same signal path.
The Valley People 440 also incorporates FM pre-emphasis compensation within the control circuits of its compressor, expander and limiter. Pre-emphasis networks for 25μs, 50μs or 75μs operation can feed the detector and control circuitry so that sensitivity increases with frequency while the audio signal itself remains flat. This allows the dynamics sections to respond more strongly to high-frequency programme content without actually equalising the signal passing through the audio path.
For stereo processing, two Valley People 440 units can be linked. The system supports master/slave linking as well as stereo coupling in which the individual compressor, expander and limiter control voltages of both channels are averaged before being applied to the control summing ports. The clipper, Dynamic Sibilance Processor and output gain sections are not affected by the control link.
The signal path begins with a balanced input amplifier feeding the Dynamic Sibilance Processor and voltage controlled amplifier. The voltage controlled amplifier is followed by the clipper circuit and balanced output stage. Separate control feeds supply the detector and compressor, expander and limiter control sections. Balanced inputs and actively balanced outputs allow the Valley People 440 to accommodate professional line-level systems, with maximum balanced output capability of +24dBm into 600Ω.
Front-panel monitoring is extensive for a 1U processor. Separate LED indicators show compressor, expander, limiter, clipper, Dynamic Sibilance Processor and overload activity. Two LED displays indicate compressor/expander gain reduction and limiter gain reduction, while another display can indicate input or output level in VU units. A front-panel bypass control allows comparison with the unprocessed signal.
Features
- Integrated compressor, downward expander, peak limiter, safety clipper and Dynamic Sibilance Processor
- Continuously variable compressor threshold, ratio, attack and release controls
- Compressor ratio adjustable from 1:1 to 20:1
- Interactive expander designed to reduce noise increased by compression
- Fixed 60:1 peak limiter with extremely fast attack characteristics
- Safety clipper with threshold automatically tracking the limiter section
- Automatic operating mode with preset compressor control parameters
- Two-mode Dynamic Sibilance Processor for selective removal of objectionable sibilant components
- FM pre-emphasis compensation for 25μs, 50μs and 75μs systems
- Control linking for stereo and multiple-unit operation