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Seminars & Articles

DC Servos and Digitally-Controlled Microphone Preamplifiers (509k)
by Gary Hebert, CTO That Corporation
AES Convention Paper 9198, 137th Convention, October 2014

Microphone preamplifiers for professional audio applications require a very wide range of gain and low noise in order to provide a high-quality interface with the vast number of available microphones. In many modern systems the preamplifier gain is controlled indirectly via a digital interface in discrete steps. Often dc servo amplifiers are employed as a means of keeping the dc gain fixed to avoid large changes in output offset voltage while the audio band gain is varied. The resulting highpass filter response varies substantially as a function of the preamplifier gain. We investigate the frequency and time-domain effects of this. We also investigate several approaches to minimize these effects.

Gary’s Convention Paper (#9198) and many other technical papers of note are available from the AES E-Library.

The 48 Volt Phantom Menace Returns (1,480k)
By Rosalfonso Bortoni and Wayne Kirkwood, THAT Corporation
AES Convention Paper 7909, 127th Convention, October, 2009

In 2001, Hebert and Thomas presented a paper at the 110th AES Convention which described the “phantom menace” phenomenon wherein microphone phantom power faults can damage audio input circuitry. This paper offers new information about the phantom menace fault mechanisms, analyzes common protection circuits, and introduces a new protection scheme that is more robust.

A Survey of Broadcast Television Perceived Relative Audio Levels (475k)
By Matt Easley, Chris Hanna, Scott Skinner, Matt Barnhill, THAT Corporation
AES Convention Paper 7896, 127th Convention, October, 2009

Perceived television volume levels can vary dramatically as audio changes both within a given broadcast channel and between broadcast channels. This paper surveys the broadcast audio levels in two large metropolitan areas (Atlanta and Boston).

A Flexible Compander IC for Wireless Microphone Applications (281k)
By Gary K. Hebert, THAT Corporation
AES Convention Paper 6337, 118th Convention, May, 2005

A new IC for implementing companding noise reduction in professional wireless microphone applications is described. Unlike existing devices designed primarily for the cordless telephone market, the new design allows straightforward, repeatable implementation of companding schemes incorporating ratios greater or less than 2 to 1, level-dependent ratios, limiters, and noise gates.

Demystifying Analog Circuits in Professional Audio Applications (286k)
By Fred Floru, Principle Engineer, THAT Corporation
AES Convention Paper 6455, 118th Convention, May, 2005

The purpose of this paper is to look at a few popular analog circuits that have a direct impact on the performance of professional audio applications. The circuits are explained with mathematical demonstrations. The impact of real life implementations on the performance specifications is explored for each circuit.

New Balanced-Input IC Achieves Very High Dynamic Range In Real-World Systems (381k)
By Bill Whitlock, Jensen Transformers and Fred Floru, THAT Corporation
AES Convention Paper 6261, 117th Convention, October, 2004

A new integrated circuit (IC) is described that uses a patented topology to achieve common-mode impedances comparable to those of an ideal transformer. As a result, the IC enables signals with very high dynamic range to be transported without contamination by system ground-voltage differences or other sources of common-mode interference.

The 48 Volt Phantom Menace (153k)
By Gary K. Hebert and Frank Thomas, THAT Corporation
AES Convention Paper 5335, 110th Convention, May, 2001

The authors encountered anecdotal evidence suggesting that field failures of existing line driver and microphone preamplifier integrated circuits (ICs) were correlated with accidental connections between line outputs and microphone inputs with phantom power applied. Analysis showed that the most probable mechanism was large currents flowing as a result of rapid discharge of the high-valued ac-coupling capacitors. Commonly used protection schemes are measured, analyzed, and shown to be lacking. More robust schemes that address these shortcomings are presented. It is concluded that the small additional cost of these more robust protection schemes is likely outweighed by the reduction in field failures and their associated repair cost.

An Improved Microphone Preamplifier Integrated Circuit (1,173k)
By Fred Floru, THAT Corporation
Paper UK103, Silicon for Audio, AES 16th UK Conference, 2001

The influence of the semiconductor process on the performance of a microphone preamplifier integrated circuit is described. Examples of active devices available in conventional junction isolated and newer bipolar technologies are shown.

An Improved Balanced, Floating Output Driver IC (1,017k)
By Gary K. Hebert, THAT Corporation
AES Convention Paper 5152, 108th Convention, February, 2000

The design and implementation of an improved balanced, floating output driver IC for professional audio applications is described. It is shown that, when the most common existing designs are used to drive ground-referred loads, the grounded output is forced into current-limiting whenever the active output clips. This results in large current spikes flowing into the ground of the receiving device. Techniques used to eliminate this problem, as well as overall performance of the resulting design, are described.

Attack and Release Time Constants in RMS-Based Feedback Compressors (981k)
By Fred Floru, THAT Corporation
AES Convention Paper 4703, 104th Convention, May, 1998

The goal of the paper is to develop mathematical models for feedback and feedforward compressors. A couple of possible configurations are explored: linear-output RMS detector with linearly-controlled VCA (linear-domain compressor) and logarithmic-output RMS detector with exponentially-controlled VCA (log-domain compressor). It is shown that the transfer functions of both configurations are equivalent. A formula for transforming the compression ratio of a log-domain compressor to that of a linear-domain compressor is derived. The differences between feedforward and feedback compressor configurations, with regard to time constants and performance, are considered.

An Improved Monolithic Voltage-Controlled Amplifier (1,073k)
By Gary K. Hebert, THAT Corporation
AES Convention Paper 4055, 99th Convention, October, 1995

The design and implementation of a monolithic voltage-controlled amplifier with an exponential gain-control characteristic is described. Sources of deviation from ideal performance, including distortion, noise, offset change with gain, and noise modulation, are analyzed.

Attack and Release Time Constants in Compressors and Limiters (1,016k)
By Fred Floru, THAT Corporation
AES Convention Paper 4054, 99th Convention, October, 1995

Although RMS detector time constants are often set by ear, an art in itself, the importance of a mathematical “tool” is explained. The mathematics of the RMS detector is revisited and models for time and frequency behavior are determined.

Digitally-Controllable Audio Filters and Equalizers (728k)
By Gary K. Hebert and Fred Floru, THAT Corporation
Paper 13-031, AES 13th International Conference, December, 1994

This paper explores solutions to varying the response of filters and equalizers using digital control techniques suitable for computer-controlled systems. High-pass, low-pass, band-pass, and parametric topologies are covered using VCAs, resistor ladders, and multiplying DACs as control elements.

Real-Time Control of DSP Parametric Equalizers (557k)
By Chris Hanna, THAT Corporation
Paper 13-041, AES 13th International Conference, December, 1994

Network control brings new possibilities to user interaction with DSP devices. One important area is to offer real-time control of filter shapes (center frequency, Q, and gain) by changing filter coefficients in real time. Textbook solutions to DSP filter problems rarely consider such dynamic situations. This paper presents observations on implementing real-time control of DSP. The work explores the update rate and granularity required to achieve perceptually smooth, real-time control.

A Building Block Approach to Network Control (440k)
By Chris Hanna, THAT Corporation
Paper 13-023, AES 13th International Conference, December 1994

Adding a network interface to a microprocessor-controlled product significantly complicates the product design. This paper describes a method of minimizing this added complexity by encapsulating the network interface in its own processor and providing a simple, standard internal communications interface between the network processor and the host processor.