Courtesy: Biamp Cornerstone

Applications in professional sound

In the years after the development of the basic mathematical construct for signal processing, many strides have taken place especially in the audio domain, where DSP has been used very effectively to bring more functionality and efficiency in audio electronics.

One of the many applications that we have seen in the professional audio space has been of that in the vocal and speech processing equipment in install applications.

Notable examples of these products include the Shure SCM 810, the Peavey Automix2 and the Audio Technica 0604. Equipped with basic fixed point DSP processors, these products provided one functionality – microphone auto mixing, and they all did this remarkably well, though the algorithms employed differed slightly lending to their own signature sound and working.

Similarly other audio functionality such as acoustic feedback elimination – a big problem in live and install sound setups also got the DSP treatment. Marketed by several manufacturers as feedback “destroyers” or feedback “killers”, these products were targeted primarily at sound rental companies and system integrators, who would employ them to remove feedback from very difficult venues and events. Essentially an adaptive filter, that would

target resonant frequencies and apply “notch” filters in real time, these devices are loved and hated just as many times for their ability to provide a quick fix but to also destroy useful signal in those frequencies.

There are two main methods used in discriminating feedback from other sounds. The first method focuses on the relative strength of harmonics. The idea is that while music and speech are rich in harmonics feedback is not.

Another method for discriminating feedback from desirable sound is to analyze feedback through some of its more unique characteristics. This can be done without analyzing harmonic content. For example a temporary notch can be placed on a potential feedback frequency. Feedback is the only signal that will always decay (upstream of the filter) coincident with the placing of the notch. However, because placing a temporary notch is intrusive some other mechanism needs to be used to identify potential feedback frequencies before a temporary notch is placed for verification. One such useful characteristic is that a feedback frequency is relatively constant over the time that its amplitude is growing. This constant frequency combined with a growing magnitude proves very useful as a precursor to the temporary notch.

DSP got incorporated into loudspeaker management as well for providing active separation of frequency bands to drive low frequency, mid and high frequency drivers with maximum efficiency.

With the ability to choose between filters to provide the ideal bandpass curve for a 2-way or 3-way loudspeaker setup, these products remain an integral part of every loudspeaker manufacturer’s arsenal till date. While most of these products are tailored to specific loudspeaker types, most of them also offer options to users to create custom filter curves to fit specific requirements.

Many independent DSP companies now manufacture precision loudspeaker management DSP products that may be adapted to a wide range of loudspeaker types and applications. Notable amongst these are some well-known names such as BSS Audio, dbx, XTA, Dolby Lake and so on.

Basic loudspeaker management processors have the capability to route audio inputs from a mixer, or another processor and essentially function as a digital crossover for driving different elements of a 2-way, 3-way or even a 4-way loudspeaker system. This function may be depicted as follows:

Other notable applications of DSP include line noise suppression/elimination and acoustic noise cancellation. While line noise suppression has been used in telecom equipment, acoustic noise cancellation is a relatively recent addition. In the 1950s Lawrence J. Fogel patented systems to cancel the noise in helicopter and airplane cockpits. Adapted subsequently by many manufacturers primarily in the aerospace industry, this has spilled over into the professional and consumer space, with most manufacturers now claiming to have at least one active noise cancellation product in their range.

Adaptive algorithms are designed to analyze the waveform of the background aural or nonaural noise, then based on the specific algorithm generate a signal that will either phase shift or invert the polarity of the original signal. This inverted signal (in antiphase) is then amplified and a transducer creates a sound wave directly proportional to the amplitude of the original waveform, creating destructive interference. This effectively reduces the volume of the perceivable noise. In the professional audio world, adaptive acoustic noise cancellation features in ceiling microphone tiles, professional video conferencing soundbars and also in-built into DSP hardware to detect and provide active noise cancellation especially on video conferencing systems.

Other DSP applications are in the embedded space within products that require the manipulation of a sound beam from an array of loudspeakers to be focused on a specific area. This is achieved using minute variations in time and phase to create a beam that may be “steered” or “shaped” to avoid hard surfaces and focus energy on listening areas. Conversely, when applied to an array of microphones, this can as a large directional microphone element which can pinpoint and capture sound from a specific spot in the room.

Coupled with user-friendly and interactive GUI, DSP is now prevalent in even the most common consumer audio devices such as sound bars and Bluetooth speakers and headphones. With sound processing now available through the convenience of mobile devices, DSP has truly transcended from the rarefied specialist domain to everyday gadgets. Now with the introduction of voice enabled devices and artificial intelligence engines, digital signal processing is poised to play an even more important role in our everyday lives.

Control of DSP hardware has been with user GUI that is tailored to intuitively change parameters, routing and by incorporating familiar interfaces such as faders and knobs from the analog world. This enables users to seamlessly transition from using analog audio gear to their digital counterparts. Further control is enabled through custom control screens that may be designed using control systems or physical faders, buttons and knobs supported through general purpose input/output interfaces on most DSP hardware units.