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DSP Made Simple
Personal audio – whether in home theater or car stereo systems — has continued to raise expectations about sound quality in public places. And that’s true whether audience members are in a theatre, auditorium …or a church. If the sound quality in your house of worship needs a little intervention, audio or digital signal processing may provide the remedy. In this article, we’ll cover some of the basics:
Back to the Basics, Part I: The Measure of Good Sound
Anyone responsible for sound has just one goal: the best sound possible. But what is good sound? Subjective as that may seem, good sound is the optimization of three components:
1. Audibility – achieving the desired effect (or a comfortable listening level) at the furthest listener. In a house of worship, an example might be making certain that the worshipper in the back row can hear the sermon or music. Is it loud enough?
2. Intelligibility –this is determined by the signal-to-noise ratio and direct-to- reverberant sound source; stated a little more simply, making certain that the signal (the voice or the music) can be heard clearly over the room’s ambient noise. DB LEVELS Is it clear enough?
3. Fidelity – the frequency response of the sound arriving at the listener’s ear that provides realistic, accurate reinforcement of speech and music. Every component of the signal chain contributes to it. Is the sound true?
We’re accustomed to hearing other, more colorful terms for describing the sound – warmth, punch, bottom – but they’re not measurable for our purposes here. “Speech that is warm but unintelligible does the listener little good.” Audio signal processors – digital signal processors – are designed to optimize a sound system for the three building blocks of good sound – audibility, intelligibility and fidelity.
Back to the Basics, Part II: A Basic Sound System
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A Basic Sound System
Digital Signal Processing converts signals from real world sources (usually in analog form) into digital data that can then be analyzed. Analysis is performed in digital form because once a signal has been reduced to numbers, its components can be isolated and manipulated in more detail than in analog form.
When the DSP has finished its work, the digital data can be turned back into an analog signal with improved quality. A DSP can filter noise from a signal, amplify frequencies and suppress others.
Types of Audio Signal Processors
Signal processors can be analog or digital, single- or multi-function or integrated with other components in a sound system. In the beginning, most were standalone devices, but over time, became multi-functional, with today’s digital signal processors (DSPs) combining a wide spectrum of functions at a fraction of the cost of individual processors. Here are some of the problem-solving features you’re likely to find in today’s DSPs:
| Volume and Gain Control | Automatic Gain Control (Speech Leveler) |
| Filters | Delay |
| Equalization | Automatic Microphone Mixers |
| Dynamics Processor | Gated Automatic Mixers |
| Compressors | Feedback Reducers |
| Limiters | Acoustic Echo Cancellers |
| Expanders and Noise Gates |
It’s sophisticated chip technology, but you can find it everywhere. DSP chips are used in sound cards, fax machines, modems, cellular phones, high-capacity hard disks and digital TVs. According to Texas Instruments, DSPs are used as the engine in 70% of the world's digital cellular phones, and with the increase in wireless applications, this number will only increase. Digital signal processing is used in many fields including biomedicine, sonar, radar, seismology, speech and music processing, imaging and communications.
What DSP Can Do
To determine whether DSP can benefit your sound system, you need to consider some of the most common problems you face in house of worship sound reinforcement. Assuming you have fairly good room acoustics, here are the problems and the DSP tools that can remedy them.
And What It Can’t
Adding DSP to your system isn’t a substitute for following accepted sound reinforcement rules. Reverberation, for instance, can’t be fixed by audio processing. Once sound energy is released by the loudspeaker, DSP has no effect. Raising the level of the sound system will only make the problem worse.
Remember:
(One way to do this is to make sure your microphone have the correct polar pattern for your application.)
For detailed information, check out these downloadable Shure education guides:
Selection and Operation of Audio Signal Processors
Audio Systems Guide for Houses of Worship
Ask the Expert:
What’s So Great About Digital Signal Processing
He has designed hundreds of sound systems for a wide variety of installations, including churches. He’s been using DSP in systems since 1995
Flexibility
“One piece of equipment, often taking up no more than one or two units of rack space, can fulfill my wildest dreams.”
“I can do a lot more signal processing that I could with separate analog gear. There’s no longer any excuse for not tuning the send to the ceiling loudspeakers and it’s easy for me to delay them so that they don’t create an intelligibility problem.”
Programming Ease
“For instance, the user interface for an equalizer looks and feels just like the controls of the analog unit it replaces. I have a real time display of the equalizer’s response as I tune it. The compressors and limiter give me control of all the setup parameters and meters, showing gain reduction dynamically with signal.”
Documentation and Backup
“I can save the file to my laptop when I’m done. I can also save different variations.”
Portability
“I can work through a design in my office, understand the free DSP and I/O available for expansion of the system – all without the need to have any contact with the hardware itself.”
Scalability
“Things like “Oh, by the way, you know we need to feed the system from the portable baptismal font that we set up at the back of the church, don’t you?” are easily accommodated.”
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St. Mary’s of the Angels Church in Chicago, Illinois was an Audio Systems Group project incorporating the Shure P4800 DSP. | |
“I can create customized pre-sets and call them up with external control signals in the form of switched contacts, logic signals and user control screens. This is perfect for churches with a variety of worship services and programs.”
Reduced Noise in the Signal Chain
“Before DSP, we had to worry about the cascading of noise contributed by each and every analog input and output stage.”
Simplified Installation
“With DSP, all we have to do is connect inputs and outputs. The rest of the wiring all happens on the computer screen.”
Cost
“Installation costs are minimal and hardware costs are much less, too. If we add the costs of a compressor/limiter, equalizer, crossover and delay, we’re even with the cost of a simple DSP that replaces them. Since the DSP can be used anywhere, the more applications you can find for it, the more you save.”
If you’d like to learn more about Jim, Audio Systems Group and the range of his talents, which extend from producing NPR’s “Jazz Alive!” to his 4 1/2 star review in Downbeat for his “Carmen McRae at Ratso’s” CD, visit http://www.audiosystemsgroup.com
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