The Fundamentals of Digital Signal Processing
Contributors: Jim Brown and Gino Sigismondi
You've done everything you can think of to keep the levels constant. You've added amps. You've moved speakers. Still, the amount of reverberation in your worship space makes intelligibility a real challenge. Then, of course, there's background noise and feedback. Believe it or not, there may be a simple solution to combating these sound quality issues. Three letters. DSP.
Whether audience members are in a theatre, an auditorium, or a church like yours, they have high expectations about sound quality. So if you're curious about the ability of DSPs to provide the remedy, read on. We'll cover some of the basics:
• DSP – Digital Signal Processing
• Signs and Symptoms: When It Can Help
• Types of DSPs
• Practical Applications
In this blogpost, Shure's Gino Sigismondi is here to tell us what Digital Signal Processors can and can't do. We also recruited expert Jim Brown of Audio Systems Group to share his vast real world experiences.
How They Work
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 their infancy, 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'll find in DSPs today:
Volume and Gain Control
Expanders and Noise Gates
Automatic Gain Control (Speech Leveler)
Automatic Microphone Mixers
Gated Automatic Mixers
Acoustic Echo Cancellers
It is highly 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 sound reinforcement. Assuming you have fairly good room acoustics, here are the problems and the DSP tools that can remedy them.
|Problem||DSP Remedy (Tool)|
|Poor tone quality||Graphic Equalizer|
|Sound source too loud||Compressor|
Automatic Gain Control
|Sound source too quiet||Automatic Gain Control|
|Varying signal levels from multiple sound sources||Compressor|
Automatic Gain Control
|Unwanted noise||Noise Gate/Downward Expander|
No Overshoot ("Look-Ahead") Peak Limiter
(due to open microphones)
|Automatic Microphone Mixer|
|Frequency response problems|
(due to misaligned loudspeakers)
|Poor intelligibility||Parametric Equalizer|
Automatic Microphone Mixer
And What DSP Can't Do
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.
• Keep unwanted sounds from entering open microphones.
(One way to do this is to make sure your microphone have the correct polar pattern for your application.)
• Turn off microphones that aren't in use.
• Keep microphones close to the sound source
• Aim directional loudspeakers away from reflective surfaces and toward listeners.
• Reduce room reverberation through structural modifications or acoustic treatments.
Then, We Asked an Expert
Jim Brown, Audio Systems Group
To gain another perspective, we contacted Jim Brown, who is the founder and principal consultant for Audio Systems Group in Chicago. He has published numerous research papers on sound reinforcement and is a contributor to pro sound magazines, including Sound & Video Contractor and Technologies for Worship.
He's designed hundreds of sound systems for a wide variety of installations and started using DSP in 1995. Jim hasn't done a system without one since and here's why:
"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."
"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 limiters give me control of all the setup parameters, 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."
"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."
"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."
"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."
"With DSP, all we have to do is connect inputs and outputs. The rest of the wiring all happens on the computer screen."
"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."
Want more information?
Selection and Operation of Audio Signal Processors
And if you'd like to learn more about Jim Brown, 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 Audio Systems Group.