Distributed Vs. Centralized Audio Processing
May 13, 2013 3:40 PM, By Don Kreski
Peak Audio founders say the distributed DSP model is obsolete.
Centralized processing debated
Zwiebel says when he shows Q-Sys to potential buyers, they often question its divergent design. “The most common negative comment I get is that customers feel they need to put their processing in many locations,” he explains. “To that I always ask, “Is it the processing or the I/O that they need to distribute?” It is almost always the I/O.
“Once they think about it, they usually admit that they do not really care where the processing lives, but to that I reply they actually do. Would they rather have all of the processing located in a secure, air-conditioned accessible space, or spread around all over the place, say up on a catwalk with the amplifiers, or in a legislative chamber that cannot be accessed during use? They always agree that yes, they would prefer to put it into their own equipment room.
“The other negative I hear is that the distributed system is more reliable, because if they lose a box, they only lose that part of the system. But in reality, when you lose a box, you usually have major problems. What if the box you lose has the input from the announcer at a stadium? Pretty much a show stopper.”
Zwiebel gives the example of a sound system that Peak Audio designed for the United States Senate back in 1999. They had five processors in the system and so, if one failed, as it did during a committee hearing he remembers, they lost only 20% of the system. “Yet that too was a show stopper. It was not possible to say, ‘Oh, you Democrats cannot talk in this hearing, but all of you Republicans can.’” The hearing had to be cancelled.
Zwiebel explains that, in an audio system, it’s the cables and connectors that are most likely to cause problems. Using five separate boxes rather than one means there must be at least five network cables and 10 network connectors added to a crucial part of the system. In a centralized system, it’s possible to eliminate these cables by putting all of the processing on one chip.
“Another benefit of centralized processing is that it simplifies design and design file management,” he adds. When a contractor deploys multiple DSP boxes, each requires its own design file or layer and the designer has to keep track of which box has what functionality. “This introduces additional complexity and can result in painfully slow system deployments, updates and compiling times, while also making troubleshooting more difficult. The centralized approach we’ve taken with Q-Sys uses one master design file for the entire system which simplifies development, deployment, management, and updating.”
Given the possibility of failure of any sound system, many designers have tried to build redundancy into mission-critical distributed systems, but it’s a difficult, expensive process. Britton says it is far simpler, less expensive, and more reliable to duplicate components in a centralized system.
For example, with Q-Sys, processing cores can be set up in pairs on the network. Should the system detect a fault in the primary core, the secondary takes over automatically within seconds. When an engineer adjusts settings, the changes are made simultaneously on both cores, so the secondary is always ready to take over. Every Q-sys product has a primary and a secondary network port. Two completely separate networks can be installed, so that if any part of either network should fail, whether it is a switch or a cable, the other network automatically and instantaneously takes over.
In addition, a centralized system can give the designer the option of adding redundant I/O frames for high-priority areas and even redundant amplifiers.
IT compatibility and AVB
Zwiebel says there has been a lot of discussion recently about AVB, or Audio-Visual Bridging, that many in the industry advocate as a network standard. One of the advantages of AVB is its ability to transfer data quickly. The AVB standard requires that total latency, or delay, introduced by the network be limited to 1.024 milliseconds.
“But standard gigabit Ethernet, using off-the-shelf components, is also fast enough to limit network latency to 1 millisecond,” Zwiebel explains. “AVB is, from an IT perspective, non-standard, requiring Layer-2 networking versus the Layer-3 standard that is Internet Protocol. As a consequence, AVB requires the use of proprietary AVB-capable switches, limiting its ability to take advantage of an existing IT infrastructure. Data traveling on an AVB network cannot pass through Cisco or HP routers, which are installed in at least 80 percent of the Ethernet networks around the world.”
Zwiebel and Britton argue that audio networks today should be completely compatible with commonly used network equipment, as Q-Sys is. In addition to low-latency audio LAN distribution, they add, an audio processing platform should support long-haul streaming over wide-area networks or the Internet.
A centralized processing platform, running on an IP network, should also follow the best practices used in the computer industry. “Audio traffic should not require a separate network infrastructure,” Britton adds. “The IT department will need to use the same quality of service settings they would use for voice over IP or IPTV, to make sure audio traffic has proper priority, but those techniques are well known now.”
Is centralized processing the wave of the future? “We think this is the way to go,” says Britton. “The centralized approach we’ve taken has yielded a very powerful processing platform, yet at the same time reduced development effort, reduced system complexity, and it will give the software a very long life as ever more powerful processors and network interfaces are brought to market by the mainstream computer industry.”
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