The advent of HD broadcasting introduces a problem for users and manufacturers of sound mixing consoles: how to deal with 5.1 surround sound. Fundamentally, surround sound requires at least three times as many channels as conventional stereo.
Channel capacity has traditionally been increased by adding commercial off-the-shelf DSP chips arrayed on more cards to perform the large number of math operations — billions every second — that sound delivery requires. This approach dictates that consoles use more energy, emit more heat and, of course, cost more money.
Power in a single chip
Calrec's Bluefin high-density signal processing system addresses the 5.1 challenge by condensing all signal processing onto one card. Processing for just 226 channels used to require 25 cards on the company's largest Alpha console. Adding the system to that console provides 480 fully equipped channel processing paths packaged as 162 stereo plus 156 mono, giving the user the ability to use up to 78 × 5.1 surround channels.
On the company's Sigma console, the signal processor provides 320 fully equipped channel processing paths packaged as 108 stereo plus 104 mono. This gives the user as many as 52 × 5.1 surround channels.
Finally, adding the system to Calrec's Omega console provides 160 channel processing paths packaged as 48 stereo plus 64 mono channels, allowing up to 24 × full 5.1 surround channels.
The advantages of Bluefin are obvious. A single card measures 8in × 10in, so it only takes up 10 percent of the space of a comparable system. It cuts the cost per channel in half so broadcasters producing HD programming can handle all the required 5.1 surround-sound signals cost-effectively.
Furthermore, broadcasters with existing Alpha or Sigma consoles can easily retrofit their desks with minimal disruption simply by replacing their existing cards with the new ones. The system's single-card design, backed by 100 percent redundancy on a spare card, also means greater reliability.
Increased audio delay, more control
In addition to DSP, the processor incorporates increased audio delay facilities, which are often required for HD production, especially when both SD and HD equipment are involved. Upconversion of SD video signals inevitably introduces delays for which audio must compensate. The high-density processor incorporates more than 19.6 minutes of audio delay divided into 432 mono legs of up to 2.73 seconds each. This delay can be positioned where needed in the audio path.
The processor also enables other processes that were impossible in the more traditional DSP core. The prefader monitor is in full surround, and the mix-minus return feeds for surround sources can be a mix-minus of the surround signal, entirely at the operator's discretion. The processor also allows full control of the surround main outputs' stereo down-mix levels, which, again, is often necessary when doing simultaneous surround and stereo mixes.
How it works
In its raw state, a field-programmable gate array (FPGA) is a silicon chip that contains a disconnected array of logic resources such as gates, arithmetic units and RAM. Equipment manufacturers make use of FPGAs by organizing and connecting their resources to perform the specific functions required.
Audio processing requires a large amount of math, but on a silicon chip, it's simple and repetitive math. In comparison, off-the-shelf DSPs satisfy a variety of uses, so they are a lot smarter than is necessary for audio signal processing. The company customized its FPGA programming to perform only the functions relevant to audio processing, thereby putting more dedicated computing power on each chip.
It sounds simple, but creating the high-density signal processing system was a difficult engineering task. A core team of five engineers constructed highly targeted circuits in the FPGA and modeled all the math calculations, which must be performed to high precision, in the console. The team discovered it could put an entire mixing console onto a single circuit card. This enables one card to replace 25, which greatly increases efficiency.
By putting all the digital signal processing into the FPGA domain, Bluefin allows broadcasters to realize advantages in cost, power, reliability and size. As Moore's Law continues to deliver improvements in FPGA density and speed, signal processing will continue to evolve.
Patrick Warrington is the technical director for Calrec.