Sound Devices Employs Blackfin® to Reduce Size/Performance Ratio in Portable HD Digital Recorders

Sound Devices, LLC builds a family of portable, field-production audio products that include mixers, preamps, computer interfaces, and recorders. The company's products are used in applications such as Hollywood feature films and documentaries, sound effects gathering, sports broadcasting, acoustical test and measurement, live music recording, and performance.

Sound Devices prides itself on delivering superb audio performance in extreme locations. Producers don't want to compromise sound quality just because they have to go to a tough location; in addition to sound quality, they want equipment that is easily portable, highly rugged, and power efficient. The last thing a radio reporter or documentary film producer trudging through the Costa Rican rainforest or hiking through the Himalayas wants to do is to carry a heavy four-track digital audio recorder on his or her back. And a recorder rendered useless due to a dead battery would be even more objectionable at the end of a long journey.

To create products that address these demands, Sound Devices turned to the Blackfin® ADSP-BF535 processor from Analog Devices, Inc. (ADI). According to Sound Devices engineer/designer Jim Allard, the Blackfin processor's high horsepower-to-power draw ratio allowed the company to "shatter the size, performance, and feature-set paradigms of previous generations of audio recorders." The resulting size/performance ratio of the Sound Devices 7-Series HD portable digital recorders is unmatched by any other audio recorder. Weighing less than three pounds each, the two-track 722 and four-track 744T recorders write and play audio files to and from an internal hard drive or Compact Flash, with either 16- or 24-bit depth at professional sample rates up to 192 kHz.

Allard said, "We investigated other signal processors on the market and chose a Blackfin ADSP-BF535 based on its high processing power, low power, and ease of programming."

"We investigated other signal processors on the market and chose a Blackfin ADSP-BF535 based on its high processing power, low power, and ease of programming."
Jim Allard,
Engineer/Designer
Sound Devices

Sound Devices On Location
Whether it's an assorted crew of production professionals sweating out the final pieces of a documentary in the Syrian Desert, or a harried few downing coffee and smoking cigarettes in the back stages of a Hollywood feature film, sound pros have their choice of using a full complement of Sound Devices' audio mixers, recorders, preamps, and computer interfaces, or they can mix and match various components. "Quite often, sound crew will use a combination of a Sound Devices 422 Field Production Mixer and the Sound Devices 744T digital audio recorder with time code for documentary film, video, and feature film production," said Sound Devices engineer/designer Jim Allard. "Or they'll use a Sound Devices 442 or a Sound Devices 302 mixer and the two-channel 722 digital audio recorder for applications such as sound effects gathering and live music recording."

The 442 Field Production Mixer contains four microphone preamplifiers and comprehensive inputs and extensive outputs, including direct outputs per channel for small applications and large, multiple-input productions alike. Sound pros can add additional microphones by purchasing a preamp for use with the 744T digital audio recorder's line level inputs. The Sound Devices 302 mixer is a compact, cost-effective battery-powered professional audio mixer for over-the-shoulder production. It interfaces with wireless transmitters and receivers, camera audio inputs, and external audio recorders, and was developed for audio for picture applications.

The 722 is a two-track digital audio recorder, ideal for audio, and the 744T is a four-track digital audio recorder with time code, which makes it ideal for portable or cart-based production jobs. As mentioned previously, both digital audio recorders record and play back audio files to and from a 40 GB internal hard drive or a Compact Flash drive. The recorders can write and read uncompressed PCM audio at 16 or 24 bits with sample rates between 32 kHz and 192 kHz. They support compressed MP3 audio playback, record from 64 kb/s to 320 kb/s, and implement Sound Devices' microphone preamplifiers for high bandwidth, high-bit rate digital recording. Both recorders feature removable, rechargeable batteries-a standard Sony-compatible Li-Ion camcorder cell. The recorders also interconnect with computers running the Windows™ or Mac OS® operating systems for convenient data transfer and backup.

Moving Data Around
"Our main function is about moving audio data from one place to another. First there are all of the processes involved with recording the audio/video. Once the audio/video is recorded, it is sent from the hard drive through the FireWire port to a computer where a sound pro can either listen to it or burn a CD," said Allard. "Because the Blackfin processor is so powerful–more than 300 MIPS–we were able to save money by doing everything with the signal processor instead of using an MCU for some of the chores." FireWire, also known as the IEEE 1394 standard, is a high-speed serial bus that is widely used for downloading audio and video files to a computer.

The Blackfin ADSP-BF535 is a 350 MHz processor, which features a Micro Signal Architecture (MSA), and is a highly integrated system-on-chip solution for digital communication and portable Internet appliances. It combines a dual MAC state-of-the-art signal-processing engine with the advantages of a clean, orthogonal RISC-like microprocessor instruction set, and Single-Instruction, Multiple Data (SIMD) multimedia capabilities in a single instruction set architecture.

Blackfin also includes on-chip peripherals that can be easily augmented in many system designs with little or no glue logic due to the inclusion of several interfaces, providing expansion on industry standard busses. The Blackfin ADSP-BF535 peripherals include two UARTs, two SPIs, two SPORTS, general-purpose timers, a real-time clock, programmable flags, a watchdog timer, and USB and PCI busses.

"We chose the particular model of Blackfin, the ADSP-BF535, specifically because of its peripherals, and especially for the fact that it features a PCI bus," said Allard. "We were able to connect our digital audio recorder to a FireWire port on a PC through the Blackfin ADSP-BF535 processor's PCI bus using off-the-shelf PCI-based FireWire chips, which was very cost effective for us. Plus we were able to utilize more bandwidth transferring audio files–such as MP3 or .wav files–from the digital audio recorder to the PC by going through Blackfin's PCI bus than if we used SDRAM. And we didn't have to tie up the memory or the disk drive during file transfers."

As mentioned earlier, the recording process involves many steps that could slow down a processor but not the Blackfin. Sound Devices leveraged the Blackfin ADSP-BF535 processor's multiple, independent DMA controllers that support automated data transfers with minimal overhead for the processor core. DMA transfers can occur between the ADSP-BF535 Blackfin processor's internal memories and any of its DMA-capable peripherals. Additionally, DMA transfers can be accomplished between any of the DMA-capable peripherals and external devices connected to the external memory interfaces, including the SDRAM controller, the asynchronous memory controller, and the PCI bus interface. DMA-capable peripherals include the SPORTs, SPI ports, UARTs, and USB port. Each individual DMA-capable peripheral has at least one dedicated DMA channel.

"Every time an audio file is recorded, DMA transfers are involved," said Allard. "First a file is sent through an A-D converter, then to the disk drive to create the audio file. It is then streamed through the D-A converter and channeled through the headphones for playback and then sent to the disk drive for storage. DMA is a great way to transfer information quickly without weighing down the processor."

Fast Time to Market
Filmmaking is a fast-paced industry, and Sound Devices took full advantage of all the help ADI could provide in the way of tools and support. In particular, Sound Devices had good things to say about ADI's VisualDSP++ integrated debugging and development environment (IDDE) and straightforward programming, which Allard said helped speed the time to market for Sound Devices' products.

The Visual DSP++ project management environment lets programmers develop and debug an application. It also includes an easy-to-use assembler (which is based on an algebraic syntax), an archiver (librarian/library builder), a linker, a loader a cycle-accurate instruction-level simulator, a C/C++ compiler, and a C/C++ run-time library that includes signal processing and mathematical functions. The debugger includes visualization features and a plotting package that enables the programmer to quickly determine the performance of an algorithm. As algorithms grow in complexity, this can speed the development schedule. The program also includes statistical profiling, which enables the developer to gather code execution metrics without interrupting the program, identifying bottlenecks quickly.

Another key aspect of the ADSP-BF535 Blackfin processor that was crucial for Sound Devices is its power management capabilities, as Sound Devices products are portable products that must operate in harsh (and remote) environments and run on removable 7.2 V (nominal) Sony M-Series batteries. To facilitate low power operation, the ADSP-BF535 Blackfin processor includes control functions that enable developers to tailor the performance and power characteristics of the processor and system to many application scenarios. It provides four operating modes, each with a different performance/power dissipation profile. In addition, dynamic power management provides control functions with the appropriate external power regulation capability to dynamically alter the processor core supply voltage, further reducing power dissipation.

Sound Devices' products are based on a core, scalable architecture that will accommodate easy updates. Allard expects Sound Devices to offer new digital audio recorders. There's no doubt in Allard's mind that the new products on the drawing board will include Blackfin, Hollywood's latest star.

For more information about Sound Devices, please visit Sound Devices .

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