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1394 TRADE ASSOCIATION TECHNICAL BACKGROUNDERDigital Audio Relies on the Features and Benefits of IEEE 1394 Morten Lave, CEO of TC Applied Technologies Like most everything else, professional audio production happens almost entirely in the digital domain these days. The digital transition has enabled amazing production capabilities on PCs using digital audio workstation (DAW) software. But with the digital transition came the need for a digital interconnect that had the bandwidth needed to move audio streams in real time along with the need for timing and synchronization capabilities that allow mixing of multiple audio channels. The 1394 or FireWire interconnect has proven to be the only interconnect that can support pro audio applications. The range of professional audio applications spans live performances to the studio. Only a few years ago, the production task was limited to studios with the huge budgets needed to buy racks of tape decks and special analog mixing equipment. The digital transition allows professional quality on much lower budgets and indeed musicians and bands can now buy their own equipment and produce material for CDs and Internet distribution. Let’s examine the type of products and the topology of a system used in professional audio. And first we should differentiate the professional space from the consumer or prosumer market. Any desktop or notebook PC today comes equipped with baseline audio capabilities and there is low-cost DAW software on the market. But the baseline capability usually means a single stereo channel in and out. In desktop PCs, soundcards may add support for a few additional channels. Moreover there are external audio I/O interfaces with configurations such as 2 input and two output channels. The professional market starts in the 8-channel input and output range. Even an individual musician needs such capability, as many non-professionals are now discovering. A larger band might need 16, 32 or even more input and output channels. The professional market is built around three basic components – the PC, I/O boxes or interfaces, and a mixer. Some manufacturers integrate both I/O and mixing in a single product. The I/O box typically includes a microphone preamp for each input channel. The devices also include A/D converters to digitize the input, D/A converters to output analog channels, and sometimes DSPs to perform other audio processing. The I/O box links with a PC via 1394. The analog outputs from an I/O box might connect to a mixer or potentially to speakers. And the mixer would generally connect to the 1394 bus also, to receive the digitized audio channels. Live session and post production A typical usage scenario illustrates the channel demand and how the 1394 interconnect is utilized. Here it is: A band might choose to record a live performance for subsequent distribution via CD or the Internet. Independent of the microphones used by the performers, the recording requires a number of dedicated microphones. You would typically use at least four or more microphones for the drum kit. You need a microphone for the amplifier/speaker used for each guitarist. In some cases - such as with an electric guitar - you can use a direct connection between the guitar and I/O box. But the robust sound produced by a guitarist is a compilation of the sounds made by the guitar, the effects added via the amplifier or a dedicated device such as a reverb or delay box, and the audio qualities of the amplifier and speaker. You also need microphones for any other amplified instruments. And you need inputs for the singers. Even a small band can require 15 or more channels. During the live performance, the production equipment must support both the concert and the recording that will be put through a separate production process later. During the concert, the I/O box would stream the channels over 1394 to a mixer used by a band member or support personnel to optimize the concert sound. The same stream would also go to a notebook PC for recording. Typically the band would want to record the raw audio as opposed to the output of the mixer used at the concert. That affords the musicians freedom to optimize the mixing process once back in a studio. 1394 attributes and pro audio requirements Realistically, the 1394 interconnect is the only viable solution for the scenario we’ve just reviewed. The reasons include bandwidth, topology, isochronous capability, timing information carried with the data, and peer-to-peer communications. It’s also important that 1394 allows scalability via additional devices that musicians can add to the bus as required. Professional audio requires significant bandwidth. The I/O boxes capture 24-bit data sampled at rates as fast as 96 kHz. The 1394 interconnect supports data rates as fast as 400 or 800 Mbps. And faster 1.6- and 3.2-Gbps versions are on the way. Generally, the 400-Mbps flavor affords the performance required in a professional audio application.
The 1394 interconnect guarantees QoS (quality of service) via its isochronous capabilities. Essentially, a 1394-based device can reserve the bandwidth it needs. Ad hoc traffic on the interconnect will not impact the real-time audio streams. Timing information is also carried along with data on the 1394 interface. That timing information is critical for the producers mixing a recording back in the studio – allowing easy synchronization of multiple channels. The scalability angle allows musicians to buy I/O boxes and mixers with the channels they presently require while knowing that they may need more channels later. It may also allow a band to carry less gear to a performance relative to what they need in a studio. The 1394 interconnect supports almost any topology including most commonly a daisy chain of devices. That makes it easy to connect a PC, I/O box, and mixer. But device manufacturers such as Presonus also utilize that topology to offer an expandable system. For example, you can link two 16-channel mixers and realize a 32-channel mixer with the 1394 interface proving the real-time connection. The peer-to-peer and synchronization capabilities also come into play in such daisy-chained scenarios. For example, the second mixer acts as a slave to the first. But the slave mixer can simultaneously transfer its output to the master mixer and the PC. USB and Ethernet Some companies have proposed using other interconnects in professional audio systems. The options are USB and Ethernet. Some of the low-end consumer-centric music I/O boxes utilize USB 2. But USB has a number of limitations in professional applications. USB 2 has a theoretical maximum transfer rate of 480 Mbps. Realistically, USB doesn’t come close to that rate even in a simple scenario with a PC linked directly to a single peripheral. But the media-access scheme and protocol are even bigger roadblocks when it comes to professional audio. USB requires that the host PC control all operations. Were an audio application to move data from an I/O box to a mixer using USB, the host would have to read the data from the I/O box and send it to the mixer. USB lacks any isochronous or synchronization capabilities. In a music application, the bandwidth wouldn’t support multiple I/O boxes or mixers. And even if it could, you couldn’t synchronize the recording back in the studio. What about USB 3? Certainly the emerging new flavor is supposed to offer much higher data rates. But not cheaply - there are very expensive modular host interfaces on the market. It’s not even clear at this point that USB 3 will follow USB 2 into ubiquitous deployment. Handset makers are balking at supporting USB 3 because of the inconvenience of the connector. Also, standard Ethernet is also incapable of serving the professional audio application -- even the 1-Gbps flavor. Ethernet lacks the required isochronous ability. The Ethernet community is attempting to address the shortcomings of the network technology in multimedia applications by adding the Ethernet AVB (Audio/Video Bridging) layer. We’ve yet to see that technology mature. But in any event it will still lack the topological advantages of 1394. Moreover, it will be a significant added expense to support Ethernet in I/O boxes and mixers. Integrating 1394 in audio products Design teams working on professional audio products have a number of options in terms of ICs that support 1394. There are numerous general-purpose 1394 ICs on the market. And there are some ICs that specifically target professional audio. TC Applied Technologies, for example, offers the Dice family of ICs. The Dice Jr product can support as many as 64 channels of audio sampled at 96 kHz. The IC includes a direct interface for A/D and D/A converters and includes a hardware mixer. Solutions can also be made using a combination of FPGA’s and other devices as done by MOTU and RME. Extending 1394 in professional audio There are compelling reasons that could lead to broader integration of 1394 technology into professional audio products. For example, it might make sense to integrate 1394 interfaces directly into instruments and even microphones. To ensure synchronization capabilities, such a microphone would need to be a slave to another product such as a mixer or I/O box. And the microphone would require an integrated jitter-free clock to support synchronization. It also makes sense to integrate 1394 into several types of speakers. For example, studio-monitor speakers would be a natural for the interface. This kind of implementation allows music to remain in the digital domain from the initial data conversion right through to the analog conversion that drives the speaker. Not only are professionals using 1394. Now, the trend among pro-am and amateur musicians is toward the FireWire interface, proven and effective across a wide range of platforms. Morten Lave, a veteran of the IC and music industries, is chief executive of TC Applied Technologies, a division of TC Electronics. He can be reached at MortenL@TCTechnologies.tc
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