We’ve all experienced it.
The SPDIF output on device A doesn’t match with the analog line inputs on pre-amp B, which don’t synchronize with the VGA-to-composite-to-component de-interlacing, re-sampling, trans-formatter box connected to the monitor from the output device, which now really wants HDMI from your 1968 turntable. The problem of non-standard, proprietary, incompatible, poor-performing interfaces has dogged audio-visual equipment for decades, and most of us have a giant rat’s nest of cabling tucked behind some piece of furniture in our house to prove it.
In the modern era, when we start trying to network AV devices, we run into a similar mish-mash of incompatible, proprietary standards. Audio doesn’t sync with video. Quality of service isn’t guaranteed. Somewhere in the chain between the camera and the microphone and the storage there is an off-the-shelf component that’s not up to the task, and it requires the whiz-kid (the one who’s on vacation this week) to debug it.
The AVnu Alliance was formed by several companies with a vested interest in solving this problem, and they’re pushing standards and coming to market with products and components to make all our high-end AV systems sing together in perfect harmony. The group is supporting the emerging 802.1 Audio-Video Bridging (AVB) standard. The idea of the AVB standard is to facilitate low-latency, time-synchronized connections through 802 networks.
The first challenge was to try to get the audio and video (or whatever signals you’re moving) to arrive at the destination synchronized. For that, we turn to the second layer of the OSI Seven-Layer Burrito (the one between the avocado and cheese, perhaps?) the “Data Link Layer” with standard 802.1AS “Standard for Local and Metropolitan Area Networks – Timing and Synchronization for Time-Sensitive Applications in Bridged Local Area Networks.” This standard time-stamps your data so that the endpoint can synchronize it with a known, external timing source – thus your HD movies won’t all seem like English-dubbed Jackie Chan movies… unless they ARE all English-dubbed Jackie Chan movies, in which case you wouldn’t really care about 802.1AS.
The ne xt problem that the AVnu stan dard addresses is stream res
ervation for quali ty of service assurance. You see, if you’re trying to stream HD Video to a giant scr een in your concert venue, and some IT gu y in the back room is downloading pirated software with bittorrent, you don’t want your video to free ze up every few seconds. It’s really annoying!
The concisely-named 802.1Qat “Standard for Local and Metropolitan Area Networks – Virtual Bridged Local Area Networks – Amendment: 9: Stream Reservation Protocol (SRP)” provides for a Quality of Service (QoS)-assured end-to-end stream reservation for AV streams. It both provides the necessary information for bridges to understand the resources required for a particular stream and manages the reservation from the source to the destination.
The third prong of the strategy is 802.1Qav – “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks – Amendment: Forwarding and Queuing Enhancements for Time-Sensitive Streams” (whoever names these standards is clearly compensated by the word). This standard specifies per-priority ingress metering, priority regeneration, and timing-aware queue-draining algorithms using the timing derived from IEEE 802.1AS (which we discussed above).
The combination of these three standards gives us a quality-assured, end-to-end, synchronized AV connection through standardized 802 networking gear. With components and systems designed to this standard, commodity networking can replace high-end, proprietary (and somewhat cranky) networking strategies in a range of professional and high-end consumer areas.
The AVnu Alliance, founded by Broadcom Corp., Cisco Systems, Inc., Harman International, Intel Corp., Samsung Electronics Co., Ltd., and Xilinx, Inc. was established to promote adoption of these standards and to eventually provide for qualification of compliant components and systems. Although the standards and the related IEEE 1722 (AVB transport protocol – media encapsulation tied in with stream reservation and timing) and IEEE 1733 (extensions to RTP for use with AVB) are not yet approved, the alliance is moving ahead with promoting them in three key areas – professional AV, automotive, and home theater. The role of the alliance is compliance and interoperability. While IEEE publishes standards, it is up to independent organizations like AVnu Alliance to take the lead on projects like plug-fests to be sure that “compliant” components actually are able to work together successfully.
“The AVB protocols have been a couple of years in development in IEEE, and this group of companies came together to form an alliance to bring the technology to market,” explains Rick Kreifeldt, VP at Harman International and Chairman of AVnu Alliance. “We see broad-market applicability for these standards and have the need and interest in areas as diverse as automotive, consumer electronics, and professional AV.”
Harman and Xilinx worked together to develop an AVB core for implementation in FPGAs. Their implementation handles timestamping between MAC and PHY and a method for implementing queues. “What is nice to us about this solution,” continues Kreifeldt “is that to us, on the product side, there is Ethernet at the bottom and up through the Xilinx core. Then, at the top, it looks like a normal PCI interface, but out the sides squirts audio and video.” Xilinx’s solution spans a broad range of their FPGA products – from the 90nm Virtex-5 and Spartan-3 to the newest 40/45nm Virtex-6 and Spartan-6. For consumer and automotive applications, which are obviously very cost-sensitive, the low-cost Spartan devices will likely get traction, whereas high-end professional and broadcast gear may opt for the additional capabilities of the higher-performance and higher-density Virtex families.
If you’re doing the simplest case – sending a single AV stream around a network, this is probably all overkill. When you run into problems (and need this technology) is when you’re moving multiple streams, dealing with resource contention, and trying to get synchronized, glitch-free service on all ends. Today, debugging such a system requires a good deal of expertise, and inevitably you’ll find that some hub or switch is mis-configured, ruining the party for everybody. Broad adoption of a standard like 802.1 AVB will hopefully allow component and system makers to focus on differentiating features in their individual products and let those of us on the consumer end plug them together to fit our needs with a minimum of hassle.
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