AV Over IP – A Primer

AVoverIP

AV Over IP is the term for technology that delivers audio visual content over Ethernet. The term also implies that content traditionally sent or switched over analog or digital switching now employs IP packets and standard Ethernet switches between the source and destination.

There are two basic AVoIP applications, Distribution and Presentation.

Distributed IP Delivery

  • Content is distributed over a large area
  • Endpoints described as encoders and decoders
  • Usually a high ratio of decoders to encoders
  • Streams are highly compressed to save network bandwidth
  • Half- to 1-second latency is acceptable, depending on application
  • Streams can provide captioning data and audio sync

Distributed content is sent over large areas, including nationwide through Netflix or over a single site from cable channels, modulated over RF coax, or via an IP network. Due the larger scale of distribution, streams should be as small as possible, able to carry ADA captioning, and don’t require instant switching for viewing. The standard format for this application is MPEG, usually MPEG2 for off-air TV and in-house RF channels, and H.264 for commercial and consumer IP-based TV.

MPEG is designed for maximum compression – H.264 can easily compress a 3Gbps (bits per second) 1080p video to a 15 mbps stream, a 200:1 ratio, and consumer streams are compressed far more. The secret sauce is called the GOP, the Group of Pictures, and only the first frame is an actual image. MPEG compresses the first frame into an image similar to a JPEG. The encoder then captures a few more frames and notes objects that move or change color, and saves just that data. This is called inter-frame encoding, as the encoder is continually cross-referencing frames. When a decoder changes a stream, it has to capture the first several frames in the next GOP and rebuild the video back to its original content. This deconstruction/reconstruction process takes time, creating about a half- to one-second delay. That’s why you experience a pause when changing MPEG channels.

Presentation IP Delivery

  • Displayed in a defined area
  • Endpoints described as transmitters and receivers 
  • Simpler to define and expand input/output configurations
  • Large streams require a dedicated IP network
  • Instant switching, about 2-4 frames
  • Streams composed of images and audio tracks, no captioning data

When a system is delivering a live presentation or event, the content on the video screens must to be in sync with what’s on the stage or podium, and cameras need to be changed instantly. Visible latency is disconcerting to the presenter and audience. This is the traditional application for AV switching systems.

The new counterpart to AV Switching is routing content over an IP network. Instead of a central switch with multi-port input and output cards, designers can define any number of individual transmitters and receivers, routed through a standard Ethernet switch.  Video frames are converted to stream packets using Motion JPEG 2000, typically compressed to 3:1 to 20:1 ratios. As the stream consists of individual images, switching is fast, with little latency. 

However, latency is a bit looser than AV’s vertical-interval switching. IP guarantees delivery, but there is no central clock to lock down timing, so there are network variables that could affect latency. As with AV switching extensive scaling at the input and output points can affect latency as well. As there is not sync data as there is in MPEG, audio could be behind – or ahead of the video, especially if the video is highly compressed and scaled and the audio just passed through. I would imagine some IP systems offer settings to minimize audio timing issues. 

What is the Effect of 4K video on commerical IP distribution systems?

For commercial systems, 4K is more of a marketing pitch than a reality. Current room and screen designs can’t take advantage of 4K, and very little content will be 4K for some time. It just isn’t needed. 

MPEG distribution technology is typically limited to 1080p. However, H.264 does support 4K and “zero latency“. For the present, 4K codecs such as HEVC (H.265) and VP9/AV1 aren’t usable for commercial applications.

 

1 thought on “AV Over IP – A Primer”

Comments are closed.