This codec primer explores and explains the essential role that video and voice codecs play in video conferencing....
Codecs let users see and hear each other in real time at different ends of the call. By reading this primer, you will learn the details of video conferencing codecs, especially the popular H.264, or MPEG-4, and as a result be better equipped to select the best system for your unified communications (UC) environment.
How this codec primer can help you
Video conferencing uses both video and voice codecs to encode and decode transmissions, so both codecs must be considered when evaluating video conferencing equipment. This codec primer allows IT professionals to learn the issues around codecs and how they factor into the rest of their unified communications and network environment before investing in video conferencing. The codecs built into the equipment you choose today will affect who can use the system, with whom, on what devices they can communicate, and the other applications that can interface with the system for years to come.
Codec primer: On codec basics
Video conferencing systems give the illusion of talking in person because they compress and decompress digital streams of audio and video in real time. The device or software that performs this function is called a codec, short for coder/decoder or compression/decompression.
Hundreds of codecs are available, which makes choosing the appropriate equipment difficult. Codecs vary by encoding techniques, supported bit rates, audio frequency spectrum, image resolution and frame rate. Other varying issues that impact the purchasing decision include codec memory and CPU requirements, royalty payments and the availability of hardware acceleration.
Matching these specifications to conferencing needs will start communications professionals on the right path. But the main challenge is ensuring that users at every endpoint of the system can interoperate, which means their codecs must be compatible.
The International Telecommunication Union (ITU) has defined some codec standards, including video codecs H.263, H.264 or H.265 and voice codecs G.711, G.729, G.723.1, G.726, G.722 and G.728. Many codecs are proprietary and work only with a certain system or application.
In recent years, a trend has emerged of royalty-free codecs, which don't require licensing payments due to patents. Such codecs include VP8 and VP9 video codecs as well as SILK and Opus voice codecs.
Codec primer: On ensuring codec compatibility
Video conferencing systems must have three elements in common in order to function:
- Users on all endpoints of the conference must be able to initiate a connection by using the same call setup protocol. Session Initiation Protocol (SIP) has emerged as the enterprise standard for initiating video conferencing and other UC applications. In some deployments, the H.323 protocol will still be used.
- Users must share a compatible video codec. In order to see other participants, each user must be able to compress and decompress images using the same technology. The dominant video codec in video conferencing today is H.264, which was developed to provide high-quality video at lower bandwidth over a wide range of networks and systems. The ITU-T standard for H.264 is also known as the ISO/IEC standard for MPEG-4 Advanced Video Coding. The two are jointly maintained to have identical technical content. Newer products may also support H.265 or VP9, both offering better compression quality than H.264.
- Users also must have compatible voice codecs at each end of the conference to code and decode sound. The most common codecs for digitizing voice are G.711 and G.722. The emergence today of high-definition voice is causing changes in this technology, however, with AAC and Opus emerging as the dominant voice codecs.
Codec primer: On navigating codec 'flavors'
While many vendors are moving toward standards and interoperability, there are still many codec options for vendors to include in their products and for customers to navigate. Products often support multiple codecs or "flavors" of the same codec, but that means maintaining multiple algorithms and code tables in the device or software memory. Switching between codecs also creates complexity, delays call setup, and increases the frequency of errors.
Proprietary codecs often require licensing fees and administrative overhead, adding another layer of cost and inconvenience. Some codecs require users to pay royalties whenever they are used. These codecs generally appear in reputable products known for reliability and scalability, but usage can be costly.
The Scalable Video Codec (SVC) extension to the H.264 standard is helping to eliminate some complexity by allowing transmissions to rely on the original encoded stream, which can then be decoded with minimal software on a wide range of clients, such as PDAs and cell phones. SVC may someday eliminate the need for video conferencing gateways and multipoint control units (MCUs) to bridge multiple users.
Codec primer: On browser support and WebRTC
Web Real-Time Communications (WebRTC) was introduced in 2012. WebRTC enables the use of real-time voice and video communications inside web browsers without the need for added installations. Many video conferencing vendors have adopted WebRTC technology in one way or another.
WebRTC adds its own complications to video codecs, as it started by supporting only VP8, a royalty-free codec that was new in the market. Today, VP8 has become the de facto codec for most WebRTC-based services and many video conferencing vendors added support to it through transcoding.
Most modern browsers today that support WebRTC support both VP8 and H.264, with some already supporting VP9 as well. This adds to the complexities of selecting video conferencing vendors and fitting them to existing deployments, as many of the current requirements include guest access via web browsers -- something that is better handled by using WebRTC instead of forcing the installation of a browser plugin.
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Tsahi Levent-Levi asks:
What’s the biggest codec challenge you’ve overcome in your enterprise?
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