PROGUEST
“Ultimately, SMPTE adopted the basic transport and synchronisation principles of AES67 and brought them into the SMPTE ST 2110 set of professional-media-over-IP standards”
Past, Present and Future: AoIP Standards in Broadcast The AES67 standard has been at the heart of audio over IP since its publication in 2013. Defining a minimum set of requirements essential to interoperability at the IP layer, AES67 fulfils the requirements of professional audio while remaining easy to adopt. Right about the time AES67 was published, SMPTE was exploring the use of IP for the transport of video, audio and related data. Any media format or media data can be encapsulated into IP, and AES67 proved that this approach could work. AES67 successfully handled key principles of operation related to IP transport – and the critical task of synchronisation, in particular – in a manner that could be applied to transport of video and data as well. Ultimately, SMPTE adopted the basic transport and synchronisation principles of AES67 and brought them into the SMPTE ST 2110 set of professional-media-over-IP standards. For the audio part (ST 2110-30), SMPTE references AES67 but adds a few constraints, mostly by removing some less important requirements mandated by AES67 and adding optional conformance levels regarding channel number and packet time recommendations, for better accommodation of typical broadcast production requirements. Because AES67 accounts only for 16- and 24-bit linear PCM formats, SMPTE created ST 2110-31 to accommodate AES3, another format originally defined by AES for serialised audio transfer on a bidirectional link
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(including MADI). AES3 has bits in addition to 24-bit PCM data that can be used to also carry other data, such as compressed or non-audio data as defined by SMPTE 337 and 338 (e.g., Dolby E, serial ADM). ST 2110-31 was built on the proven RAVENNA AM824 format for bittransparent AES3 transport over IP. Progressing with PTP Both AES67 and SMPTE ST 2110 are based on Precision Time Protocol (PTP) version 2.0, which dates back to 2008. Officially known as IEEE 1588, PTP outlines a standard protocol for synchronising clocks connected via a network. The release of the backwardcompatible PTP version 2.1 in 2019 introduced new features that allow broadcasters to take advantage of new and future IP-based audio and video solutions without the need to replace all their version 2.0 infrastructure. Broadcasters working with PTP v2.1 will benefit from enhanced accuracy, flexibility, robustness and security. Enhancing Control via NMOS While AES67 and other standards for IP audio address the transport layer, it is more difficult to agree on a common standard for upper-layer functionality. Nevertheless, recent efforts by AMWA with NMOS – and IS-04, IS-05 and IS-08 specifically – address discovery, registration and connection management, along with audio channel mapping. These specifications support the move to a common control layer, in turn allowing broadcasters to use
a single control interface for multiple devices, even when those devices themselves have proprietary interfaces. Crossing the WAN Together, AES67 and SMPTE ST 2110-30 and -31 cover virtually any scenario for transporting audio across IP networks. Both standards are designed to work in real-time production environments, mostly running on local or tightly administrated corporate networks. However, successfully transporting AES67 across a provider’s IP link without complete transparency or the guaranteed performance of a local area link can be challenging. For this reason, AES SC-02-12-M (the same group which developed AES67) is in the process of finalising an engineering document describing the challenges associated with running AES67 across wide area links, including cloud-based distribution. Embracing Immersive Audio With object-oriented audio becoming more popular, the AES X241 working group has been examining the use of pure AES67 to distribute audio objects along with a separate stream for audio object metadata, with PTP ensuring accurate synchronisation as these elements are reassembled upon delivery. SMPTE is looking at the same challenge, albeit through a wider lens, with ST 2110-41 – ‘fast metadata transport’. The expectation is that these efforts will ultimately yield results that are consistent across AES and SMPTE standards. Andreas Hildebrand is a RAVENNA evangelist.
