Over the past weeks, we’ve looked at the most successful industry-wide attempts to create a cross-manufacturer broadcast audio networking standard, including the efforts of the AES-X192 working group, the Ravenna-aligned manufacturers and the AVnu Alliance.
However, some proprietary broadcast audio networking standards have already evolved into fully featured protocols, albeit mostly manufacturer-specific ones.
Calrec’s Hydra and Hydra2 networks were developed to address specific industry needs and have evolved to meet broadcasters’ rapidly changing requirements. At this point in our exploration of network technologies we will refer to these technologies to illustrate some of the challenges which are singularly characteristic of broadcast infrastructures.
As explained in the earlier ‘Compromise & Capability’ installment of this blog, it is both a strength and a potential weakness of the Layer 2 and Layer 3 audio networking protocols that they are able to make use of off-the-shelf hardware. Strengths include the fact that Layer 2 protocols can make use of standard, affordable network cabling and hubs; Layer 3 protocols can use off-the-shelf IP-based routers and bridges, like Internet data traffic, and may therefore be multicast over a wide geographical area.
However, the downside is that predictability — and therefore reliability — may be compromised when data is routed via such industry-standard Internet hardware. And put bluntly, most responsible broadcasters are not prepared to run the risk of their content being delivered with the reliability of a YouTube video.
Proprietary protocols like Calrec’s Hydra also exhibit this compromise.
The most recent manifestation of Calrec’s standard, Hydra2, is a wholly proprietary Layer 1 protocol that is unable to use industry standard hardware above standard networking cable. As such, all of its routing and control hardware and software has to be specifically made for use with Hydra2, which makes it relatively expensive. However, the advantage is improved reliability, efficiency and predictability, and for many broadcasters this is a worthwhile trade-off. Modern professional broadcast audio networks, such as those installed in a multi-studio broadcast centre, may be routing tens of thousands of audio channels simultaneously. The ability to do so efficiently and deterministically, without any bandwidth restrictions, whilst keeping latency low and with plenty of scope for swift recovery in the event of component failure, is highly valued.
In part the low latency of Hydra2 derives from the data structure chosen for it. As a Layer 1 protocol, its data is not packaged in Ethernet Frames. Instead, audio is sent in 512-sample chunks with no headers, but with some capacity for control data. This method of data-packing is highly efficient, and because the start of the data is coincident with the leading edge of the audio sample clock, it contains implicit synchronisation information. The predictable arrival times of the data almost completely removes the need for buffering which reduces network latency — Calrec reckons with an 11-sample latency for an AES3 signal passing from from input to output across a Hydra2 network.
The Layer 1 proprietary nature of Hydra means that proprietary routers had to be developed — but again, this has a performance advantage. Each Router Core (as Calrec terms them), has a matrix that works like a synchronous TDM router, and can route one input to all 8192 outputs simultaneously if required.
Next week we’ll continue looking at some of Hydra’s specific features and the benefits they offer broadcasters, including redundancy and system control.
Calrec’s Network Primer series is in association with