Audio Over IP - Making It Work - Part 1

Building and operating IP networks is much more than just about saving money on infrastructure costs. Its success is deeply rooted in the ease of flexibility, scalability, and inter-connectivity that it can provide. And one of the greatest benefits of IP is that the protocol and underlying hardware is independent of the data being carried, therefore, distributing and interfacing between different formats is easier than ever.

Traditional broadcast systems consisting of SDI, MADI, or AES work perfectly in isolation but become incredibly complex when signals need to be routed between them. De-embedding audio from the SDI feed to multiplex it into the MADI stream, or even distribute down an AES cable, increases cost and specialization. And differing sampling formats provide even greater challenges as we must configure each piece of equipment to the studio standard before transferring across the node.

Monitoring presents its own challenges since we would need systems with many different interfaces to access the whole of the signal chain. Sometimes, not all interfaces are available on one monitoring solution so two or more would need to be installed resulting in increased costs, complexity, and valuable real-estate in the studio or outside broadcast vehicle.

Rigid Systems Lack Scalability

Broadcast systems have stood the test of time and continue to deliver reliable high-quality audio. However, the price we pay is lack of flexibility. Rigid systems make scalability difficult and adopting new formats extremely challenging.

Moving to IP solves many of these challenges. Audio signals of different formats and sample rates can be distributed over the same network, as the physical interfaces needed for interconnectivity consist of standard “off the shelf” types often found in the IT industry.

IT Continues to Innovate

Fiber infrastructures deliver unparalleled bandwidths with modern routers increasing from 10Gbps to 100Gbps in recent times, and 400Gbps and more in the near future. Riding on the back of IT research and development, broadcasters can take advantage of the new speeds and innovation, with the certainty that there is always downward pressure on price.

Interfacing to telecommunication companies (telco’s) in broadcasting has long been a painful and expensive task, requiring specialized interfaces due to the limited adoption of SDI, MADI and AES in wider industry, thus resulting in greater complexity and increase in costs.

IP is Ubiquitous

IP is now the go-to interface for telco providers and is ubiquitous in the IT industry. An abundance of data circuits championed by IT has reduced prices and increased bandwidth and availability. Moving to IP systems allows broadcasters to hook into telco’s with ease and reduced costs, taking advantage of the downward pressure on prices demanded by IT customers.

One of IP’s greatest strengths is that the datagrams it sends are independent of the underlying hardware. IP is equally happy to send its data over ethernet, as it is over frame relay or Wi-Fi. And as a user, we do not know or care about the physical layer of connectivity between different nodes assuming our need for low latencies and jitter is respected.

Dia 1 – IP moves independently of the underlying hardware and communication channels.

Dia 1 – IP moves independently of the underlying hardware and communication channels.

To make IP systems easier to understand and interface to, the Open Systems Interconnection (OSI) model breaks network operations into seven layers to give an abstracted view and demarcation of the structure of networks. A User Datagram Protocol (UDP) will work on any IP system regardless of the source - assuming the manufacturer has complied with the specification for IP.

Ethernet is Pervasive

Ethernet networks are the most generic form of distribution for data streams within a broadcast facility. Speeds continue to increase as switch manufacturers plough more money into research and development. Data rates of 10Gbps are easily affordable with 25Gbps, 40Gbps, and 100Gbps in mainstream use. Simplistically, one 10Gbps ethernet or fiber channel can carry three uncompressed, 3G-SDI services.

Distribution of data takes place at both layer-2 and layer-3. Local Area Networks (LAN's) are often associated with layer-2 protocols such as Ethernet, and layer-3 routes packets between layer-2 networks to provide Wide Area Networks (WAN's).

Domain Routing

Although we must set the source and destination addresses within each device, such as the camera, microphone, or monitor stack, the internal network will only switch at level-2. It’s only when an IP datagram is routed outside of the network, or domain, that IP routing occurs.

IP datagrams distributed within a studio tend to travel over a common medium such as ethernet’s copper CAT6, but when the signal leaves the station it may well be converted to fiber. The underlying physical medium and its associated addressing changes, but the IP source and destination addresses stay the same.

To make multiple copies of camera outputs in a traditional workflow, we would use a distribution amplifier with each output going to a unique destination. In effect, we’ve provided "one to many" mapping.

In IP the same concept is possible but, instead of using distribution amplifiers, we use multicasting. Multicasting is a protocol used in switches, routers, and end devices such as cameras, microphones, and monitors.

Dia 2 – IGMP Host Membership Query Messaging.

Dia 2 – IGMP Host Membership Query Messaging.

Multicasting uses the principle of destination opt-in or opt-out. Using the Internet Group Management Protocol (IGMP), a destination device communicates with a router to request a copy of a multicast data stream. If the multicast stream is available on that router, the stream will be sent to the requesting port. If it is not, then the IGMP request is sent further upstream until the source device is found.

Switch Duplicates Datagrams

If a cameras output is to be sent to the production switcher and monitor stack, then both these devices will send an IGMP message to the switch to request copies of the stream. The switch will duplicate the multicast stream and output it to the requesting port.

The alternative to multicasting is for the camera to be manually configured to send its datagrams to each requesting device. In this case there will need to be two distinct streams of data leaving the camera, one for the production switcher and one for the monitor stack. In doing so, we’ve already doubled the data rate coming from the camera, and adding another destination such as a video recorder will triple its output. With HD and UHD outputs, this method soon becomes unsustainable on the link between the camera and switch.

SDN’s Overcome Limitations

Multicasting is an efficient method of distribution as the switches duplicate the datagrams, resulting in the load being taken off the camera, optimizing its IP output. However, IGMP relies on configuration information being typed into the routers resulting in complex network administration. To overcome these limitations, broadcasting is looking to Software Defined Network (SDN) techniques.

The flexibility and scalability offered by IP is unprecedented. We’re not just saving money by using commercial-off-the-shelf products, but also by providing a hardware interface that is independent of the data travelling on it. The result is future-proof broadcast operations.

Let us know what you think…

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