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TDM: The Third Way - Part 2
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In Part 1 we looked at how TDM provides a compromise to deliver the flexibility and scalability of IP, while at the same time providing the ease of use of SDI. In this article, we look at how TDM deals with latency and frame accurate timing.
Television is a synchronous delivery mechanism designed to take advantage of some of the features of the human auditory (HAS) and visual systems (HVS). In today’s world, audio is digitally sampled and there are also no real moving pictures in television, just a sequence of still images played back very quickly to give the illusion of motion, hence the reason we have frame rates. However, both the HAS and HVS are very sensitive to disturbances in the sampling rate. If there are any discontinuities or significant variance in the timing of the sampling, viewers certainly notice them, and often experience stress.
Even in the world of IP where packets are asynchronously distributed, timing is of the utmost importance for broadcasters. SMPTEs ST2110 has helped alleviate many timing issues by using PTP (Precision Timing Protocol), however, this has come at the expense of substantial higher complexity and cost. Designing and building a reliable PTP compliant network is also a demanding and complex task.
SDI can be thought of as synchronous because the sampling clock is built into the transport stream. For the receiver to correctly display the images, and decode the sound where audio embedding is used, the receiver’s clock must be fully synchronous with the sender’s clock. That is, it must be phase and frequency aligned. The major advantage of this alignment is that latency is largely limited to the propagation time of the signal traversing through the cable.
In this context, ST2110 systems along with PTP are synchronous in that the sender and receiver clocks must be phase and frequency aligned. However, the challenge with IP and PTP is that it is susceptible to excessive jitter if the network is not correctly designed. This includes provisioning for symmetrical paths for the timing messages and PTP aware switches that take into consideration the buffer time of the switch. If not considered, the buffer time can greatly influence the receiver’s ability to fully synchronize resulting in lost packets or increased latency.
TDM systems simplify the timing as the synchronization is built into the protocol that links the nodes. As the network is self-managed, each of the nodes is aware of the topology wrt to the other nodes (through neighbor detection) in the network and is both able to determine the distance (= travel time) between them and the physical connectivity employed. This leads to deterministic and low latencies between the nodes which in turn enables highly accurate timing planes to be established. Not only does this keep latency low, but also negates the need for excessive time dependent buffers.
The Riedel MediorNet not only establishes highly accurate synchronization between the nodes that make up the network, but it also provides TDM in the specialist application of broadcast television. Consequently, not only are the nodes accurately referenced to each other, but each node also can provide a source of video reference for cameras and other image generating devices. In effect, the sync-pulse generator is distributed to each network node. Using the (distributed) system manager, the reference signals in each node are automatically timed to take into consideration the latency of the network, so every video (and audio) signal appears at a preselected node frame and line timed. Using the preselected node as the input to the production switcher will reduce the need for frame synchronizers and contribute to keeping overall latency low.
Fig 1 – the top line shows evenly gapped packets being distributed in a low-jitter TDM network leading to low latency. The bottom line shows a high-jitter IP network with the packets dispersed leading to either dropped packets or the need to include a larger buffer, which in turn will lead to much higher latency.
Another advantage of the TDM approach is the reduction in cabling and an overall simplification of the network. Each node acts as both a router and signal gateway so that no other physical equipment is required to make the network work. QSFPs using high-speed fibers provide four 100G links on the MicroN UHD allowing 12G UHD/4K signals to be distributed throughout the network with incredibly low latency. It’s also worth remembering that QSFP fibers allow signals to travel in both directions, so unlike SDI, the 12G UHD/4K signals can travel to and from each node. The TDM nature of the network significantly reduces the amount of cabling as multiple video, audio, control, and monitoring signals are multiplexed over fewer fiber cables.
As well as exchanging timing information between each other, the nodes also share management data in the form of control and monitoring information. A user-friendly and easy-to use application running on a PC communicates with every node throughout the network to provide configuration, control and monitoring functionality. Furthermore, the heartbeat messaging system continuously polls each device so that in addition to real-time monitoring, system status information can be accessed quickly by the engineer.
As each node is connected, the other nodes on the network automatically detect any new nodes to provide autoconfiguration, after which, the relevant information automatically displays on the monitoring software. All the timing and routing parameters are calculated in real-time by the nodes leaving the engineer with a complete plug-and-play infrastructure once they finish cabling the system up.
One of the challenges of building any broadcast infrastructure, whether a permanent installation or ad-hoc outside broadcast, is the need to create test signals. Every engineer knows that trying to fault find in a massive auditorium is a daunting task due to the physical separation between cameras, monitors, microphones, and sound consoles, etc., and that’s before the audience arrives. The MediorNet nodes not only deliver real-time monitoring, but also provide test signal generators. From the control software, engineers can select audio and video test signals allowing the testing of connection cabling from the node to the device, and testing of the devices themselves.
Jan Eveleens, Director Business Development Video Solutions.
As each of the MediorNet nodes are designed to work in broadcast specific applications, they provide services that are normally found in auxiliary equipment. These include frame-syncs, embedders, de-embedders, delay compensation, up/down cross conversion, color correction, and multiviewers. The powerful processing capabilities of the nodes allows advanced video and audio processing thus reducing the complexity (and cabling) of the infrastructure and further empowering the concept of decentralized networks. This extends to the concept of software defined infrastructures as in addition to these software-controlled video processing functions, each node provides video and audio routing, thus empowering a completely flexible and scalable infrastructure.
Recently, Plazamedia staged the UEFA Euro 2020 for Deutsche Telekom using Riedel’s TDM infrastructure with MetroN routers, 13 MicroNs, and nearly 20 MicroN UHD frames. The MicroNs formed a redundant ring each with 200G bandwidth and distributed 33 UHD signals, four of which originated from the IBC in Amsterdam. Furthermore, the system routed nearly 70 3G video signals and the audio layer handled 10 fully saturated MADI connections in addition to all the embedded tracks.
TDM is proving to deliver all the flexibility and scalability of IP, but with the simplicity of use of SDI. The decentralized managed philosophy of the network design combined with the plug-and-play capability makes TDM easy to operate and manage for both fixed and ad-hoc installations.