Precision Timing: The Case for PTPPosted on May 30, 2018
With the transition to IP networks for all aspects of the signal processing path, accurate timing becomes more difficult, due to the fundamentally asynchronous, non-deterministic nature of packet-based networks. Fortunately, a solution is available, in the form of IEEE 1588 Precision Time Protocol (PTP). When used properly, this technology can synchronize device clocks to within nanoseconds across a large network with many hundreds of nodes. When these clocks are derived from GPS (Global Positioning System) signals, PTP can provide a very accurate and stable time base for all types of signals within modern media operations.
The simple answer is that PTP is required by modern media standards, including SMPTE ST 2110 for IP video production and AES67 for IP audio. But that doesn’t explain the reasoning behind the decisions of the standards groups. To delve deeper, it makes sense to begin with a brief look at the history of video and audio signal processing and to see why synchronization and timing are so important.
First and foremost, video signals are built on a foundation of timing. Each frame of a video signal is built up of thousands or millions of pixels, each of which requires a data value to be generated by a camera or delivered to a display in the correct order and at precisely the correct time. Video signals contain sync signals that indicate the start of each video scan line and the locations of every line within each video frame. These sync signals are used by the receiving device to properly place each pixel in the appropriate location within the video image. In traditional video networks based on SDI, sync signals are normally generated centrally and distributed to each device in the form of a “black burst” or “tri-level sync” signal to “genlock” the devices to “house sync.” For digital audio, a similar function is performed using a DARS (Digital Audio Reference Signal). In traditional studio architectures, distributing these sync signals requires separate, overlay networks that connect to every device, adding considerable cost and complexity to the overall system.
With Ethernet networks, which are inherently bi-directional, it is possible to distribute a common time signal using IEEE 1588 PTP to every device without a separate overlay sync network. Each device can then accurately synchronize its internal clock to a single master clock. This clock time can then be used to accurately achieve media signal synchronization by means of the SMPTE Epoch, explained below.
The figure below shows a network made up of multiple devices that all share a common clock using PTP derived from GPS. Using this clock, each device can be referenced to the SMPTE Epoch, and hence to each other, without any need for an overlay clock distribution network. Contrast this with a traditional setup, which would have required one sync distribution path for video and another one for audio. Clearly, a PTP-based solution is much simpler.
For additional information about precision timing, its benefits, and applications download It's About (Precision) Time white paper
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