Using Ethernet in the HD studio
Jun 1, 2008 12:00 PM, BY GAËL MACÉ AND MICHAEL JOHAS TEENER
In last month's article, we examined some advantages of moving to a common network infrastructure based on new-generation Ethernet protocols. In addition, we discussed some of the underlying protocols being developed to support an IP-based production environment. Now it's time to describe in more detail the next-generation HD studio with genlock, real-time switching and studio network management.
Studio genlock over Ethernet
The video genlock signal exactly synchronizes the frame rate of video equipment with a reference signal also called genlock. Each piece of equipment has to be in perfect sync so there are clean takes and transitions in live switching, editing and post production. Without genlock, switching between sources results in a momentary loss of image stability while the monitor or device tries to lock itself to the new signal. Without synchronization between all sources, the images may roll either vertically or horizontally, or break up completely.
Genlock synchronizes four key video signal attributes: vertical, horizontal, frame and color synchronization. These are all normal parts of a standard composite video signal. When properly combined, the result is a correctly displayed image. A standardized signal, which includes all of the attributes but without any actual video image, is known as black burst. In the absence of black burst, many genlockable devices will instead accept a standard composite video signal for synchronization.
In order to genlock two video sources, at least one must have a genlock input. The other signal source may be used as the master, from which the sync signal must be derived. Alternatively, a variety of sync generators are available, which either produce black burst from an incoming video signal, or generate their own internal black burst references for all connected genlockable cameras in the system.
The problem
The first production goal is to synchronize any camera signals so that video coming from different cameras can be cut and mixed without roll, jump or chroma shift.
The constraints of how precise synchronization must be are strong, down to the pixel level. This requires a timing precision in the range of several tens of nanoseconds. This precision level was originally required by legacy analog equipment, which was quite sensitive to frequency chroma shift and offered minimal buffering.
With the migration to a digital world, past constraints are not so severe. Digital color transmission does not depend on any frequency, and buffering is no longer a problem. Even so, time constraints have not disappeared. For example, if two unsynchronized cameras are shooting the same action, the display of their two pictures on the same screen may exhibit a slight delay between the two images. There may also be some stutter as frames are either dropped or repeated.
Figure 1. As with standard video chains, AV sync must be maintained throughout the entire system. End-to-end delay should be limited to no more than two frames.
Click for full image
The global latency caused by the overall production chain (from the camera's head to the output of master control) must be imperceptible to the human eye. Throughout the entire IP studio, there should be an appearance of exact synchronization among video equipment. In addition, the camera operators must see the video on their monitors without delay as the scenes are shot. An example system block diagram demonstrating genlock is shown in Figure 1.
A system's global, end-to-end delay, from capture to the output of the video switcher, should be constrained to one or two frames. For synchronization reasons, this delay is typically an entire multiple of a frame. Because a video switcher and its video effects circuits usually make use of these one or two frames, the latency inherent to the network and its core equipment must be minimal — preferably, less than one frame.
Furthermore, as with every multimedia flow, jitter is a problem because it increases buffering. The input buffers of a current video switcher are typically proportioned to manage no more than two or three video lines of jitter (a few tens of microseconds). Because packet buffering increases latency, the network itself must have low jitter.
Finally, in a production environment, the boot time needs to be short and accurate. When video equipment is connected to the infrastructure, especially for a live event, it needs be able to start operation in less than one minute. Also, the synchronization systems and related servo mechanisms must have short convergence times.
A layered approach
Figure 2. Synchronizing IP-based AV networks is more complex than with analog or digital video networks. The solution requires layered synchronization. The IEEE 802.1 AS system will provide the network timing signal, but the video equipment needs to add counter values, which can be used to develop genlock.
Click to enlarge
The main difficulty in synchronizing equipment over an Ethernet/IP network is that packet transmission time over the network is not constant. The consequence is that there is always a time difference between the instant at which a packet is received and the instant at which it was intended to be received. This difference corresponds to the transmission jitter. While its average value may be zero, for each packet, its value is not zero.
A key component of IEEE 802.1AS is the ability to provide an accurate network timing service. This feature will ensure the transmission of a clock with limited jitter, typically in the submicrosecond scale. Further filtering of the 801.1AS clock has been shown to meet the requirements of uncompressed HD video.
On top of this network layer, a video application could transmit counter values at which synchronization signals (genlock) should occur. This layered synchronization is shown in Figure 2 on page 104.
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