TIMING AND SYNCHRONIZATION IN BROADCAST VIDEO

1.Introduction

Digitization of video signals has been common practice in broadcast video for many years. Early digital video was commonly encoded on a 10-bit parallel bus, but as higher processing speeds became practical, a serial form of the digitized video signal called the Serial Digital Interface (SDI) was developed and standardized. Serialization of the digital video stream greatly facilitates its distribution within a professional broadcast studio.

In a studio with multiple cameras, it is important that video signals coming from multiple sources are frame aligned or synchronized to allow seamless switching between video sources. For this reason, a synchronization signal is often distributed to all video sources using a master synchronization generator. This allows video switching equipment to select between multiple sources without having to buffer and re-synchronize all of its input video signals. In this application note, we will take a closer look at the various components that make up a broadcast video system and how each of the components play a role in the synchronization chain.

2. Digitizing the Video Signal

A video camera uses sensors to capture and convert light to electrical signals that represent the three primary colors– red, green, and blue (RGB). Horizontal, vertical, and field synchronization (HVF)
The sample rate needed to capture video depends on its resolution. Note that the color difference (PbPr) signals are sampled at half the rate of the luma (Y) signal since human vision is more sensitive to changes in light intensity than changes in color. The three 10-bit luma and color difference video data streams are multiplexed to form a single higher rate 10-bit YCbCr stream, which is then serialized and encoded to meet SMPTE standards. Since the resultant SDI datastream is sampled using the timing generator, it becomes synchronous with the studio’s master sync generator. This is referred to as genlocking in the video world.


3. Genlocking - Synchronization of Video Equipment

In a studio with multiple cameras, it is desirable to synchronize all video sources to facilitate downstream video processing such as switching, editing, keying, fading, etc. This process is known as genlocking the video equipment and is achieved by synchronizing all of the video sources to a common synchronization signal.The master sync generator provides a common synchronization reference to all equipment
that generates a video source.


4. The Need for Synchronization Within Video Network Components

Many components are involved in the production of the final broadcast video signal. Video switchers accept video sources from multiple sources such as cameras and video storage devices. Distribution amplifiers send video signals around the studio and perform the job of a repeater to help clean up the signals they distribute. Timing generators and frame synchronizers ensure that all video sources are synchronized making the process of switching, keying, and editing possible. Video routers provide a single point for switching and routing video signals to and from other studios. Complicating the process further, many of these components also must support the multiple standard definition and high definition video formats that are available today. In this section, we will take a closer look at the various components that make up a video network and investigate their timing and synchronization requirements.


5. Jitter Requirements for Video Clocks

As the video signal travels through the interconnecting wires and the components that make up the video network, the digitized signal will accumulate jitter. If jitter becomes excessive, the video signal will deteriorate to the point where it is no longer usable or recoverable. For this reason, jitter reduction circuitry is included at several points throughout the video path to maintain signal integrity.

The process of recovering video from the SDI signal requires extracting both the data and a sampling clock from the datastream. This is known as clock and data recovery (CDR). Since the clock is recovered from the SDI datastream itself, it will track its jitter but only within the loop bandwidth of the phase-locked loop (PLL) based CDR circuitry. Jitter that occurs above this loop bandwidth will not be tracked; it will be filtered. Although recovering a jitter filtered clock sounds like a good idea, it can cause decoding errors if there is still excessive jitter present on the data. For this reason, SMPTE has specified limits on the jitter content of SDI signals.


6. Simplifying Timing and Synchronization in Video Networks
There is no doubt that timing and synchronization plays a critical role in successful processing and distribution of the video signal within a studio. Because of the multiple standardized video formats available today, both the asynchronous oscillators (XOs) and the phase-locked loops (PLLs) that make up the timing path need to be flexible enough to support multiple clock rates and frequencies. Often PLLs need to translate (or convert) between noninteger related frequencies as in the case of a timing generator that needs to synchronize to an NTSC HSYNC rate and generate an HD-SDI sample clock for an SDI serializer (e.g., locking to 15.75/1.001 kHz to generate 74.25/ 1.001 MHz requires a multiplication ratio of 4719000/1001 or 4.71428571428571).


7. Conclusion
We have seen that synchronization plays a critical role in the broadcast studio. Not only is it necessary within the video signal itself so that the receiver knows how to frame the pictures that it receives, but it is also necessary at the physical level to ensure proper serialization and de-serialization of the video signal. The oscillators and phaselocked loops that make synchronization possible in these video systems need to provide multiple frequencies to support today’s variety of standard definition and high definition resolutions. Not only is flexible frequency conversion important, but controlling jitter with properly placed jitter filters is critical in successful recovery of the video signal as it propagates through the video equipment within a studio.


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