By Manuel Mota, João Risques, Synopsys, Inc.
Analog video interfaces are essential components of digital home and personal entertainment systems. This is mainly due to their ability to deliver very high image quality with very low power consumption while maintaining compatibility with most modern and earlier-generation video devices where analog video interfaces predominate. In the past, analog video interfaces were implemented using external components. With the proliferation of richer multimedia content in advanced consumer electronics such as DVD players, digital TVs and set-top boxes, integrating high-speed analog interface intellectual property (IP) including analog video interfaces and other multimedia analog and digital interfaces into systems-on-chip (SoCs) has become critical to achieve the necessary processing power and image quality.
Video formats can be divided into the following resolution categories: standard TV (PAL, NTSC, etc.), HDTV (e.g., 1080p) and widescreen VGA formats (e.g., 1920x1200) for PC graphics. All of these formats require the accurate transmission of color, brightness and synchronism information over a long cable. To accomplish this, the video transmitter must be able to transmit the video signal reliably and independent of the quality of the transmission cable. Conversely, the receiver must be able to accurately receive the signal and decode the synchronism information in order to re-create a high-quality image.
Maintaining competitiveness in today's consumer electronic products market requires integrating increased functionality into the same digital SoC, which, in turn, poses key design challenges in terms of power dissipation, form-factor reduction and ability to handle multiple video sources.
This two-part series discusses the implementation of analog video interfaces that can be embedded into complex SoCs. Part one focuses on the transmitter part of the analog video interface, which is essentially a digital-to-analog converter with video performance, and discusses the characteristics of the source and of the transmission medium. Part two provides a detailed review of the analog video interface receiver, which is based on a video analog front-end. Key characteristics are covered, highlighting the special features embedded into the IP that allow it to recreate a high-quality image on the destination side. Particular emphasis is placed on the features embedded into the IP that provide SoC designers with high-quality, low-power and compact-area video solutions. These analog IP components can be easily integrated into a wide variety of SoCs for video electronic products, making them an advantageous alternative to external components.
In the first part of this article, we discussed the general aspects governing the need and the selection of solutions for analog video interfaces, and detailed the most relevant technical aspects of the transmit side of such interfaces the video digital-to-analog converter (video DAC or VDAC). In this second part, we will focus on the receiver side of the interface the video analog front end (video AFE or V-AFE).
Video analog front end circuits (also known as video analog-to-digital converters [video ADCs] or video decoders) are essential components of digital home and personal entertainment systems. Despite growing use of emerging digital interfaces, analog video interfaces still play a prominent role in multimedia applications. Due to their high image quality and especially low power consumption compared to some popular digital video interfaces, there is a thriving market for consumer devices that use analog video as their primary connection. Additionally, computer displays and home entertainment equipment will continue to support the large installed base of set-top boxes, VCRs, DVD players, digital cameras and other equipment that only provide video in an analog format. To meet this requirement, system-on-chip (SoC) designers must incorporate high-performance video AFEs that can capture and digitize video from both xVGA computer video sources and a wide range of analog display formats, including the increasingly popular 1080p high-definition TV (HDTV or simply HD) format.
Typical implementation of an HD analog video interface
Click on image to enlarge.
As described in Figure 1, the video AFE "receives" the analog video signal that is typically sourced from a video DAC and transmitted over a 75 Ohm cable. Although this configuration is commonly used, video analog signals may also be originated differently another possible arrangement is used in TV broadcast, where the video AFE can be used after the radio frequency (RF) down conversion and the resulting video signal can be fed directly to the analog front end. Overall, as the signal source and transmission channel is often unknown and not normalized, AC coupling transmission is needed to create electrical isolation between the two sides of the system.