The basics much more than an ADC
At its simplest, a video AFE consists of one or more ADCs and a series of clamping, signal conditioning and filtering circuits. In some applications, such as HDTV and PC graphics, the video AFE also contains synchronism (sync) processing circuitry that is used to extract the timing and frame sync information embedded within the analog signal, as illustrated in Figure 2.
Simplified video analog front end block diagram
Click on image to enlarge.
An ADC's sampling frequency and resolution varies by application, with standard-definition TV (SDTV) requiring 10-12 bits at 27/54 megasamples per second (MS/s) and 1080p HDTV signals requiring 10 bits at 148.5 MS/s. Computer display applications require 8-10 bit resolution, with sampling frequencies as high as 205 MS/s to capture the full resolution of a WUXGA/60 Hz or UXGA/75 Hz resolution display. The variations in ADC requirements, however, are relatively small compared to the differences in the requirements for the analog portion of the video AFE's signal chain.
Regardless of the application it is supporting, the video AFE's analog signal chain is critical to its overall performance and, in most cases, a more demanding portion of the overall design effort than just the ADC. To understand why, let's look at the things that must be done to a raw video signal before it can be cleanly digitized by the ADC:
- Clamping: A video signal often has a DC component that must be removed or brought to a normalized level that is acceptable to the ADC input. In SDTV applications, this is handled by a charge pump clamp, but more sophisticated techniques (such as bottom or mid-scale clamping, sync-tip and others) are also required for computer monitors and HDTVs.
- Gain and offset control: A programmable gain amplifier (PGA) is used to adjust the incoming signal's swing to match the ADC's input range and control each color component's saturation (in component video systems) or the image brightness/contrast (in composite video systems).
- Clock recovery and sync generation: These circuits extract the sync signals embedded within the analog video signal and provide the digital data stream with the timing required to produce a properly framed image. The sync circuit's ability to recover timing information from weak or noisy signals plays a major role in the video AFE's overall performance and its ability to function under real-world conditions.
- Filtering: Programmable filters and de-glitching circuits are used to clean up the sync signal(s) being sent to the video AFE's sync and clock recovery circuits. In addition, HDTV applications require a very sharp (3rd order) low-pass anti-aliasing filtering to be applied to the video stream to suppress unwanted sampling artifacts and improve noise performance by strongly attenuating out-of-band noise that could otherwise be folded back to baseband during the analog-to-digital conversion.
A video AFE's capability to deliver high-quality video images relies not only on the performance of the ADC but also depends greatly on the remaining functions' quality and capability to operate according to different video formats' requirements.