Much-hyped PC/TV convergence is becoming a reality in many display products. Consider an LCD PC monitor with a TV input, or a big-screen TV coupled to your home PC for Web surfing. Also, projectors for business presentations are shrinking to the point where they can easily be connected to your DVD player in a temporary home-entertainment setup. Al-though the simultaneous support of video (TV) and PC inputs might be obvious to the end user, the hardware and software designers must contend with a multitude of video-interface standards and the instruments upholding them.
Historically, PC and TV formats developed with little in common. Video targeted over-the-air transmission. Consequently, it's a bandwidth-limited signal that has black and white (luma) and color (chroma) information frequency-multiplexed into one composite-video signal using interlaced video scanning. Standards evolved separately in Europe and the U.S. for composite video: PAL and SECAM in Europe, NTSC in the U.S.
However, graphics signals developed for a point-to-point connection between a PC and a monitor aren't bandwidth limited. They require separate red, green, and blue (RGB) components, with video-synchronization signals carried on dedicated lines. While there's a common framework for PC graphics signals with no regional differences, several industry-standard formats exist, including legacy IBM and MAC graphics formats.
Enter digital TV (DTV), which introduces component, not composite, video transmission using the YCbCr color-space representation (Y = luma; Cb and Cr are chroma difference signals), instead of RGB. DTV also unlinks the video and transmission formats; the same transmission system can be implemented for several formats—actually up to 18 variations for terrestrial DTV. Some of these are the common 1080I and 720P high-definition (HDTV), 480I standard-definition (SDTV), and 480P enhanced-definition (EDTV) formats.
Creating A Video/Graphics Front End: Today, designers are building all-format video front ends using several available ICs: an analog-input video multiplexer, a (digital) video-decoder IC, and a high-speed triple-graphics ADC analog front end (AFE). These front ends handle the following signals (Fig. 1):
- CV: composite video (PAL/NTSC/SECAM) that combines luma (Y) and subcarrier-modulated chroma (C) in one signal. The color subcarrier for NTSC is at 3.58 MHz, while for PAL it's at 4.43 MHz.
- Y and C: the luma and modulated chroma components of S-Video. This signal carries Y and C on separate signals. Thus, CV = Y + C.
- YCbCr or YUV: component video with luma and both color-difference signals (red minus luma and blue minus luma) on three separate signals. So, C of S-Video = subcarrier-modulated CbCR. U and V are scaled components of Cb and Cr, respectively, prior to subcarrier modulation.
Presently, real-world video interfacing remains very much analog. All TV-based equipment requires analog video inputs. Recent market research suggests that more than 80% of today's flat-panel PC desktop monitors have analog interfaces.
Any display application that requires both video and graphics inputs needs a "dual" front end. The separate AFE is necessary because the front-end ADCs in the video decoder IC can't handle the high sampling clocks for enhanced and high-definition formats, or for PC graphics (see the table).
An independent sync-separator IC is needed to operate with formats that carry video synchronization embedded in the Y component, such as all component DTV formats, and some graphics formats. Another complexity is that sync-separator ICs are traditionally developed for SDTV sync formats, not for HDTV. PC formats, on the other hand, carry their video syncs on separate Hsync and Vsync signals. Once all formats are digitized, a video back end takes care of both scaling and video de-interlacing, such as in a flat-panel display, or video compression in video-storage applications.
Design Issues With Discrete ICs: The AFE will digitize either graphics to RGB or component video to YCbCr (because the component analog video interface uses YCbCr). But all displays require RGB-style signals for the internal display element, whether it's an LCD panel, a CRT, or a digital micromirror device for DLP-based projection equipment. Because the clamping circuit in the AFE must be configured to properly digitize either RGB or YCbCr component video or graphics, the digital back end must detect the color space used by the source, and initialize the video clamp. If necessary, conversion to RGB is performed via a matrixing operation.
Second, PC monitors have a linear transfer characteristic, while traditional CRT-tube TVs need to be overdriven in the lower-amplitude (black) regions. Their I/O relationship is approximated by a power-function: light output = (voltage input)γ, where γ represents the nonlinear light output versus voltage input relationship of the picture tube. Because of a tube's low light output in dark areas, its gamma number is greater than 1 (usually about 2.5). Thus, low-intensity areas of the picture (near black) are compressed, and high-intensity areas (near white) are expanded.
To compensate for this, the video signal is emphasized prior to transmission using an inverse gamma curve. This process is known as gamma correction. The designer potentially faces the need to "de-gamma" the video signal when the display has a linear transfer characteristic (γ = 1), as in most flat-panel displays, rather than the traditional nonlinear characteristic of CRT displays.
Finally, there's the issue of video-input synchronization and format detection. Today's PC graphics signals don't carry their own identification, so the AFE needs to detect which graphics format goes to the display, then properly initialize the front end to the correct sampling frequency and phase. Graphics signals are stair-stepped, nonbandwidth-limited signals. So, in addition to controlling the sampling frequency, the clock phase needs to be set to avoid sampling during pixel transitions.
Today's AFEs demand external detection of the incoming format (based on video sync frequencies and pixel-analysis) and only provide low-level access to the phase-locked loop's (PLL) frequency/phase controls. They don't offer an integrated "auto-lock" algorithm.