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When making a purchase decision, most consumers balance their wants and their needs. Within a certain price range, and given a set of features and expected performance levels, consumers make informed decisions to purchase or not to purchase a product. Computer and computer accessory purchases conform to this decision-making model.

One of the newest accessories developed for personal computers is the Liquid Crystal Display (LCD) Monitor. When compared to their Cathode Ray Tube (CRT) counterparts, LCD Monitors offer space savings, lower radiation emissions and lower power consumption while delivering comparable video performance with a sleek, cutting-edge look. Advancements in technology, increased demand and the declining costs of producing the monitors have driven LCD price points to levels that allow average consumers to explore either bundling an LCD monitor into a new computer system or replacing an old CRT monitor. Given the purchase decision-making model, how does the consumer know whether to buy a digital, or an analog monitor?

Consumers typically consider several key factors when making purchases: performance, compatibility, and cost. The type of interface becomes a key consideration in monitor purchases. The standard red, green and blue (RGB) analog interface faces a growing challenge from the recently developed digital-only interface, and many published articles have debated the pros and cons of analog and digital.

Following are block diagrams and discussions of each interface. Rather than exploring supporting hardware such as micro controllers and back lighting, the discussion focuses exclusively on differences between the solutions.

Analog

The current RGB analog mass-market solution (see Fig. 1) takes the separate digital video data RGB from the computer and presents them to the DAC. Data are converted to analog and transmitted to the monitor along with the horizontal and vertical synchronization signals.

Fig. 1 Analog Solution

Inside the monitor, the pre-amp provides amplification, clamping and offset adjustments. The pre-amp can be implemented discretely or integrated pre-amps can be used. Most pre-amps currently available on the market were designed for use in CRT applications and have not been optimized for operation within LCD applications. As a result, pre-amp induced inefficiencies and errors occurring in an LCD environment will degrade the video performance.

The crucial next step is performing the analog-to-digital conversion. During the conversion process, the limited resolution of the converter introduces errors. The errors include dc components such as linearity and offset, or ac components like sparkle or bit errors. While these non-ideal properties may seem significant when reviewing datasheets, if the defect is a random one, the human eye is very forgiving. With a 60-Hz refresh rate of the LCD panel, the human eye will filter a sparkle code if it’s not excessive. Designers must be aware of ADC input bandwidth limitations and, if insufficient, the user will see the effects on the monitor.

An ADC with poor input bandwidth will significantly degrade the viewable performance of the LCD monitor, with video switching from white to black within one pixel. Since the analog signal swings full-scale, the ADC with poor input bandwidth results in pixel degradation. The edges between pixels will not be sharp and will be seen as fuzzy or gray lines where a vertical line of black is next to a vertical line of white. Typically, it is recommended that the ADC have an input bandwidth of 1.5 times the sampling clock, which itself is determined by considering monitor resolution and refresh rates. For example, an XGA (1024x768) monitor with an 85-Hz refresh rate needs a 89-MHz clock and an ADC with an input bandwidth of at least 133 MHz:

Fs = (Horizontal x Vertical x Refresh Rate) / 0.75) where 0.75 is the active video factor

      = (1024 x 768 x 85) / 0.75 = 89.13 MHz,

so the input bandwidth is 89.13 x 1.5 = 133.7 MHz

Within the analog interface, a data clock is required to provide synchronization between the LCD monitor and the incoming signal from the graphics controller. Synchronization is derived by the phase lock loop (PLL) using the horizontal sync pulse from the computer to create internal clock signals for the ADC and digital controller chip. To ensure the ADC samples at the correct time, phase adjustment is needed. This may require monitor adjustment by the user for the most optimal visual video performance. The PLL also introduces phase noise or clock jitter into the monitor that will cause visible artifacts demonstrated by a monitor that has a gray background appearing "noisy," with significant variations in brightness. Usually the visual effect on the LCD screen is a specific area on the LCD screen looking either washed out or brighter.

In the analog system, once the signal is converted to a digital data stream, LCD panels often require appropriate scaling and frame rate conversion. Up or down scaling is performed to match the screen size of the panel. Frame rate conversion sets the refresh rate to the required panel frequency, typically 60 Hz. Signal degradation resulting from analog-to-digital conversion may be exaggerated during the scaling process. Additionally, substandard graphics controller cards, poor shielding of the cable from the graphics controller card, and bad connectors can degrade signal performance, resulting in errors throughout the data conversion process and yielding visible image degradation.

Digital

The digital interface block (see Fig. 2 transmits computer data directly without unnecessary data conversion to analog -- and subsequently back to digital -- removes the associated possibility for error.

Digital does not benefit from the same universal standards that analog solutions enjoy. Competing standards include low-voltage differential signaling (LVDS), PanelLink, transmission minimized differential signaling (TMDS) and digital interface standard for monitors (DISM.) Each proposed transmission technique has merits, but until a single standard is adopted, and gains momentum, computer manufacturers must wager on the most probable long-term solution; in light of the rapid changes in the computer industry, the chances of choosing correctly are remote. Each standard fragments the market keeping digital solutions relatively expensive.

Fig. 2 Digital Solution

Solution Cost

The analog system requires a pre-amp, high-performance ADC, and PLL. These additional costs are embedded in the overall LCD Driver board and are borne by the consumer. The total cost of the solution must be viewed from the end product level. The consumer is typically concerned about their financial outlay, not the cost of the analog front-end at the board level.

When comparing retail prices of analog and digital LCD monitors, the list price for Digital 15 inch XGA - TFT LCD monitors were listed between $913.00 and $1299.00. Analog 15 inch XGA - TFT LCD monitors were listed from $929.00 to $1359.00. While analog monitors tend to be more expensive, digital monitors require the additional purchase of specialized digital video graphics cards and cables. The retail price of an ATI Rage 4-Mbyte digital video card is $64.53, and this cost is included in price comparisons. The cost continues to rise if users choose not to install the digital video card into the PC themselves, but decide instead to pay a professional to install the card.

When purchasing a new PC that bundles in a digital LCD monitor, the compatibility issue becomes less of a concern: Unless at some point in the future the user wants to upgrade to a larger monitor. For the over two hundred million PC users worldwide that have standard analog RGB interfaces, compatibility will be a concern when buying a digital LCD monitor. The inconvenience of installing a new video graphics card may drive consumers away from adding a digital LCD monitor to their system.

Compatibility and ease-of-use issues extend to ratified standards that will eventually drive the adoption curve of digital monitors to mainstream. With the adoption of a single standard, the demand for digital monitors will certainly increase as the long-term trend will turn towards users demanding superior performance from their monitor solution. Currently, digital monitor solutions are priced competitively, and in the long-term, digital has advantages in its ability to be scaled and optimized. Process improvements continue, and the explosive growth in LCD panel consumption will continue to drive the overall cost of these solutions down. In the meantime, analog will continue to set the standard for monitor compatibility, and its availability will influence buying decisions. Ultimately, the decision whether to buy an analog solution or a digital solution comes down to consumers and their buying patterns.

About the Authors

John Norris is a staff engineer in the Communications Products Business of Harris Semiconductor in Palm Bay, Florida. He joined Harris in 1986 and is the Program Manager of LCD Monitor products. Mr. Norris earned a master’s degree in business and a bachelor’s degree in electronic engineering technology.

Ken Mann is a Senior Marketing Manager in the Communications Products Business of Harris Semiconductor in Palm Bay, Florida. He joined Harris in 1984 and is responsible for product marketing of the company’s portfolio of data acquisition and DSP Products. He has earned an A.S. in Electrical Engineering, a B.S. in Business and an MBA.

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