Analog circuit design makes HDTV different

As high-definition television (HDTV) display technology matures, the quality of analog video and audio processing will eventually differentiate between televisions. Digital processing in HDTVs, including image scaling, deinterlacing, and MPEG demodulation, continues to play an important role, but these functions will soon become standardized. The analog processing of video and audio signals in HDTVs directly affects the user's "TV viewing experience," yet HDTV manufacturers are rapidly seeking improvements in their television set analog circuit designs to increase sales and profits. This article will discuss some of the issues and design considerations for high-performance analog signal processing in HDTVs.

In liquid crystal (LCD) TVs and plasma (PDP) TVs, the flat panel displays themselves have traditionally been different in key performance aspects of HDTV TVs. The seventh and eighth generation LCD flat panel displays cost billions of dollars to develop, but recent strength in PDP displays has forced some companies to form joint ventures to share their financial resources and technology. Currently, almost all leading manufacturers of LCD and PDP TVs are working with other major manufacturers of LED /' target='_blank'> displays.

This has resulted in a "level playing field" in terms of LCD and PDP led /' target='_blank'> display performance. Several competing HDTV models from different TV manufacturers now use displays produced by these joint ventures.

To support older devices, an emulation interface is required

Even in the emerging stage of high-definition digital broadcasting , HDTV still requires both analog and digital interfaces. Composite, component, and RGB signal analog interfaces are needed to support older standard-definition (SD) devices such as video recorders (VCRs), camcorders, DVD players, and personal computers (PCs). Because users expect their SD TV programs to have the same quality as high-definition (HD) TV programs, high-performance analog interfaces must be designed. Integrated multiformat video decoders, such as the ADV7403 from Analog Devices, include a high-performance video decoder for composite video signals and a three-channel analog-to-digital converter (ADC) for digital component analog video and RGB.

When designing analog interfaces, it is important to provide clean, low-noise power supplies and to use short printed circuit board (PCB) traces from the connector to the device inputs. We also recommend using a PCB design with a complete ground plane to minimize overall PCB noise and reflections. Short trace lengths on digital outputs (both data and clock) help reduce the possibility of high-frequency signal reflections that can corrupt signal integrity.

Since the analog YUV signals recorded in DVD movies have 10-bit resolution (which can represent more than one billion different colors), it is important to choose an analog interface that accurately reproduces colors using a 10-bit (or higher) resolution ADC. For example, Analog Devices' ADV7403 multi-format video decoder integrates a 12-bit, 140 MHz ADC to maximize the improvement in image quality.

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Audio poses more challenges to HDTV

Audio processing in HDTVs presents several interesting design challenges. The trend toward smaller TV cases has necessitated a reduction in speaker size while reducing the space needed to dissipate heat from the amplifiers. Since HDTVs are often used as home theaters, the audio circuitry in the TV must support the latest surround sound and bass enhancement algorithms. HDTVs must accomplish these major advances in SD audio while meeting ambitious goals of low cost and early time to market. Cathode ray tube (CRT) display SD TVs can only reproduce stereo sound (2.0 audio) and have large TV cases that easily accommodate appropriately sized speakers. HDTVs with LCD or plasma displays have narrow TV cases that can only accommodate smaller speakers, limiting acoustic performance. HDTV programming is broadcast in surround sound (5.1 audio), so audio designers must design high-quality audio products to work within these constraints while still meeting stringent cost requirements. Using programmable audio processors, designers can now develop a single-speaker audio platform that can be integrated into different HDTV models that can use speakers of various sizes. Using the latest psychoacoustic research, design engineers have developed new audio processor algorithms that use two or three speakers in an HDTV to give the listener the experience of a full multi-speaker surround sound system.

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To properly support popular third-party virtual surround and surround sound algorithms, the audio processor must include an audio codec and a digital-to-analog converter (DAC) with excellent analog performance. For example, Analog Devices' ADAV400 with active equalization technology (used to compensate for small speakers) supports popular third-party algorithms and the analog performance required to produce rich, deep sound. Graphical software tools such as SigmaStudio allow full control of the entire sound environment.

Finally, Class D audio amplifiers minimize the size of heat sinks and other external components, allowing them to fit into the tight spaces of new HDTV products. The large picture tubes used in CRT TVs left audio designers with ample chassis space, allowing CRT TV audio designers to use Class AB amplifiers, which are inefficient and typically require large heat sinks. By using efficient Class D audio amplifiers such as the Analog Devices AD1991 (with >85% efficiency at full power), designers can fit their audio amplifier circuits into their tight spaces without compromising audio quality or power output.

HDMI is the interface for watching the latest high-definition DVDs

High-Definition Multimedia Interface (HDMI) is quickly becoming the preferred digital interface for connecting HDTVs to set-top boxes, DVD players or recorders, audio and video (A/V) receivers, and personal media players. Since the transition from analog interfaces (composite, component, and RGB signals) to digital interfaces (such as HDMI) is expected to take several years, it is essential that HDTVs support both interfaces. Dual-interface products (such as the AD9380 from Analog Devices) are a cost-effective solution for supporting both analog interfaces for older devices and future HDMI interfaces.

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AD9380 8-bit dual analog interface and HDMI display interface block diagram

The HDMI interface operates at gigahertz data rates, so system designers must use high-speed analog design techniques. To meet the requirements of the HDMI compliance test specification, the PCB traces between the HDMI connector and the receiver input must be carefully controlled so that the differential input capacitance is less than the specified value. The length of the PCB traces between the HDMI connector and the device input should be as short as possible to maintain the integrity of the TMDS signal. We recommend using a single ground plane for any PCB board with an HDMI interface. Using multiple ground planes can be detrimental because small independent ground planes can create long ground loops. The traces used for the digital outputs should also be as short as possible because short traces generally reduce the possibility of reflections.

Designing with high-performance analog components plays an important role in designing HDTVs, surpassing similar solutions in terms of picture and sound quality. Users expect all of the following features in the next generation of HDTV sets: vivid pictures, loud sound, and reasonable prices. HDTVs designed with the latest analog and HDMI interfaces, Class D audio amplifiers, and programmable audio processors will meet user expectations for features and performance. Stay tuned to your favorite retail electronics store and keep an eye out for all the latest wall-mounted display HDTV sets. You will see and hear the difference that high-performance analog design can make in your TV viewing experience!