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Advanced software for smooth DVD decoding

Posted: 16 Mar 2002 ?? ?Print Version ?Bookmark and Share

Keywords:dsp? dvd? decoding? mpeg4? ccir601?

Digital videodisk playback is a complicated process that requires decoding many different complex data types and coordinating the flow of time-sensitive data through a complex series of transformations. One of the major functions of a DVD player is the playback of compressed audio and video from the DVD disk.

At the heart of a DVD player is the multi-function DVD chip. This block typically incorporates functions such as an MPEG-2 video decompressor, a DSP, a 32-bit microprocessor, I/O for audio, infrared for control and a built-in TV encoder or digital video output for a TV encoder. Some microprocessors may have a memory-management unit, but some DVD chips have removed the MMU to reduce cost.

The DVD loader includes the laser, pickup and servos. The loader also has a DVD controller to drive the servos and spindle, read data from the optics and present a suitable data interface to the DVD chip. Although there is no rule defining which loader interface a particular DVD player will use, market segments favor certain interfaces: Higher-end, multi-speed or PC-focused loaders use an ATAPI/IDE interface; lower-end, single/double-speed or consumer-focused loaders use proprietary serial or 8-bit interfaces to reduce cost.

Some DVD video packets are encrypted with the Content Scrambling System (CSS). DVD chips incorporate a CSS decryption block to offload decryption processing from the microprocessor. The video processor off-loads common graphics chores from the microprocessor. De-interlacing duties for progressive-scan systems, video scaling, frame-rate conversion, video mixing and zoom are all handled by the video processor.

By using a combination of perceptual, predictive and entropy-encoding algorithms, MPEG compresses video by approximately 95 percent. Decompressing the resulting video is very compute-intensive and so, most DVD chips incorporate hardware MPEG video decoding.

Decoding scheme

Most MPEG video decoders, for example, will decode MPEG video from VCDs and supervideo encoders and convert CCIR-601 or -656 digital video from the DVD chip's video output into analog signals for a television. The digital video data is presented to the encoder as 8bit data, but many video encoders oversample and interpolate the data to 10-bit for improved video quality before converting it into composite, S-video or component video signals. The encoder must support Macrovision copy protection for DVD compliance. Progressive-scan video outputs are not yet common among mid- to low-end DVD players, but some DVD chips, such as the CS98100, support progressive-scan video capabilities.

All DVD players must have analog audio outputs, both stereo or 5.1 channel digital audio outputs, S/PDIF and standard optical TOSLINK. Some multi-channel audio D/As are specifically tailored for the consumer DVD market and others are tailored for the high-end DVD player markets. The digital audio outputs are encoded using IEC 60958 for uncompressed pulse-code-modulated audio or IEC 61937 for AC3, Digital Theater Sound (DTS) or otherwise compressed audio.

An embedded RTOS runs on the microprocessor to provide basic functionality for a range of functions including booting, memory management, I/O, task control, messaging, semaphores, timers, peripheral drivers and communications stacks. The microprocessor also handles demux audio, video and sub-picture data.

Compression capabilities

Compressed audio on a DVD is stored as MPEG-2, Dolby Digital (AC-3) or DTS. Some DVD players can decode other audio formats such as MPEG-1 Layer 3, WMA and AAC. All of those encoding methods are perceptual coders and give compression ratios of around 5 to 10:1. The coders use knowledge of human auditory perception to remove sounds below the ear's threshold (a very soft 80Hz tone), frequency-masked sounds (a 1kHz tone might mask a 1.1kHz tone that is 40dB down), channel-masked sounds (a loud sound from the center channel can mask a soft sound in another channel) and temporally masked sounds (a quiet sound that follows a loud sound). Also, unused bandwidth from one channel can be allocated to a busier audio channel. The resulting bitstream is then entropy-encoded to further reduce the bandwidth.

Because the software is specific to the chip and to a single application (the DVD player), the DVD chip vendor will often develop much of the required software and license it to the DVD system maker. The system maker can then tailor and brand the software to meet specific player needs.

Today's large DVD manufacturers use proprietary virtualization algorithms or preferred equalization algorithms, so the DSP must be programmable and the DVD chip vendor must either provide tools for OEMs to develop DSP software or in-house programmers to adapt software to the OEM's needs.

? Alson Kemp

Cirrus Logic





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