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Digital VideoDigital Video

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Digital VideoDigital Video

Until the arrival of the Pentium processor, in 1993, even the most powerful PCs were limited to capturing images no more than 160 x 120 pixels

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Digital VideoDigital Video

When processor speeds finally exceeded 200MHz, PCs could handle images up to 320 x 240 without the need for expensive compression hardware

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Digital Video FormatsDigital Video Formats

There are numerous video formats:M-JPEGMPEGAVIQuickTimeDV

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M-JPEGM-JPEG

Motion-JPEG not surprisingly based on the JPEG still image format

Stores every frame as a compressed bitmap image

Typical compression ratios of between 2:1 and 12:1

Can be applied in hardware or as a software codec

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MPEGMPEG

Moving Picture Experts Group standards for compressing motion video and audio signals using DCT (Discrete Cosine Transform)

The MPEG formats are asymmetrical - it takes longer to compress a frame of video than it does to decompress it

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MPEG CompressionMPEG Compression

MPEG uses both spatial DCT compression (as JPEG) and temporal compressionTemporal compression involves removing data

that does not change between consecutive frames

MPEG video streams consist of a sequence of sets of frames known as a GOP (group of pictures)

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MPEG CompressionMPEG Compression

Each GOP (typically 8-24 frames), has only one complete frame represented in full, which is compressed using only spatial compressionThe spatially compressed frame is just like a

JPEG still and is known as an I frame (intra frame)

Around the I frame are temporally-compressed frames, representing only change data

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MPEG CompressionMPEG Compression

During encoding, prediction techniques compare neighbouring frames and pinpoint areas of movement and define vectors for how each will move from one frame to the next

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MPEG CompressionMPEG Compression

By recording only these vectors, the data which needs to be recorded can be substantially reduced

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MPEG CompressionMPEG Compression

There are two types of vector frames:P frames (predictive), refer only to the

previous frameB frames (bi-directional) rely on previous and

subsequent frames

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MPEG CompressionMPEG Compression

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MPEG CompressionMPEG Compression

I B B P B B P B B I

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MPEG CompressionMPEG Compression

This combination of compression techniques makes MPEG highly scalable

Using longer GOPs with more B and P frames, reduces data rates

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MPEG-1MPEG-1

MPEG-1 (White Book Standard) was introduced in 1993Supports video coding at bit-rates up to about

1.5 Mbit/s and virtually transparent stereo audio quality at 192 Kbit/s

Provide video resolution of 352x288 at 25 fps This produces video quality slightly

below the quality of conventional VCR videos

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MPEG-2MPEG-2

Resolutions of 720x576 and 1280x720 at 50 fps, with full CD-quality audio

This is sufficient for all the major TV standards, including PAL, and even HDTV

MPEG-2 is used by DVD-ROMs, digital satellite and CableMPEG-2 can compress a 2 hour video into a few

gigabytesDecompressing an MPEG-2 data stream requires only

modest computing powerEncoding video in MPEG-2 format requires

significantly more processing power

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MPEG-4MPEG-4

MPEG-4 is a standardised way to define, encode and playback time based media

It can be used in numerous applications:VideoDelivering 2D still imagesControlling animated 3D modelsHandling two-way video conferencesStreaming video, etc.

Standardized in October 1998

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MPEG-4MPEG-4

Provides standardised ways of representing units of aural, visual or audio-visual content, as discrete “media objects”

These can be of natural or synthetic origin, for example, they could be recorded with a camera or microphone, or generated with a computer

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Video CodecsVideo Codecs

These are the algorithms that handle the compression and decompression of the digital video

Every architecture has certain codecs built-in and some codecs are common to many architecturesE.g. QuickTime originally used Sorenson video

codec, but now supports MPEG-2, Cinepak etc.

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CinepakCinepak

Cinepak – developed by SuperMac Technology

Vector quantization based codec developed specifically to deliver 24-bit video in quarter screen (320 X 240 pixel) windows from files restricted to single-spin CD-ROM data rates

Cinepak is a highly asymmetric codec, with a compression process that takes 300 times longer than decompressionBetter than Indeo for action sequencesPoorer than Indeo for "talking head" and other low-

motion sequences

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IndeoIndeo

Indeo – developed by Intel Indeo is a proprietary blend of colour

sampling, vector quantization, and run-length encoding

One of Indeo’s valuable characteristics is its scalability. Indeo will deliver a movie clip at higher frame rates when more processing power is availableIt is slightly faster than Cinepak at

compression but is still slow

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Video File FormatsVideo File Formats

Like audio and image files, video files also have a series of diverse formats:.mov.qt.avi.mpg

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QuickTimeQuickTime

A video and animation system developed by Apple Computer

In QuickTime, a structure of time-based data is called a movie, hence the file extensions .mov or .qt

The QuickTime application can create, display, edit, copy, and compress video data in most of the same ways that text and still-image graphics are currently manipulated

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QuickTimeQuickTime

Its open architecture supports many file formats and codecs, including Cinepak, Indeo, Motion JPEG and MPEG, and is extensible to support future codecs, such as DVCAM

In February 1998, the ISO standards body gave QuickTime a boost by deciding to use it as the basis for the new MPEG-4 standard

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QuickTimeQuickTime

Besides processing video data, QuickTime can handle still images, animated images (sprites), vector graphics, multiple sound channels, MIDI music, 3D objects, virtual reality panoramas and objects, and even text

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AVI (Video for Windows)AVI (Video for Windows)

Audio Video Interleave A special case of the RIFF (Resource

Interchange File Format), defined by Microsoft

Video for Windows supports several data compression techniques, including RLE, Indeo, and Cinepak

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DVDV

In the late 1990s a new generation of entirely digital cameras and camcorders emerged, and with them a new video format, Digital Video (DV)

It uses a spatial compression technique (each frame being compressed on an individual basis rather than being compared to adjacent frames) based on the DCT

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DV CompressionDV Compression

DV can compress different parts of each frame to different ratiosSo, the blue sky in an image backdrop can be

brought down to, say, 25:1, while the complex forest in the foreground, which needs more detail, is reduced to only 7:1

In this way DV can optimise its video stream frame by frame

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