Audio and video capture
Audio and Video capture is a process of storing AV on the hard disk of a personal computer using AV capture cards. AV capture cards are normally PCI card or AGP (Accelerated or Advanced Graphic port) graphic card that allow a computer to capture composite, component video or directly as UHF modulated signal from a terrestrial or a satellite antenna. Video capture cards must be distinguished from video editing cards; the latter have dedicated hardware for processing video beyond the analogue-to- digital conversion.
Video capture starts with a fast A-D converter, providing YUV 4:2:2 data. Its sampling rate (hence final picture quality) can usually be selected in software, primarily to suit the resolution available from the source; and the computer/HDD capabilities. Typical resolution settings are 384 X 288 pixels (low band/VHS/Video-8 standard) and 640 X 480 (NTSC) and 768 X 576 (PAL), corresponding to standard television with zoom and other facilities. High-definition capture cards are also available with 1920 X 1080i picture resolutions. Following sampling and quantisa- tion, data compression takes place to reduce the bit-rate; this is essential to reduce the amount of hard disk space for the program. Most analogue capture cards work to Motion-JPEG (M-JPEG) standard, with which the data rate can be reduced by a factor of up to 100, though of course there’s a trade-off between compression rate and moving-image quality. At compression ratios up to 5:1 the results are very good; between 5:1 and 15:1 some deterioration in quality is perceptible, depending on motion rates and picture ‘busyness’. For compression ratios higher than 20:1, MPEG-2 algorithms are used: Main Profile@ Main Level MP@ML for SDTV and Main Profile @ High Level (MP@HL) for HDTV. The com- pressed video data is now applied to the computer’s data bus, along with audio data, ideally produced and processed on the same card to avoid lip-synchronisation problems.
Editing
The editing process is conducted by a special software program with which the user’s requirements are specified in terms of trimming and combining clips, adding transitions, effects, titles and so on. Sophisticated editing programs offer many effects and facilities: batch capture, animation, ‘morphing’, ‘paints’, chroma-key, filters, image re-sizing etc. The final pic- ture/sound programme schedule is built up on a timeline, a series of horizontal on-screen rows, each representing a video, audio or effects track, and progressing in time from left to right. This timeline can be scaled as required, ranging from the entire required ‘movie’ to just a few frames, seconds or minutes. All these instructions are stored with frame/timecode markings, but not yet executed. Editing is followed by a process called rendering.