film. One of the main purposes of ‘striping’ tapes is to record a continuous control track for the pictures and audio to be added later – as in insert editing. Control tracks are not used in disk recording and nonlinear editing.

Convergence (Stereoscopic) In human eyesight, convergence is the ability of our eyes to divert eye optical axes horizontally in an inward direction. The convergence ‘near point’ is the closest point which is still possible to perceive one image. In practice, the eyes can easily converge inward but have much less ability to diverge outward, as it is something we don’t do in life and only when looking at 3D images that have positive parallax beyond the individual human interocular. In cameras – ‘toeing’ of the cameras (to simulate the eyes converging) to focus on a depth point in the scene, either in front of, behind or at the point of interest. The ‘convergence point’ is where the axes of toed in cameras align on the Z-axis. Convergence can be adjusted in post production by horizontal movement. Note that sometimes the term ‘vergence’ is used to describe both convergence and divergence. Convergence pullers are camera-crew members on a Stereoscopic shoot who are responsible for setting up and shifting the convergence during a shot. See also: Parallax

Corner pinning

A technique for controlling the position and rotation of pictures in a DVE by dragging their corners to fit a background scene: for example, to fit a (DVE) picture into a frame hanging on a wall. Corner pinning was developed by Quantel as a practical alternative to precisely setting the many parameters needed to accurately position a picture in 3D space. This works well with graphical user interfaces, e.g. pen and tablet. It can also be combined with the data derived from four-point image tracking to substitute objects in moving images, for example replacing the license plate on a moving vehicle. See also: Tracking

Co-sited sampling

This is a sampling technique applied to color difference component video signals (Y, Cr, Cb) where the color difference signals, Cr and Cb, are sampled at a sub-multiple of the luminance, Y, frequency – for example as in 4:2:2. If co-sited sampling is applied, the two color difference signals are sampled at the same instant, and simultaneously with a luminance sample. Co-sited sampling is the ‘norm’ for component video as it ensures the luminance and the chrominance digital information is coincident, minimizing chroma/luma delay.

CRC

Cyclic Redundancy Check – an advanced checksum technique used to recognize errors in digital data. It performs the same function as a checksum but is considerably harder to fool. A CRC can detect errors but not repair them, unlike an ECC. A CRC is attached to almost any burst of data which might possibly be corrupted. On disks, any error detected by a CRC is corrected by an ECC. See also: Asynchronous, Checksum, ECC.

Cross conversion

Changing video material from one HD format to another. For example, going from 720/60P to 1080/50I is a cross conversion. Note that this example involves both changing picture size and the vertical scan rate from 60 Hz progressive to 50 Hz interlaced. Similar techniques are used as for standards conversion but the HD picture size means the processing has to work five or six times faster. See also: Down-res, Frame-rate conversion, Standards conversion, Up-res

CSMA/CD See: Ethernet

Cut (edit)

A transition at a frame boundary from one clip or shot to another. On tape a cut edit is performed by recording (dubbing) the new clip at the out-point of the last, whereas with with true random access storage no re-recording is required – there is simply an instruction to read frames in a new order. Simple nonlinear disk systems may need to shuffle, or de-fragment their recorded data in order to achieve the required frame-to-frame access for continuous replay.

D1 A format for digital video tape recording working to the ITU-R BT.601, 4:2:2 standard using 8-bit sampling. The tape is 19 mm wide and allows up to 94 minutes to be recorded on a cassette. Despite the advantages, D1 use was limited by high cost and is rarely found today. However the term ‘D1’ is still occasionally used to imply uncompressed component digital recording – ‘D1’ quality. See also: D2, DVTR

D2

A VTR standard for digital composite (coded) PAL or NTSC signals. It uses 19 mm tape and records up to 208 minutes on a single cassette. Neither cassettes nor recording formats are compatible with D1. Being relatively costly and not offering the advantages of component operation the format has fallen from favor. VTRs have not been manufactured for many years. See also: Component, D1, D3, DVTR

D3

A VTR standard using half-inch tape cassettes for recording digitized composite (coded) PAL or NTSC signals sampled at 8 bits. Cassettes record 50 to 245 minutes. Since this uses a composite PAL or NTSC signal, the characteristics are generally as for D2 except that the half-inch cassette size allowed a full family of VTR equipment to be realized in one format, including a camcorder. D3 is rarely used today.

D4

There is no D4. Most DVTR formats hail from Japan where 4 is regarded as an unlucky number.

D5 A VTR format using the same cassette as D3 but recording uncompressed component signals sampled to ITU-R BT.601 recommendations at 10-bit resolution. With internal decoding, D5 VTRs can play back D3 tapes and provide component outputs. D5 offers all the performance benefits of D1, making it suitable for high-end post production as well as more general studio use. Besides servicing the current 625 and 525 line TV standards the format extends to HDTV recording by use of about 4:1 compression (HD-D5).

D6

A little used digital tape format which uses a 19mm helical-scan cassette tape to record non-compressed HDTV material. The Thomson VooDoo Media Recorder is the only VTR based on D6 technology. The format has passed into history.

D7 This is assigned to DVCPRO.

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GLOSSARY OF TERMS