Before using a video editing application you must get the original video material into your computer. For this you’ll need some kind of video connection, either in analogue or digital form.
You can connect the output of a video cassette recorder (VCR) or camcorder to the analogue video input of your computer or its video capture card. However, if you have a Digital Video (DV) recorder you’ll get much better results by transferring the material over a FireWire connection.
A digital video circuit ensures that video data is conveyed without any distortion to the signal.
The FireWire connection on a DV recorder, also known as an iLink, DV terminal or an IEEE 1394 interface, conveys both audio and video signals. In addition, the tape transport of the recorder can be controlled over the FireWire link by using an editing application such as iMovie.
This interface is used for connecting professional digital hardware, including BetaSP video recorders and cameras. It normally conveys non-compressed 10-bit data, usually via BNC connectors. The data can be in high definition (HD-SDI) or standard definition (SD-SDI) format.
An analogue video circuit can cause some degradation of a video signal, although this isn’t usually a problem. Two incompatible systems are commonly used for analogue connections:-
This type of circuit consists of a single wire surrounded by a wire mesh screen or shield. The colour information in RGB form is superimposed onto a combined brightness and synchronisation signal, which means the interface can cause some loss of quality.
On domestic equipment this type of connection is often made via a phono plug, also known as an RCA plug or PIN plug, as illustrated below:-
Professional equipment usually employs a BNC coaxial connector for composite video, providing a reliable twist-and-lock connection. The 75 ohm version is the most common, although it can easily be plugged into a similar 50 ohm socket, sometimes resulting in jammed or damaged connectors.
The S-Video interface, as originally introduced with the S-VHS video cassette recorder (VCR), overcomes some of the limitations of composite video connections. The signal is separated into its luminance component (Y) and chrominance component (C), which are then conveyed separately via a 4-pin mini-DIN connector, wired as shown below. This kind of plug also engages with the 7-way mini-DIN socket that’s fitted to some video cameras.
You can convert an S-Video signal to composite from using this simple circuit:-
This type of socket, commonly fitted to television receivers and other audiovisual devices, was developed in France by the Syndicat de Constructeurs d’Appareils Radiorecepteurs et Televiseurs (SCART), although it’s also known as a Peritel (Peritelevision) connector. The plug has 20 pins, although the case itself is considered to be pin 21. The connector carries audio signals, as well as RGB video and composite or S-Video information, and is wired as shown below:-
|Pin||Audio signal||Video signal||Control signal|
|7||Blue (75 Ω)|
|8||Function Switching *|
|11||Green (75 Ω)|
|14||Data Bus Ground|
|15||Red In/Chrominance (C) ‡|
|16||Blanking/Composite Sync †||RGB Control •|
|18||Blanking/Composite Sync Ground †||RGB Control Ground|
|19||Composite Out/Luminance (Y) Out + Sync Out ‡|
|20||Composite In/Luminance (Y) In + Sync In ‡|
* 0-2 V (TV mode), 5-8 V (Wide screen AV mode) or 9.5-12 V (Normal AV mode)
• 0-0.4 V (Composite input select) or 1-3V (RGB input select)
† Used for RGB Control in RGB-capable devices
‡ S-Video input option for Arrangement 2 connector but not Arrangement 1
Modern television receivers often have two SCART connectors. The Arrangement 1 socket is normally used for a satellite receiver or a similar device, whilst an Arrangement 2 socket accepts S-Video signals and operates in wide screen mode with a video cassette recorder (VCR).
A standard SCART cable is normally wired pin-for-pin on every circuit, with exception of the following pairs of pins that are swapped:-
|Plug A||Plug B||Circuit|
|3||6||Audio Left/Mono (A to B)|
|6||3||Audio Left/Mono (B to A)|
|1||2||Audio Right (A to B)|
|2||1||Audio Right (B to A)|
|19||20||Video Composite/Luminance (A to B)|
|20||19||Video Composite/Luminance (B to A)|
All of the remaining circuits either carry information in both directions or in the direction determined by the actual devices. A video source providing RGB outputs, for example, would normally be connected to another device that accepted these signals. Note that although separate video luminance circuits are provided in each direction, the common chrominance circuit operates both ways.
Special SCART adaptor cables are commonly required for connecting SCART-equipped appliances to other devices that have alternative connections, such as BNC sockets or phono sockets. Sometimes this demands unorthodox solutions. For example, if you connect a composite video circuit to pin 20 of a SCART plug, it’s common practice to also fit a 1 kΩ resistor between this pin and pin 16, thereby providing the necessary blanking signal.
To create and edit a movie on a computer, or to digitise images from an analogue video input, you must have suitable video hardware. This can be in one of the following forms:-
Many modern computers have both composite and component video inputs and outputs, as well as audio inputs and outputs. However, older machines can only handle images of
320 × 240 pixels at a frame rate of 30 frm/s, equivalent to a quarter-sized NTSC screen, giving the same resolution as a low-resolution VGA monitor. Later models accommodate full-format video, with an image size of
640 × 480 or
768 × 576 pixels, running at 30 or 25 frm/s for NTSC and PAL respectively.
1024 × 768, set the zoom to give
256 × 192pixels.
Although you can load analogue video material via a video converter or video capture card (see below) it’s much cheaper to connect analogue signals to a DV camcorder and then select
Record on the camcorder: the pictures will then travel automatically to your computer via FireWire.
If you haven’t got a PowerPC-based machine, or need more advanced facilities, you can use a video capture card. Most modern cards accommodate full-format video, although older cards are limited to unacceptably low frame rates such as 15 frm/s.
Video capture cards come in various forms, including the following:-
If your computer has a Universal Serial Bus (USB) port, you can use a low-cost video to USB converter to provide a video input. In fact, some adaptors provide inputs and outputs for both video and sound, as well as a TV tuner in some instances. Unfortunately, due to the speed of USB, the video capture facility can be limited to
352 × 288 pixels at 25 frm/s. Devices that convert USB video to analogue are better, usually giving a full-sized picture, such as
768 × 676 pixels at 25 frm/s
A video to FireWire converter works in the same way as a USB converter. However, since FireWire has a greater bandwidth, it usually gives better quality. Devices of this kind, also known as a FireWire digitiser, sometimes include a TV tuner and FM radio.
ePanorama website at www.epanorama.net
©Ray White 2004.