Connection Types

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VidBlaster is very versatile and allows connection of any type of camera connection. Here we sort out the different connection types sported by cameras, camcorders and other video devices (excluding IP Cameras), ranging from cheap consumer cameras to high-end professional studio equipment.

Contents

PC Serial connections

These are high-speed, general-purpose serial connections, used on computers for a variety of purposes, including the connection of various peripherals including scanners, printers and disk storage. They are not explicitly designed to carry video, and therefore incur overheads that are not found in dedicated video connections.

USB

Typical USB connector
PCI expansion card
Cardbus expansion

Introduction

The probably most used and cheapest interface for connecting cameras to a computer is USB. The USB interface allows connection of cameras in many different ways. USB Webcameras can be connected directly to the computer by USB and is immediately available as a source.

There are also USB capture devices, or "Frame Grabbers", allowing you to use other type of video connections, such as composite, component, VGA etc, as a "webcam".

USB cameras ranges from low quality $10 devices to $3-400 full HD cameras.

Webcam hardware brands

Limitations

The USB interface is probably the most flexible for use with computer video. But there are hardware limitations. Most desktop and laptop computers are equipped with anywhere between 4 and 10 USB inputs making you believe you can actually connect this amount of USB cameras.

This is rarely true. Even though there a lot of physical connectors they most often share the same USB controller. This means that all devices will share the same bandwith.

In short terms this would mean that the higher bandwith one camera use, fewer cameras can be connected.

For example (Theoretical examples):

There are ways to overcome the Bandwith limitations. By adding USB expansion cards you can add 1-4 USB ports to your computer.

For Desktop systems there are PCI cards that gives you some extra USB ports and for Laptop systems there are Express cards. These are relatively cheap ($20 and up) and a good way to expand your system.

See also - A discussion on how different connection methods can affect video quality, in the article about Working with Chroma Key .

IEEE1394/Firewire

6 & 4 pin 1394 connectors
Firewire was developed by Apple in the early '90s as a possible replacement for the SCSI bus, using a 6 pin connector, but it was Sony who first used it for a video product by incorporating into their new digital cameras that used the "DV" tape format, where they named it i-Link and used the smaller 4-pin connector. The DV video format uses lossy compression at around a 5:1 ratio, combined with either 4:1:1 or 4:2:0 YUV colour sampling. If a DV camera is connected to a PC by firewire (1394), then it's this chroma-reduced compressed stream that is received by VidBlaster. "DV quality" recordings are still regarded as being within the realms of "broadcast quality", provided a broadcast-quality camera front end (lens, sensor, etc) is used, but even at this level they are not regarded as being ideal as a source for chroma keying. In a live environment, it may be better to take an analogue YUV feed from a higher-end DV camera that's to be used as a chroma key source than to use a 1394 feed.

HDV can also be transferred over 1394 in its compressed format. HDV uses 4:2:0 YUV sampling (and in the 1080i version is anamorphicly sampled down to 1440x1080 pixel size), and uses MPEG-2 Long-GOP compression to give the same data rate (~25Mb/s) as DV/DVCAM/DVCPro, allowing it to be recorded on the same size of tape used for DV recordings. As such it is something of a half-way house between SD & HD, allowing cameras such as the Sony Z1 to record in both SD DV/DVCam and HD HDV formats.

Analogue video connections

BNC connector (Male)
In the world of analogue connections, RGB or YUV component offer the best chroma quality, followed by Y/C (S-Video), followed by composite. There is a common misconception that composite video is limited to "VHS quality", probably because VHS tape machines only offered this type of connection, and people forget that analogue NTSC & PAL broadcast TV uses composite encoding to good effect. However, what is true is that there's a lot of consumer-quality composite video devices (like EasyCap USB interfaces) which are not of "broadcast quality", whereas devices that handle YUV are more likely to be aimed at higher quality applications. If restricted to using composite connections, then better quality capture cards like those from Viewcast & Blackmagic will help preserve quality. Although there is a difference between these various analogue connection methods, potentially there is a bigger quality spread between the outputs from different camera models. Using a composite feed will be inferior to YUV from a high quality camera, but changing from composite to YUV from a low quality camera may make little difference.

Composite

Typical A/V camera cable

Composite is so named because it combines luma (brighness) and chroma (colour) infomation into a single signal that can be sent down a single co-ax cable. In the early days of video, there were no colour cameras and TVs were monochrome (black & white), so the video signal only carried the luma (brightness) information, along with sync pulses to mark the start of each line and frame. When colour TV was invented, it was important to maintain compatibility with existing TVs and transmission systems, so the PAL & NTSC systems used an encoding method whereby a low bandwidth colour (chroma) signal was added into the basic luma signal, resulting in a "composite" signal. Composite is still useful for standard def signals due to it's simplicty, near universal support, and simple cabling requirement (ie. single core coax). Composite connections are usually via BNC connectors on broadcast equipment, or RCA/phonos on consumer equipment, and can also be found as one of the options in SCART connectors.

Note - The A/V output on consumer/semi-pro camcorders is usually composite video, characterised by a yellow (RCA/phono) connector, with audio colour-coded white (and red, if L+R stereo).

S-Video

4 pin S-Video connectors

S-Video, or Y/C connections, use the same colour encoding method as composite, but keep the luma (Y) and chroma (C) signals seperate, which helps preserve the integrity of both, since the mixing/unmixing process used in composite video inevitably causes some degradion, particularly in consumer-grade encoders and decoders. S-Video connections therefore require twin co-ax connections, which can be in the form of a single combined cable (usually with a 4-pin Mini-DIN connector), or a pair of cables with similar connectors to composite cables. The Y signal is just like a composite signal, before the days of colour, and if connected to a composite input will result in a monochrome image.

Component

Whilst component video connections from computers (eg. from VGA cards) are typically RGB (Red/Green/Blue), in the world of broadcast and TV cameras, "component" usually means YUV (or YPbPr) component. The 'Y' is the same luma (brightness) signal as it is in a Y/C (S-Video) signal, but now there are two colour-difference signals (also known as U & V) which result in a much higher bandwidth colour signal, with far more colour detail, than in either Y/C or composite encoding. In terms of identifying YUV connections, the Y connector is colour-coded green, with U & V red & blue.

Connecting the Y signal on it's own to a video input will result in a monochrome picture, since the Y signal also carries the sync pulses, whereas if either the U or V signals are connected individually, no picture will be detected. Component cabling requires three co-ax cores, so is heavier and more costly than the equivalent composite cabling, but it offers the ultimate in analogue video connection quality.

Digital video connections

Both SDI and HDMI are serial digital connections explicitly designed for real-time video transmission, unlike firewire or USB. They can carry uncompressed SD or HD video signals, with embedded audio, and offer the highest quality option for any camera connection.

SDI

The SDI (Serial Digital Interface) standard was developed in the broadcast industry as a way to pass high quality uncompressed digital video over the same cabling as that used for composite video connections, hence allowing TV stations to upgrade their systems without renewing their entire cabling infrastructure. It runs at 270Mb/s, usually with the 4:2:2 colour-sampled video, with either 8 or 10bit data values, and up to 16 channels of embedded audio. Technically, SDI is only for standard definition, but the term is often used as an abreviation for HD-SDI, the high-definition version which, not surprisingly, requires a higher bandwidth (up to ~3Gb/s for 1080p) and a higher grade of cabling for longer runs. SDI cabling usually uses BNC connectors on high-grade co-ax (eg Belden 1694A) and is therefore quite robust, and can be used with maximum distances of over 100M at 720p/1080i, or over 400M at SD resolutions. Powered analogue-to-SDI converter boxes can be used in conjuction with analogue cameras, in order to benefit from the advantages of SDI, and also to allow connection to SDI capture cards like those from Blackmagic.

HDMI

HDMI plug

HDMI is a development of the DVI connection standard used on PC graphics cards, but with extra features very much geared towards TV and multimedia use. It shares some features with SDI/HD-SDI, in that it can carry most of the same uncompressed digital signal formats used by SDI, such that there are plenty of SDI<>HDMI signal converter boxes available to allow interconnection between the two formats. However, HDMI is primarily designed for consumer products, so the connectors are less robust and non-locking (unlike BNCs), and the signal can incorporate HDCP (High-bandwidth Digital Content Protection), meaning that some outputs (eg. from DVD & BluRay players) will not be accepted by HDMI inputs on PC video capture cards, to prevent commercial recordings being digitally copied. However, when used with HDMI sources like video cameras, HDCP will not be present in ther signal, but the other limitaion is in cable length, which is typically limited to 15M or less, without the use of active extender units. Converting HDMI to SDI/HD-SDI near to the source will allow for the much longer cable runs which might be needed for coverage of conferences or sports events.

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