Digital Video Signal Types

Digital Video Signal Types

If a video signal is transmitted digitally, it does not suffer from the same afflictions as with analog video. Current methods for transmitting and storing digital video are all high quality.

Summary Of Quality

There are two ways to transmit digital video: composite or component. Composite signals are no longer in common use, and today's methods all involve digital component signals. This leads to the best possible transfer of a video signal from one source to another. As with analog component video, the video signal is separated into a luma (brightness) component, and two color difference components (chroma). All digital component connections use a single cable, which keep the luma (brightness) and chroma (color) components separate, which results in good color resolution and reproduction.

Composite

Although digital in nature, this is inferior to today's methods. This was more common in the early days of digital video. But because of the nature of composite video, the color information is not as pristine, and is less than ideal for post production applications like chroma-keying. An example of a composite digital tape format is D2.

Component

The best flavor of digital video storage and transmission. Luma, and both color difference components are maintained separately resulting in high color resolution. Used in SD and HD video.

SDI: A single cable is used, a 75 ohm coax cable with a BNC connector, but all three signals are kept intact and separate. For SD video, this connection is called SDI, standardized in SMPTE 259M. For HD, a similar standard is used, called HD-SDI, standardized in SMPTE 292M. These types of connections keep the luma and chroma components distinct, ideal for post production applications like chroma-keying. Hi Def video recorded in RGB 4:4:4 is transmitted by 2 HD-SDI cables, each carrying half the color information. This is referred to as dual-link HD.

 
FireWire: (The IEEE 1394 standard developed by Apple. Also dubbed iLink by Sony) A connection type where video, audio and timecode can all be passed through a single cable. Although there are different FireWire protocols, they pass digital component video "uncompressed". This doesn't mean the video being transmitted is uncompressed, but the transfer through FireWire is like a file transfer of digital data, so no extra compression or quality loss happens during this transmission. These connections are found on many different newer technology cameras and decks, including DV, DVCAM, DVCPRO 50, DVCPRO HD, an HDV. FireWire 400 allows for data rates up to 400 Mbit/sec. FireWire 800 allows for data rates up to 800 Mbit/sec. At the current time, these "native" formats that connect directly via FireWire only use 8-bit encoding (as opposed to higher-end 10-bit formats). Although there is an I/O solution for Final Cut from AJA called the Io or the Io HD which will transfer 10-bit video through a FireWire connection. In addition, Panasonic has introduced a new 10-bit compression format called AVC-Intra that can be recorded to P2 cards with some of their newer cameras. This format can also use FireWire to connect to the computer and transfer 10-bit video.

 
HDMI: (High Definition Multimedia Interface) A somewhat new interface used for transmitting uncompressed digital signals. This connection is increasingly being found on digital (Hi Def) televisions, cable and satellite set-top boxes and DVD players, both standard def and the next-gen DVD standard called Blu-ray. In addition, some HDV and AVCHD cameras have HDMI connections. In post production environments it is increasingly being used as a display connection between different video interfaces. One cable is used to transmit video and several audio channels. Type A HDMI is backwards compatible with single-link DVI, a connection type commonly found on newer graphics cards in computers. This allows a DVI output from a computer to connect to an HDMI display by means of an adapter. HDMI connections are also now being found on video interfaces for post production from companies like AJA and Blackmagic Design for use with Final Cut Pro and other editing software.