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December 2005 - page 3
CONNECTORS (JACKS)
 RCA Jacks (phono -as opposed to phone- or CINCH/AV connectors) were introduced by RCA Corp. in the late 1940s as an analog audio interface during the introduction of the long-playing (LP) 33 1/3 RPM record. They began to replace the older (phone) jack plugs used in home audio and telephony with the popularity of the cassette tape by the early ‘70s. An RCA jack consists of a center male pin that carries the signal and a surrounding metal ring that carries the ground. Because the center pin makes contact with the receiving jack before the grounding ring does, loud hum and buzz will be heard if components are powered during connection/disconnection. When carrying audio, white or black are used to represent the left channel and red is used for the right channel. In video, yellow is used to signify composite video, while Green signifies Component Y, Red is Component R-Y and Blue is Component B-Y. Digital audio is supposed to be marked as SPDIF (Sony Philips Digital InterFace), but is usually marked digital out, Dolby Digital or 5.1 Digital Output. RCA jacks are the most widely used consumer connector today, even as power and speaker connectors.
Tip Ring Sleeve (TRS), the three-conductor version of a phone “jack plug” and its corresponding socket, is sometimes referred to as a stereo headphone jack. Telephone switchboard operators formerly used these jacks until the mid-‘80s to connect one caller with another before the age of computer-controlled automated switching. In audio, the tip is usually for the left channel, the ring for the right channel and the sleeve is used for the ground. Insulating rings are used as spacers between the contacts to prevent shorting of the signals together. It is also possible for the arrangement to apply to a monaural balanced signal, providing many of the advantages of the XLR connector. Until the popularization of the RCA phono jack in the early 1970s, The TRS phone jack was the most widely used and recognized method of connecting two devices to route a signal.
XLR Connector (originally the Cannon X series, followed by the addition of a latch, then a rubber gasket) is a multi-pin balanced design for rugged and professional analogue (usually three-pin) and digital audio connection, as well as for professional and broadcast video applications. Examples include DIN and Mini-DIN families, such as the S-Video connector (sans latch), the DB25 connector for multi-track recording and computer printers from a decade ago. They are also used by Neutrik as a loudspeaker connection called the Speakon connector. Home audio utilizes the three-pin XLR; male connectors are used for the output, while female connectors are used for the input (unlike the RCA or TRS Jacks). Normally, Pin One = Ground, Pin Two = normal polarity (hot) and Pin Three = inverted polarity (cold). The standard impedance is 110 ohms in the case of video usage. This balanced method produces Common Mode Noise Rejection, which eliminates any unwanted noise that is common to both normal and inverted signals – and caused by EMI/RFI in particular.
Toslink is an optical interface using a plastic fiber optic cable and the Sony/Philips Toslink Red Diode transmitter/receiver software combination. It consists of plastic grip connector that snaps into place at the component ends of the fiber optic cable, providing an approximately tight optical link that introduces a high degree of jitter (time based uncertainty) due to its simple design. Originally used for digital two-channel audio in the ‘80s, it is now a commonplace connection for DVD players, computers and video game systems that feature Dolby Digital and DTS Surround Sound. Seen initially as an ideal non-electrical connection method, it has since been refined by the use of jitter-reducing circuits at the receiver end called FIFO – (First In First Out), which re-clocks the incoming jittery signal to produce a clean version of the digital source. Lengths should not exceed 12 feet without using glass fiber optic cables due to light loss.
ST Fiber Optic is a precision designed and created glass fiber optic system using a bayonet connection that transfers digital audio, using red laser light (c. 680 nM) at full bandwidth with extremely low inherent jitter. Originally designed and used by the telephone industry in the early ‘60s, it has completely revolutionized the amount of information that can be sent down a single connecting fiber or glass wire. As a connection method for digital audio, it is only surpassed by certain carefully executed balanced designs using a three-pin XLR connector, but these cannot transmit signal over anywhere near the same distance without the use of differential amplifiers in the signal path.
Mini Din (S-Video PS-2) is from the family of smaller DIN connectors with three to eight pins. The concept of a direct interface cable was ubiquitous from the era of the stereo tape recorder of the ‘60s, which required two inputs and two outputs to be usable. Video is stored on VHS as separate color and luminance information, so a natural upgrade from a single composite video connection was the four-pin S-Video cable, first used in late 1985. This method eliminates the need for a comb filter in the display device (or scaler), thus eliminating certain interlace artifacts like stair-stepping and zippering between bright colors while providing more actual picture information at the same time. An S-Video connection is almost always preferable to a composite video connection on RCA phono jacks.
BNC, “The Bayonet Neill-Concelman” connector, is used to terminate both coaxial video and digital audio cables. They are commonplace in professional video, amateur radio, the Serial Digital Interface (SDI) and for almost every piece of test equipment created during the last 45 years. It belongs to the bayonet family of twist on connectors, unlike the RCA jacks or TRS plugs. The BNC is perhaps the best unbalanced connector because of its extremely tight electrical tolerances, which include dead-on 75 ohm performance.
DVI, “Digital Visual Interface,” is a standard developed by the Digital Display Working Group, designed to maximize the connectivity between a source and its display. Originally intended for easing computer set-up, newer HDTV televisions adopted this connector several years ago, just before the HDMI connector was sanctioned by the Advanced Television Standards Committee (ATSC) for the same purpose in television. The two kinds of connectors carry the same digital HDCP copy-protected information, but the DVI architecture and software have proven to be more robust in the short term. A single DVI link actually consists of four twisted pairs of wire (red, green, blue and a clock) to transmit 24-bit data per pixel. A single DVI link can transmit a maximum resolution at 60Hz of 2.6 megapixels. Not surprisingly, the DVI connector has provision for a second link, doubling the available bandwidth at 60 Hz to 5.2 megapixels. The DVI connector is capable of handling analog (DVI-A), digital (DVI-D), or a combination of both (DVI-I), and it is possible to have audio as well, though this is not being implemented in consumer televisions.
 HDMI, High-Definition Multimedia Interface, is an uncompressed digital audio/video interface adopted by the industry to ease the transition from analog to digital A/V and television. The connectors are small and rectangular, featuring plug and play software that, like USB, allows for easy interchanging of the connections while components are turned on and in use. HDMI must utilize the HDCP copy management system and all manufacturers are required to purchase a license in order to manufacture devices that incorporate the interface. HDMI and DVI-I (with HDCP) are essentially the same digital signals, using a different connector on the end.
Special thanks to Ultralink-XLO for help compiling all of the data needed for this feature.
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Resources:
Jeremy Kipnis – video calibrator in Connecticut - (203) 938-3767
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