Crosstalk

In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, or channel, to another.

In cabling

In structured cabling, crosstalk can refer to electromagnetic interference from one unshielded twisted pair to another twisted pair, normally running in parallel.

Near end crosstalk (NEXT)
NEXT is a measure of the ability of cabling to reject crosstalk. Interference between two pairs in a cable is measured at the same end of the cable as the interfering transmitter. Crosstalk is undesirable. In crosstalk, the signals traveling through adjacent pairs of wire in twisted-pair cabling interfere with each other. The pair causing the interference is called the “disturbing pair,” while the pair experiencing the interference is the “disturbed pair.” Channel NEXT is the NEXT value measured between one wire pair and another in the same cable; NEXT is measured at both ends of the wire. The NEXT value for a given cable type is generally expressed in decibels per feet or decibels per 1000 feet. NEXT value varies with the frequency of transmission. The higher the NEXT value, the greater the cable’s ability to reject crosstalk at its local connection. Generally specifications for cabling (such as CAT 5) include the minimum NEXT values.[1]
Power sum near end crosstalk (PSNEXT)
PSNEXT is a NEXT measurement which includes the sum of crosstalk contributions of all adjacent pairs.[1] It is the algebraic sum of near-end crosstalk (NEXT) of three wire pairs as they affect the fourth pair in a four-pair cable (e.g., Category 6 cable). The specification was developed to directly address the effect of transmissions on multiple adjacent pairs on the pair being tested and is relevant to all connecting hardware and associated communications cables.[2]
Cabling bandwidths in excess of 100 MHz (Category 5 cable bandwidth) make consideration of PSNEXT more important. Gigabit Ethernet through Cat-6 uses all four wire pairs simultaneously and bidirectionally. The additional wire pair usage and growing bandwidth increase the need to keep NEXT in check.
PSNEXT is a way of measuring NEXT in the ends of cables due to their close proximity. The (cited) SMP white paper states that the testing process for PSNEXT consists of measuring all pair-to-pair crosstalk combinations and then summing all of the values for each pair.
Far end crosstalk (FEXT)
Interference between two pairs of a cable measured at the other end of the cable with respect to the interfering transmitter.[1]
Equal level far end crosstalk (ELFEXT)
An FEXT measurement with attenuation compensation.[1]
Alien crosstalk (AXT)
Interference caused by other cables routed close to the cable of interest.[3]

In audio

In stereo audio reproduction crosstalk can refer to signal leaking across from one program channel to another, reducing channel separation and stereo imaging. This is an electrical effect and can be quantified with a crosstalk measurement.

Interchannel crosstalk applies between the two channels of a stereo system, and is usually not very important on modern systems, though it was hard to keep below the desired figure of -30 dB or so on vinyl recordings and FM radio.

Crosstalk between channels in mixing consoles, and between studio feeds is much more of a problem, as these are likely to be carrying very different programmes or material.

Crosstalk measurement is made on audio systems to determine the amount of signal leaking across from one channel to another.

The IBA drew up a weighting curve for use in crosstalk measurement that gives due emphasis to the subjective audibility of different frequencies, as shown here. This is still in use, despite the demise of the IBA, and in the absence of any international standards is worth adopting.

Other examples

In telecommunication or telephony, crosstalk is often distinguishable as pieces of speech or signaling tones leaking from other people's connections.[4] If the connection is analog, twisted pair cabling can often be used to reduce the effects of crosstalk. Alternatively, the signals can be converted to digital form, which is much less susceptible to crosstalk.

In wireless communication, crosstalk is often denoted co-channel interference, and is related to adjacent-channel interference.

In integrated circuit design, crosstalk normally refers to a signal affecting another nearby signal. Usually the coupling is capacitive, and to the nearest neighbor, but other forms of coupling and effects on signal further away are sometimes important, especially in analog designs. See signal integrity for tools used to measure and prevent this problem, and substrate coupling for a discussion of crosstalk conveyed through the integrated circuit substrate. There are a wide variety of possible fixes, with increased spacing, wire re-ordering, and shielding being the most common.

In full-field optical coherence tomography, "crosstalk" refers to the phenomenon that due to highly scattering objects, multiple scattered photons reach the image plane and generate coherent signal after traveling a pathlength that matches that of the sample depth within a coherence length.

In stereoscopic 3D displays, "crosstalk" refers to the incomplete isolation of the left and right image channels so that one leaks or bleeds into the other - like a double exposure, which produces a ghosting effect.

See also

References

  1. 1 2 3 4 "Category 5 / 5E & Cat 6 Cabling Tutorial and FAQ's". lanshack.com. Retrieved 2013-01-05.
  2. ↑ "Component Level PowerSum Compliance and RJ21X Connectivity Solutions (Superior Modular Products White paper)" (PDF). Archived from the original (PDF) on July 16, 2006. Retrieved 2008-09-13.
  3. ↑ Eliminating alien crosstalk, Communications News, February 2009, archived from the original on 2010-02-09
  4. ↑ Federal Standard 1037C glossary Archived 2011-10-24.
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