(Note: This article was originally written as part of an EE Times feature on Interconnects, Passives and Electromechanical components.)
Audio is a field with its own share of standard connectors, from line-in/out and headphone TRS connectors, and binding posts and banana plugs for connecting loudspeakers to amplifiers in consumer applications to the balanced XLRs in professional audio. But probably the most familiar audio connection component is the ubiquitous RCA "phono" jacks/plugs used in consumer A/V equipment.
Originally introduced by the Radio Corporation of America in the 1940s to allow monaural phonographs to be connected to amplifiers, they later became widely adopted as a simple and inexpensive way to interconnect components in consumer audio (and later, video) systems. But was it a good choice?
While audio interconnects - especially the cables themselves - receive a good deal of attention from audiophiles who fret about such things as "skin effect" and wire and dielectric material, most audio design engineers recognize that the RCA connector itself is by far the more critical - and limiting - component. The parts suffer from several drawbacks:
1.) When making a connection, the ground connection is made *after* the signal connection (and vice versa when disconnecting). While this doesn't affect normal operation, if the connectors are connected/disconnected while the associated equipment is powered on this could result in amplifier and speaker damaging bursts of noise.
2.) Unlike the balanced connections used in most professional audio equipment, an RCA connection is unbalanced, resulting in more susceptibility to noise. While the typically shorter cable lengths used in consumer audio makes this less of an issue in most applications, noise is still a common enough problem that some might argue for balanced connections in consumer gear. But this would come at a cost - both in dollars and in circuit complexity.
3.) With RCA connectors, each signal requires a separate plug. This can obviously contribute to the familiar "rat's nest" of wiring, but in some cases can even create problems where the spacing of stereo input/output jacks on equipment is unable to accommodate some heavier-duty interconnects equipped with larger size cables and plugs.
4.) Some argue that the lack of a characteristic 75-ohm impedance is also an issue with RCA connectors. For analog audio this is hardly a problem, but for video and - maybe - digital audio, the point is more valid. In the latter case, almost all consumer digital audio equipment has adopted the use of the RCA connectors (along with the more expensive and less robust TOSLINK optical connector) for the S/PDIF interface - despite the fact that the interface standard calls for a 75-ohm impedance connection.
This could be achieved with 75-ohm BNCs, or even RF coaxial F-connectors, but no engineer familiar with transmission line design would specify an RCA connection here as an optimal solution. Ultimately, though, does it really matter? Not according to some, who point out that with a maximum bandwidth of about 12 MHz and a wavelength of about 10 ft, the S/PDIF signal simply won't be affected by typical RCA connector/cabling lengths. But the debate continues.
The RCA connector's deficiencies haven't gone unrecognized in the audio industry. For example, at least one manufacturer (Canare) makes a "75-ohm" RCA plug designed to minimize impedance mismatches in video and digital audio applications. However, while this may be an improvement over other connectors in this respect, the inherent dimensions of any RCA-type connector prevent it from ever achieving true 75-ohm status.
Other attempts to improve the RCA plug have also been tried, including at least one "reinvention." While these may (or may not) offer improved construction quality and some theoretical electrical benefits - often at an exorbitant price and accompanied by dubious (to say the least) technical claims - they're still basically RCA-compatible connectors with the same drawbacks listed above.
For now, it looks like consumer audio remains stuck with the RCA phono plug. However, with the increasing popularity of digital media - and interface standards like HDMI - perhaps it has seen its best days.
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