Audio is a field with its own share of standard connectors, including line-in/out and headphone TRS connectors, binding posts and banana plugs for connecting loudspeakers to amplifiers in consumer applications, and the balanced XLRs used in professional audio. But probably the most familiar, yet most questionable, audio connection component is the ubiquitous RCA "phono" jack/plug used in consumer A/V equipment.
Introduced in the 1940s to allow monaural phonographs to be connected to amps, the RCA connector found wide adoption as a simple and inexpensive way to interconnect components in consumer audio (and later, video) systems. But was it a good choice?
Audio interconnects, especially the cables themselves, receive a good deal of attention from audiophiles, who fret about such matters as the "skin effect" and wire and dielectric material. But most audio design engineers recognize that the RCA connector itself is by far the more critical-and more limiting-component.
|Introduced in the 1940s, RCA connectors became widely adopted as a simple and inexpensive way to connect consumer audio and video components, but poor grounding, the lack of balanced connections and other issues make it clear that they may not have been the best choice.|
|Unlike their RCA counterparts, XLR connectors have a balanced differential input and output, making them a better choice for professional audio applications.|
The parts suffer from several drawbacks:
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, amplifier- and speaker-damaging bursts of noise can result if the connectors are connected or disconnected while the associated equipment is powered on.
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 make 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 that would come at a cost, both in dollars and in circuit complexity.
Each signal requires a separate plug. This can obviously contribute to the familiar "rat's nest" of wiring, but in some cases it can even create problems in which the spacing of stereo input/output jacks on equipment is unable to accommodate some heavier-duty interconnects equipped with larger cables and plugs.
Some argue that the lack of a characteristic 75-ohm impedance is also an issue. For analog audio this is hardly a problem, but for video and (maybe) digital audio, the point is more valid. Almost all consumer digital audio equipment has adopted the RCA connector (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. That 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, 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 feet, the S/PDIF signal simply won't be affected by typical RCA connector and cabling lengths.
The RCA connector's deficiencies haven't gone unrecognized in the audio industry. At least one manufacturer (Canare) makes a "75 ohm" RCA plug designed to minimize impedance mismatches in video and digital audio applications. While that may be an improvement over other connectors, 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 been tried, including at least one "reinvention." While they may offer improved construction quality and some theoretical electrical benefits-usually at an exorbitant price and accompanied by dubious technical claims-they're still basically RCA-compatible connectors, with the same drawbacks.
For now, it looks like consumer audio remains stuck with the RCA phono plug. But with the increasing popularity of digital media (and interface standards like HDMI), perhaps the RCA connector has seen its best days.
Rich Pell is editor of EE Times Group's AudioDesignLine and Embedded Internet sites.
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