The iPhone can be a useful audio recording tool. You can use the built-in microphone for voice notes and field recordings with reasonably high-quality results. However, to take advantage of the iPhone's digital audio recording potential -- or simply to "patch in" external audio equipment -- you'll need an adapter cable. Using an RCA adapter, you can record audio with any app that you'd use with the built-in microphone, but you'll have much more flexibility about the source and sound quality.
The RCA Standard
Although audio connections come in many formats, the most common jack used in both professional and consumer audio equipment is RCA. You can find the typical red and white pair on the inputs and outputs of everything from home stereo equipment to HDTVs and professional video cameras (often joined by a yellow RCA plug for analog video signals). RCA jacks are also used for some digital connections, but these will be of no use to your iPhone adapter. Similarly, although the RCA plug is also sometimes called a "phono plug," the RCA jacks used for turntables frequently depend on a preamplifier for proper volume levels, and therefore may be unsuitable for direct connection to your iPhone.
RCA Adapter Considerations
The most common RCA-to-iPhone adapter is a cable consisting of one, two, or three RCA plugs on one end and a 3.5 mm TRRS plug on the other end. TRRS stands for "tip ring ring sleeve" and is distinctly different from a standard TRS headphone plug, having an extra ring to provide microphone input and stereo headphone output from a single accessory. Make sure that the adapter you choose supports the microphone input channel; a standard Y-shaped, red-and-white RCA adapter may only provide headphone output.
Mono Versus Stereo
No matter which RCA-to-3.5 mm adapter you use, you'll only be able to create mono (single-channel) recordings using the 3.5mm jack in your iPhone. If stereo recording is a must-have feature for you, there are alternative adapters available for the iPhone. Products such as Tascam's iXJ2 or Line 6's Mobile In connect to the dock port on the iPhone and provide line inputs. In the case of the iXJ2, the inputs are are a pair of 3.5 mm jacks -- compatible microphones can be connected directly, or a pair of adapters may be used to connect RCA or other jack formats.
Once you have everyting connected, it's just a matter of launching your chosen iOS audio app and pressing "Record." This includes anything from Apple's own Voice Memo or GarageBand to countless third-party digital recording apps. You may want to keep an eye out for an app that has the option to set the recording level. Most apps are designed to work with the iPhone's built-in recorder, and other audio sources may be significantly more quiet or loud.
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