Just to complicate things, TRRS plugs come in two different types, with the mic connection being in a different place. Technically, these plugs are known as TRS (Tip Ring Sleeve) connectors, and you will need a TRRS version to make mono recordings. In this case, you need a jack-plug with three rings on the end, which is capable of making four connections, ie there’s one extra band for the mic. ![]() (The third way is via Bluetooth, but this seems to be very limiting.) Microphone jacks There are two main ways to fit an external microphone to a smartphone: either it plugs into the headphone/mic socket, or connects via a microUSB or similar port. Again, HD Voice offers better quality, but both handsets have to support it, and so does all the network equipment between the two phones. In fact, the Enhanced Full Rate (EFR) codec used for GSM voice calls offers a maximum of 12.2kbps, which is a fraction of what you’d get from a landline. When it comes to network problems, mobile phones are designed to use very low bit-rates. (The Lumia 1020 is one of the few smartphones that records good quality stereo.) Some smartphones have more than one built-in mic: one is used for recording while others are generally used to detect and remove background noises. Handsets that support HD Voice ( Wideband audio) or 4G’s Voice over LTE ( VoLTE) should have better audio capabilities, though they still can’t provide the twice-CD-quality stereo of a handheld recorder. Worse, most only support mono recording, not stereo. Mobile phones also operate with a very restricted frequency range, perhaps 300Hz to 3.4kHz, and use low-cut filters and noise gates to suppress background noise. The microphones are tiny and you might well have a problem locating them. When it comes to handset design, smartphones are optimised for on-screen operations such as apps and browsing, not for voice.
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