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Basic Sensors in iOS by Alasdair Allan

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Chapter 3. Using Audio

The main classes for handling audio in the SDK are in the AVFoundation and Media Player frameworks. This chapter will provide a brief overview of how to play and record audio using these frameworks.

The Hardware

Whilst most phones have only one microphone, iPhone 4 has two. The main microphone is located normally on the bottom next to the dock connector, while the second microphone is built into the top near the headphone jack. This second microphone is intended for video-calling, but is also used in conjunction with the main microphone to suppress background noise.

In comparison the iPad 2 has a single microphone, but there is a difference between the two models which could lead to a difference in audio recording quality between the 3G and WiFi-only models. On the WiFi-only model, the microphone hole is built-into the back of the device, whereas on 3G models, it’s built into the antenna casing. There are suggestions that this difference may lead to cleaner audio recordings with the WiFi model, with the 3G model sounding muffled and echo-prone by comparison.

Both the iPhone 4 and the iPad use an Apple branded Cirrus Logic 338S0589 for their audio DAC, with a frequency response of 20Hz to 20kHz, and audio sampling of 16-bit at 44.1kHz.

All of the current iPhone, iPad and iPod touch models use a 2.5mm 4-pole TRRS (tip, ring, ring, sleeve) connector which has a somewhat unorthodox mapping to the standard RCA connector as shown in Table 3-1.

Table 3-1. Mapping between ...

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