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Sound System

The 16-bit stereo audio circuitry provides sound input through the built-in microphone, the audio line in, the USB port, and sound output through built-in stereo speakers and the midrange-enhancer speaker and the audio minijack.

All audio is handled digitally inside the computer, including audio data from the optical drive and devices connected to the USB and FireWire 400 ports. Sound data is converted to analog form only for output to the internal speakers and the audio line out.

The sound circuitry handles audio data as 44.1 kHz 16-bit samples. If audio data sampled at a lower rate on another computer is played as output, the Sound Manager transparently upsamples the data to 44.1 kHz prior to sending the audio data to the sound circuitry.

In this section:

Audio Line Out
Audio Line In
Internal Microphone
Internal Speakers

Audio Line Out

The audio line out is located on the left side of the computer. The jack accepts a standard stereo mini-plug.

The stereo audio signals at the jack are configured to drive a pair of low-impedance stereo headphones. External powered speakers may also be connected to the audio line out.

The audio signals on the audio line-ou have the following electrical characteristics:

• output impedance: 16 ohms each channel

• minimum recommended load impedance: 32 ohms each channel

• maximum level: 1.5 V rms (4.4 V P-P)

Audio Line In

The 12-inch PowerBook G4 has a stereo audio line in jack located on the left side of the computer next to the line out.

The audio inputs are designed to accept high-level audio signals: 2 Vrms or +8 dbu, which is the standard output level from CD and DVD players. The output level of some consumer audio devices is lower, often 0.1 Vrms or –10 dbu. Sound recordings made on the 12-inch PowerBook G4 with such low-level devices have more noise than those made with high-level devices. The user may obtain better results by connecting an amplifier between the low-level device and the computer’s audio input jack.

The audio input jack is a 3.5 mm miniature phone jack with the signals connected as follows:

The sound line in has the following electrical characteristics:

• maximum input signal amplitude 2 Vrms (5.65 Vpp), +8 dbu peak

• input impedance at least 47 kilohms

• channel separation greater than 60 dB

• recommended source impedance 2 kilohms or less

• ground noise rejection greater than 40 dB

• frequency response 5 Hz to 20 kHz, +0.0, –0.5 dB

• distortion below –80 dB

• signal to noise ratio (SNR) greater than 90 dB (unweighted)

Internal Microphone

The computer has a built-in microphone located on the top left part of the case next to the escape key.

Internal Speakers

The 12-inch PowerBook G4 has a pair of stereo speakers located on the back of the cpu case and an internal midrange-enhancing speaker. The sound system provides parametric equalization for the speakers. The computer turns off the sound signal to the speakers when headphones are connected to the audio minijack.

Sound effects and output from other audio sources can be specified in the System Preferences Sound panel. An output device is displayed on the Sound panel when the computer detects that it is plugged in. The system default setting is the internal audio controller. Once the default is changed to a different device, it will remain the default as long as the device is plugged in.

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