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Sound System

Under the control of the system software, the sound circuitry can create and record sounds digitally. It can receive audio signals through the audio input jack and send audio signals to the internal speaker, the headphone jack, the audio output jack, and the Apple speaker minijack.

Audio signals from the audio input jack are converted to digital data internally. All audio is handled digitally inside the computer, including audio data from the CD or DVD drive and from devices connected to the USB and FireWire ports. Audio data is converted to analog form for output to the internal speaker, the headphone jack, and the Apple Pro Speaker minijack.

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.

Plugging-in some components in the sound system mutes other components, as shown below:

In this section:

Audio Input Jack
Headphone Jack
Audio Output Jack
Apple Pro Speakers Minijack

Audio Input Jack

The Power Mac G4 has a stereo audio line-in jack on the back panel. Low level consumer products operating below -10 dbu require a pre-amp.

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 Power Mac 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 input jack 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)

Headphone Jack

The Power Mac G4 has a stereo headphone jack on the front of the enclosure. The headphone jack is suitable for connecting a pair of headphones or amplified external speakers. When a plug is inserted into the headphone jack, the internal speaker and the Apple Pro Speakers (if connected) are muted.

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

The headphone jack has the following electrical characteristics:

• full-scale output level (open circuit) 1.5 Vrms (4.5 Vpp), +4 dbu peak

• source impedance is 10 ohms

• channel separation greater than 60 dB

• recommended load impedance 32 ohms or greater

• distortion, 32 ohm load, is better than –80 dB (0.5%)

• frequency response, 32 ohm load, 20 Hz to 20 kHz, +0.0, –0.5 dB

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

Audio Output Jack

The Power Mac G4 has a stereo output jack on the back of the enclosure. The audio output jack is suitable for connecting amplified external speakers.

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

The audio output jack has the following electrical characteristics:

• full-scale output level (open circuit) 1.5 Vrms (4.5 Vpp), +4 dbu peak

• source impedance less than 10 ohms

• channel separation greater than 60 dB

• recommended load impedance 1 kilohm or greater

• distortion is –80 dB (0.02%)

• frequency response, with 10 kilohms load, 5 Hz to 20 kHz, +0.0, –0.5 dB

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

Apple Pro Speakers Minijack

The Apple Pro Speakers minijack is a stereo 2.5 mm miniature jack. It has a smaller diameter than the headphone jack so that the user cannot inadvertently plug headphones into it.

The electrical characteristics of the Apple Pro Speakers minijack are optimized for use with Apple Pro Speakers. The Apple Pro Speakers include an internal ROM that enables the computer to identify the speakers. Speakers other than the Apple Pro Speakers should not be connected to the Apple Pro Speakers minijack.

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