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Audio System

The 15-inch PowerBook G4 computer supports a audio system with both digital and analog audio. The new optical digital audio input and output capability features Sony/Phillips Digital Interface (S/PDIF) input and output. S/PDIF technology results in a clean audio signal with no added noise to or from an external audio device.

Under the control of the system software, the audio circuitry digitally creates and records sounds. The 15-inch PowerBook G4 computer can receive input only from either the analog input or the digital input. However, it can output simultaneously to digital and analog devices: the internal speaker and combination the headphone out and S/PDIF out port.

By default when components are plugged into the combined optical digital audio output/headphone out port, the audio system mutes the internal speaker.

The headphones and optical digital audio output are only muted when selected in the System Preferences. Muting and audio options are set in System Preferences:Sound:Output.

The analog and digital audio circuitries are not independent. Different audio streams cannot be played to the analog and digital circuitry. The selection of digital or analog output is performed through Sound pane in System Preferences.

The audio circuitry and audio device drivers handle audio data in multiple formats. Both digital and analog audio circuitry handle audio input and output data at sample rates of 32.0 kHz, 44.1 kHz, 48.0 kHz, 64.0 kHz, 88.2 kHz, and 96.0 kHz at sample depths of 16 bits and 24 bits.

If audio data sampled from another computer at a lower rate is played as output on the 15-inch PowerBook G4, the Core Audio (Mac OS X’s OS Level Audio API) transparently up-samples the data to the currently set sampling frequency prior to sending the audio data to the audio circuitry. To maximize audio fidelity, the Core Audio samples are stored as 32-bit floating point. The Sound Manager exists as a Carbon compatibility layer, but developers are encouraged to move their applications to Core Audio for maximum performance and fidelity, because the Sound Manager is capable of representing samples only as 16-bit values.

For more information about audio API’s on Mac OS X, visit the Apple audio technologies developer web page at

http://developer.apple.com/audio/

In this section:

Optical Digital Audio
Audio Inputs
Audio Outputs
Digitizing Audio

Optical Digital Audio

The digital I/O circuitry automatically performs input clock recovery on an incoming data stream. This enables bit-accurate copies of the digital data.

Audio signals from the audio input port 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, combination headphones and optical digital audio output, or external speakers.

For details on the optical digital audio input and output electrical specifications, refer to “Audio Line In and Digital Optical Audio Input Specifications” and “Headphone Out and Digital Audio Output Specifications.”

The 15-inch PowerBook G4 computer automatically locks its internal audio hardware to the incoming audio stream to synchronize the audio subsystem to an external device supplying the audio stream. This function allows audio and video to play in sync with the external audio or video device. The data format for signals transmitted over the optical cable is S/PDIF protocol IEC 60958-3.

Since the 15-inch PowerBook G4 does not support a hardware sample rate converter, core audio services provide the sample rate conversion.

External Clocking

When the 15-inch PowerBook G4 computer detects a digital input stream, it is automatically locked to“External” clocking and the computer audio circuitry will track and follow the outgoing digital sampling rate and lock the internal audio hardware to the sampling rate of the external device. The external clock must be stable enough to be locked onto, otherwise the digital circuit will signal an error and the driver will fall back to using the internal clock. The digital input always follows the external clock.

Internal Clocking

When the 15-inch PowerBook G4 computer is set for “Internal” clocking, the computer audio circuitry will run using the computer’s internal clock. Internal clocking is used when there is no digital input.

Audio Inputs

The audio system accepts inputs from the following sources:

• the built-in microphone

• the combined optical digital audio input/audio line in port

• a CD or DVD in the optical drive

• a digital audio device connected to a USB or FireWire port

The audio line in and optical digital audio input port and the microphone preamp share a dedicated analog input channel in the audio circuitry; the other inputs send digital data. The analog input can be set for play-through or recording. The digital inputs can be selected or mixed by the audio audio circuitry.

The computer also accepts digital audio data from the SuperDrive or from devices connected to the USB or FireWire ports. Audio data from those sources can be sent to the audio system to be converted to analog form for output to the speakers and the combination headphone and optical digital audio output.

Built-in Microphone

The built-in microphone is located on the left speaker grill.

The audio signal from the built-in microphone goes through a dedicated preamplifier that raises its nominal 30 mV level to a nominal 700 mV RMS signal to the audio circuitry. That signal level assures good quality digitizing without driving the analog input into clipping.

Audio Line In and Digital Optical Audio Input Specifications

The combination audio line-in and optical digital audio input is a 3.5 mm mini jack that accepts line-level stereo signals. It also accepts a stereo miniplug-to-RCA cable adapter for connecting stereo equipment to the computer.

The audio line in signal connections are

• tip: audio left channel

• ring: audio right channel

• sleeve: audio ground

Based on playback of a 1 kHz, -1dBFS 24-bit sine wave playback, 24-bit 44.1 kHz output sample rate (unless otherwise specified below) the digital audio input and output have the following electrical characteristics (nominal specifications):

• Fsi – input sample rates (external clock mode): 32 kHz, 44.1 kHz, 48 kHz, 64 kHz, 88.2 kHz, or 96 kHz

• Fsi – input sample rate (internal clock mode): 16 kHz – 96 kHz

• bits per sample: 16-bit or 24-bit

• SNR (external clock mode): >130 dB

• SNR: >110 dB

• THD+N (external clock mode): < -130 dB (0.00003%)

• THD+N: < -110 dB (0.0003%)

Modem Activity Audio Signals

Modem activity audio signals from the communications slot are sent to the audio circuitry as 8-bit digital data.

Audio Outputs

The audio system sends audio output signals to the built-in speakers and the combination optical digital audio output and headphone out port.

Headphone Out and Digital Audio Output Specifications

The combination heaphone and optical digital audio output port is located on the left side of the computer. The headphone out provides enough current to drive a pair of low-impedance headphones. It can also be used as a line-level output.

The headphone out has the following electrical characteristics:

• impedance suitable for driving standard 32-ohm headphones

• output level 2.0 V peak-to-peak (1.41 V RMS)

• signal-to-noise (SNR) 90 dB unweighted (typical)

• total harmonic distortion (THD) 0.01% or less

During playback of a 1KHz, full-scale sine wave (S/PDIF output format, 44.1KHz output sample rate, 24-bit sample depth, unless otherwise specified) the digital audio output has the following nominal specifications:

• Jack Type: 3.5mm Analog/Optical Combo Jack

• Output Data Formats: S/PDIF (IEC60958-3), AC3

• Output Sample Rates: 32KHz, 44.1KHz, 48KHz, 64KHz, 88.2KHz, 96KHz

• Bits per Sample: 16 or 24 (S/PDIF),16 (AC3)

• Frequency Response: 20Hz – 20KHz, +/-0dB

• Signal-to-Noise Ratio (SNR): >130dB

• Total Harmonic Distortion + Noise (THD+N): <-130dB (0.00003%)

• Channel Separation: >130dB

Internal Speakers

The computer has two internal speakers, one on either side of the keyboard. The computer turns off the audio signals to the speakers when an external device is actively connected to the optical digital audio /headphone out and during power management.

Digitizing Audio

The audio circuitry digitizes and records audio according to the audio input sample rate and bit depth selected in Audio MIDI setup. If a sound sampled at a higher or lower rate on another computer is played as output, the Sound Manager transparently up-samples or down-samples the sound to the audio output sample rate and bit depth selected in Audio MIDI setup prior to outputting to the audio circuitry.

When recording sound from a microphone, applications that may be affected by feedback should disable sound play-through by calling the Sound Manager functions.

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