Audio
The Power Mac G5 computer supports a sound system with both digital and analog audio. The new digital capability features Sony/Phillips Digital Interface (S/PDIF) input and output using optical connectors. 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 sound circuitry digitally creates and records sounds. The Power Mac G5 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, the headphone jack, the audio output jack, and the S/PDIF output connector.
By default, when components are plugged into the headphone jack or the rear line-out, the sound system mutes the internal speaker.
The headphones, rear line-out jack, and S/PDIF output are only muted when selected in the System Preferences. Muting and sound options are set in System Preferences:Sound:Output. Selecting the “Allow Line Out and Digital when headphones are attached” checkbox allows operation of the analog and digital output ports when headphones are attached, enabling monitoring of the optical digital transport.
The analog and digital sound 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 sound circuitry and audio device drivers handle audio data in multiple formats. Both digital and analog sound circuitry handle audio input and output data at sample rates of 32.0 kHz, 44.1 kHz, and 48.0 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 Power Mac G5, 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 sound 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/
Optical S/PDIF Audio
Digital data is transmitted to and from the digital audio I/O using optical cables. The physical connectors, commonly referred to as TOSLink, are for both input and output and conform to IEC60874-17. The TOSLink friction-lock type F-05 connectors are available from pro-audio, musician’s supply, hi-fi and other retailers. The 7.5 mm digital optical TOSLink input and output connectors are located on the back of the enclosure.
The digital I/O circuitry can either perform input clock recovery on an incoming data stream or can sample using the internal clock. If samples are clocked using the internal clock, the data is run through the sample-rate converter on the digital circuitry. To enable bit-accurate copies, the external clock should be enabled in Audio MIDI Setup. Developers of applications that need this capability will need to provide access to the input clock recovery control.
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 headphones, line output jacks, or external speakers.
For details on the S/PDIF digital input and output electrical specifications, refer to “Digital Optical Input/Output Specifications”.
The Power Mac G5 computer also has the ability to lock 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.
The Power Mac G5 computer’s audio subsystem has built-in sample-rate conversion (SRC) that allows the incoming audio to have a different rate than the playback audio, enabling the audio to come in the digital-in connector. For the best digital input quality in its purest form, set the audio clocking preference to “External Clock.”
External Clocking
When the Power Mac G5 computer is set to “External” clocking, the computer audio circuitry will track and follow the incoming 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.
Internal Clocking
When the Power Mac G5 computer is set for “Internal” clocking, the computer audio circuitry will run using the computer’s internal clock. Incoming digital audio is automatically sample-rate converted to the selected output sample rate using the sample rate converter in the audio chip. The sample rate converter is high quality and does not use CPU bandwidth for audio conversion.
Digital Optical Input/Output Specifications
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 sound input and output have the following electrical characteristics (nominal specifications):
* Fsi – input sample rates (external clock mode): 32 kHz, 44.1 kHz or 48 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 (internal clock mode, 16 kHz < Fsi < 96 kHz): < -110 dB
* THD+N (external clock mode): < -130 dB (0.00001%)
* THD+N (internal clock mode, 16 kHz < Fsi < 96 kHz): < -110 dB (0.0003%)
Analog Audio Line-in
The Power Mac G5 has a stereo audio line-in jack on the back panel. The audio inputs are designed to accept high-level audio signals: 2.2 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 0.316 Vrms or –10 dbV. Sound recordings made on the Power Mac G5 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:
Tip Left-channel audio
Ring Right-channel audio
Sleeve Audio ground
Based on a 1 kHz, 2VRMS sine wave input, 24-bit 44.1 kHz input format, 0 db input gain and no weighting, the line input has the following electrical characteristics :
* maximum input voltage: 2.2 Vrms (+8 dBu)
* input impedance: > 47 kilohms
* bits per sample: 16-bit or 24-bit
* channel separation: greater than 75 dB
* frequency response: 20 Hz to 20 kHz, +0.5 dB/–3.0 dB
* THD+N distortion: below < –85 dB (no weighting)
Analog Audio Line-out
The Power Mac G5 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:
Tip Left-channel audio
Ring Right-channel audio
Sleeve Audio ground
Note: For best results, line-out devices should be plugged into the line-out jack. Headphone impedance is 32 ohms and line-out impedance is 1 kilohm.
Based on a 1 kHz, -1 dBFS sine wave playback, 24-bit 44.1 kHz output format, -1 db output level and 100 kilohm load no weighting, the line output has the following electrical characteristics (nominal specifications):
* output voltage: 1.4 Vrms (+4 dBu)
* source impedance: less than 24 ohms
* bits per sample: 16-bit or 24-bit
* channel separation: > 65 dB
* THD+N distortion: < –80 dB (0.01%) no weighting
* frequency response: 20 Hz to 20 kHz, +0.5 dB/–3.0 dB
Analog Headphone Jack
The Power Mac G5 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 is muted.
The headphone jack is a 3.5 mm miniature phone jack with the signals connected as follows:
Tip Left-channel audio
Ring Right-channel audio
Sleeve Audio ground
Note: The rear line-out is not designed to power headphones. Headphone impedance is 32 ohms and line-out impedance is 1 kilohm.
Based on a 1 kHz sine wave playback, 24-bit 44.1 kHz output format, -1 dB output level and 100 kilohm load no weighting, (unless otherwise specified) the headphone output has the following electrical characteristics (nominal specifications):
* output voltage: 1.4 Vrms, (+4 dBu)
* output impedance: 24 ohms
* bits per sample: 16-bit or 24-bit
* channel separation: > 65 dB
* frequency response: 20 Hz to 20 kHz, +0.5 dB/–3.0 dB
* THD+N distortion: < –80 dB (0.01%)
* output power (into 32 ohms): 20 mW
L'interface audio des G5
L'interface audio des G5
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