The Audio Library (AL) programming interface provides an application programming interface for developing audio applications. The Audio Library is provided in the IRIS Development System option. The Audio Library is documented in the IRIS Digital Media Programming Guide. This guide, as well as the Apple® Audio Interchange File Format – AIFF-C Specification, are available from Silicon Graphics in the IRIS Digital Media documentation kit. Also, the various audio library calls, for example AFclosefile(3dm), are documented in the digital media reference pages (section 3dm).

To select the input or output sampling rates, pull down the Rate menu and select either Input or Output, then choose the appropriate rate. Figure 3-2 shows the Input sampling rate menu.

Figure 3-2. Changing the Sampling Rate (Input Shown)

The Output rate sampling menu has the additional option of matching the Input sampling rate regardless of what the Input sampling setting. This is useful because apanel is not a sampling rate conversion tool. If the input and output rates do not match, the resulting audio signal is pitch-shifted and distorted.

Sampling-rate conversion is performed by the graphical tool soundfiler and by the command-line tool sfconvert. See audio(1) for an overview of the various tools.

Be aware that audio applications will change the sampling rates as needed and there is no way to “lock” the sampling rate so that this does not happen. Many applications return the sampling rate to the previous setting, but some applications do not.

By default, the left and right sliders of both the input and output controls are ganged together. If you want independent control over the sliders, select the Options menu and click on the check box that corresponds to the sliders you want to control. Figure 3-3 shows the Options menu.

Figure 3-3. The Audio Panel Options Menu

In two-channel mode, you can select from one of the three input channels using the Input pulldown menu. Available input channels are digital I/O, line-level input, and microphone input.

You can switch the microphone input between mono and stereo modes using the Options menu.

To adjust the headphone volume, use the output sliders.

The line-level output, and digital I/O signal levels are not adjustable.

If you want to see the signal level of a sound file that you are playing (for example, using playaifc or soundeditor), switch the input mode to Digital. This causes the meters to display the signal level of the sound file as it plays, provided:

Do not turn on monitoring (by checking the Monitor box) while listening to a sound file, as this will cause feedback.

In four-channel mode, the microphone and headphone jacks have different functions than in normal, stereo mode in order to provide four-channel, line-level input and output. Table 3-1 shows the function of each connector in four-channel mode.

You can switch the input scale between simple 1 to 10 gradations and decibel calibrations by running apanel with either the -decadescale or -dbscale options. See apanel(1) for more information.

You can save and load audio panel configurations through the File menu. Select the Save option to save the current apanel configuration in the default filename (apanel.parameters) in your home directory. The Save As option prompts for an alternate file name.

To load a configuration, select Open from the File menu.

Audio panel configurations are not automatically loaded when apanel starts. You have to specifically open a file that contains audio panel configuration settings.

The audio connector bodies and strain-relief attachments cannot be longer than about 1.75 inches to 2 inches (4.4 cm to 5 cm). The exact length depends upon where the audio connector housing is installed, which jack you are using, and the flexibility of the audio cable you are attaching. Also, rackmount systems will typically provide greater clearance and deskside systems for connector housings. See Figure 3-4.

Figure 3-4. Audio Cable Connector-Body Limitations

To avoid damaging the audio cables and connectors, use right-angle audio connectors whenever possible.

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Warning: Do not operate a Challenge or Onyx system with the door open or removed. The door channels air for proper flow through the system and protects connectors and back-panel components from accidental damage. If an audio connector does not allow the door to close properly, you should obtain a right-angle connector or other adapter cable to allow you to close the system door.

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Serial digital transfers can be made for any sampling rate between 30 kHz and 50 kHz. If you want to use other sampling rates, you must use the analog interconnect (line-level).

Avoid using mono plugs in the stereo connectors, or cables that short some of the signals together. Avoid using cables with built-in attenuation.

The analog input circuitry for microphone and line-in recording has a software-controlled gain setting. The input circuitry accommodates signal levels of up to 8.4 Vpp (peak-to-peak) at the line-in connector. This is in excess of the levels produced at most modern consumer equipment line-out connectors.

To produce the best possible recordings, turn up the output control as far as possible without exceeding 8.4 Vpp (if the source machine has an output control). Start with the apanel input gain level setting at the “factory preset” level, which corresponds to “8” on the scale. Adjust the input gain level setting so that loudness peaks of the input material lights up almost the entire level meter. You can obtain the best results with a low input gain setting and a signal that registers nearly the entire span of the meter.

In order to minimize distortion and crosstalk effects between the headphone amplifiers and the more sensitive portions of the analog audio system, reduce the headphone volume level as much as possible during critical analog recording. Changing the headphone volume does not affect the line-out levels.

The analog audio instrumentation circuits in the Audio/Serial Option are very sensitive to the implementation of an analog ground voltage reference. You can achieve the best performance by letting the workstation be the only machine in your system of audio equipment that connects its system electrical ground to the safety grounding conductor.

If this is not possible, you may have to use signal isolation transformers. Note that the serial digital audio ports are already isolated by transformers within the machine, so that ground-reference problems do not typically apply to serial digital audio connections.

If your workstation is connected to a monitoring system such as a power amplifier driving speakers or headphones, turn the monitoring system volume down or off before you power the workstation on or off.

Do not wear headphones that are plugged into the workstation during power on or off cycles.

Do not record or monitor from an input connector that has nothing plugged into it.

Never connect a high-power output, such as the speaker output of a power amplifier, to any of the audio connectors on the Audio/Serial Option.

The Stereo Serial Digital Audio interconnect is a coaxial digital interconnect that makes noiseless audio connections to DAT machines, professional audio equipment, CD players, other workstations, and so forth. It corresponds to the electrical characteristics of AES3-ID-DRAFT, for distribution of digital audio using 75-ohm video cable. The interconnect is transformer coupled to prevent ground loops and provide isolation.
It supports up to 24-bit, 50 kHz sampling. It is based on the following standards:

The sample rate is encoded in the incoming stream, and the input sample rate may serve as sample clock source for analog in, analog out, and digital out. The digital output is nominally set to consumer-level configuration.

Table 3-3 lists the specifications for the serial digital audio input and output:

Table 3-4 lists the specifications for the line-level input.

Table 3-5 lists the specifications for the line-level output:

Table 3-6 lists the specifications for the microphone input:

Table 3-7 lists the headphone output specifications: