This Chapter provides an overview of the Audio/Serial Option, describing the various connectors and basic functions.
The Audio/Serial Option provides the following capabilities:
Audio Library-compatible audio input and output.
Audio capabilities that are functionally equivalent to those provided on the Indigo2 and Indy, including dedicated line-level in and out, serial digital audio in and out, headphone output, and microphone input.
Line-level audio input and output in either two-channel mode or four-channel mode.
Six serial ports, switchable through software between RS-422 and RS-232.
Serial port speeds of up to 38400 baud using standard utilities (such as getty and cu) and up to 115,200 baud through user-developed applications. User-developed applications can use either STREAMS or, for lower latency, memory-mapped I/O.
 | Caution: Although the Audio/Serial Option serial ports can function at speeds beyond 115,200 baud, such higher rates are not supported. Operating beyond 115,200 baud can result in loss of serial characters and overall degraded performance. |
Figure 1-1 is a block diagram of the Audio/Serial Option board, showing the serial subsystem and its relation to the audio subsystem.
The serial subsystem provides six DB-9 female serial connectors, each of which are selectable in software between RS-232 and RS-422 modes. Figure 1-2 shows how the ports are arranged on the connector plate.
For a description of the pin-outs of the serial connectors, see Appendix A, “Connector Pinouts.”
 | Note: In RS-422 mode, the pin-outs of the Audio/Serial Option ports do not match those of the built-in RS-422 connector on CHALLENGETM and OnyxTM deskside and rackmount systems. See “Differences Between Existing Challenge and Onyx Serial Ports”. |
Table 1-1 shows how the ports correspond to tty device files.
Table 1-1. Correspondence of Serial Ports to Device File Names
RS-232 Device File | RS-422 Device File | Memory-Mapped Device File |
|---|---|---|
/dev/tty[dfm]72 | /dev/tty4[dfm]72 | /dev/aso_mmap/tty72 |
/dev/tty[dfm]73 | /dev/tty4[dfm]73 | /dev/aso_mmap/tty73 |
/dev/tty[dfm]74 | /dev/tty4[dfm]74 | /dev/aso_mmap/tty74 |
/dev/tty[dfm]75 | /dev/tty4[dfm]75 | /dev/aso_mmap/tty75 |
/dev/tty[dfm]76 | /dev/tty4[dfm]76 | /dev/aso_mmap/tty76 |
/dev/tty[dfm]77 | /dev/tty4[dfm]77 | /dev/aso_mmap/tty77 |
The audio subsystem provides the following inputs and outputs:
Headphone output (stereo)
Microphone input (mono or stereo)
Line-level input
Line-level output
Serial digital input/output
For detailed electrical specifications, impedances, and output levels, see “Audio Specifications” in Chapter 3.
All the audio jacks accept 3.5-mm stereo plugs, which can use mono plugs where appropriate. Figure 1-3 shows the layout of the audio connectors.
The heart of the Audio/Serial Option audio subsystem is the HAL2, a high-density ASIC on the audio module that performs the basic I/O functions. Figure 1-4 is a block diagram of the audio subsystem.
The audio module provides the following features:
Stereo line-level analog audio input and output.
Stereo headphone output.
Microphone input that supports stereo and mono microphones.
Serial digital audio input and output, supporting up to 24 bits per sample.
Industry-standard sampling rates of 8, 11.025, 22.5, 16, 32, 44.1, and 48 kHz, and others.
Microphone input with DC power.
Simultaneous input and output.
Independent input and output sampling rates.
Output rate can be synchronized to the serial digital audio input rate.
Silicon Graphics Audio Library (AL) application programming interface (API).
Low-latency operation.
Four-channel mode supports four-analog-channel input and four-analog-channel output, simultaneously.
Analog audio input may be synchronized to serial digital audio input rate.
Sampling time base is generated by Bresenham's algorithm, allowing near-continuous choice of sample rates.
The audio module is built around a central controller chip, the HAL2 ASIC, two audio CODEC chips, an AES transmitter chip, an AES receiver chip, a microphone input circuit, a headphone amplifier circuit, and a four-channel mode switching relay.
The HAL2 chip is a 1-micron, 28K-gate, CMOS gate array that contains the data path and control logic to interface the P-bus and the audio devices on the module. The major functional blocks connected to the HAL2 are the two CS4216 CODECs, the CS8401 AES3 transmitter, the CS8411 AES3 receiver, the headphone gain circuit, the microphone input circuit, and the four-channel mode switching relay.
Notable features of the HAL2 design:
Three independent clock sources:
AES out
analog in
analog out
Each of the clock generators can select from three different time bases:
44.1 kHz master clock
48 kHz master clock
AES3 external clock (through the serial digital jack)
The clock generators scale the input clocks by rational fractions.
Each audio device has its own DMA channel and clock generator.
The audio devices attach with conventional three-wire serial interfaces.
In four-channel mode, each four-channel DMA data stream consists of interleaved, synchronized audio data.
The audio module uses a pair of Crystal Semiconductor CS4216 stereo audio coder/decoders (CODECs). These chips are monolithic CMOS mixed-signal devices. Both of the Analog-to-Digital Converters (ADC) and Digital-to-Analog Converters (DAC) are 64x-oversampling delta-sigma 16-bit converters. They also contain on-chip reconstruction and anti-aliasing filters, programmable input gain, and programmable input source switching. The filter responses track the sampling rate, a significant advantage over the older fixed-response, low-pass, analog filter designs.
In the normal mode of operation, the CODEC A DAC is used for analog output and the CODEC B ADC is used for analog input. The CODECs can use independent sample-rate clock generators from the HAL2 so that the analog input sample rate and the analog output sample rate may be selected independently. The analog input (to CODEC B) is selectable from either the line or microphone inputs under software control. The analog output signal (from CODEC A) is routed both to line-out and to the stereo headphone circuit.
The audio system provides an enhanced mode of operation that extends from two to four the number of simultaneously active analog input channels. The graphical audio control panel utility, apanel, can switch the audio system between two-channel mode and enhanced, four-channel mode. This can also be done programmatically by applications.
In enhanced, four-channel mode, both CODECs are synchronized to the same sample rate and are used simultaneously for input (ADC) and output (DAC). CODEC B's input (ADC) comes from the microphone input, while CODEC A's input (ADC) comes from the line input. CODEC A's output continues to be routed to the line output, but CODEC B's output is routed at line-levels to the headphone jack.
The audio module provides a 3.5-mm stereo phone jack for a set of headphones. See “Audio Specifications” in Chapter 3 for output power ratings and other characteristics, and Appendix A, “Connector Pinouts,” for the tip-ring-sleeve connections.
In four-channel mode, the headphone output is switched to line-level output.
The microphone input is a 3.5-mm stereo phone jack that accepts mono or stereo signals. See “Audio Specifications” in Chapter 3 for information on input sensitivity and other characteristics, and Appendix A, “Connector Pinouts,” for a description of the tip-ring-sleeve connections.
In four-channel mode, the microphone input can be used either for microphone-level signals or switched to line-level input.