audio(1) audio(1) NAME audio - Introduction to audio facilities DESCRIPTION An IRIS Audio Processor is included with the Onyx2, O2, Octane, and Tezro systems. It is an optional upgrade for Fuel systems using USB Audio or a DM8 PCI Option card, Onyx300, Onyx3000, Onyx350, and Onyx4 systems using a DM8 PCI option card, or for any system using a RADical PCI option card. Audio capabilities include digitizing sound from a microphone or from standard audio line-in using the stereo analog to digital converters, receiving and transmitting data via the serial digital audio interconnects, and sound output to headphones, internal loudspeaker, and standard line-out using the stereo digital to analog converters. Input gain, headphone/loudspeaker level, input and output sampling rates, and input sources are all independently specified with software. Audio software utilities provided with the workstation allow for immediate use of the built-in audio capabilities. Application programs use the audio system through the Audio Library (AL), an application programming interface accompanying the Digital Media Development Software (dmedia_dev.sw). UTILITY PROGRAMS IRIS Digital Media Execution Environment (dmedia_eoe.sw.tools) includes a number of utility programs that allow you to record, play back, and convert sound files. These programs are provided to make your workstation audio immediately useful. For more information, please see the manual page for each of the utility programs listed below. apanel(1) audio control panel playaifc(1) AIFF, AIFF-C sound file player playaiff(1) installed as a symlink to playaifc sfplay(1) sound file player recordaifc(1) AIFF, AIFF-C sound file recorder recordaiff(1) installed as a symlink recordaifc aifcinfo(1) displays information about AIFF-C, AIFF files sfinfo(1) displays information about sound files aifcresample(1) sampling rate conversion utility aiff2aifc(1) converts an AIFF file to an AIFF-C file aifc2aiff(1) converts an AIFF-C file to an AIFF file aifccompress(1) converts an AIFF or AIFF-C file to a compressed format aifcdecompress(1) decompresses an AIFF-C file sfconvert(1) sound file conversion utility soundeditor(1) sound editing with graphical user interface soundfiler(1) sound file conversion with graphical user interface passthru(1) user-level audio input monitor utility The audio execution environment (dmedia_eoe.sw.audio), in addition to required IRIX audio software support, contains the following utility programs: portstat(1) displays status of all open audio ports on system rbview(1) graphically displays status of audio ports AUDIO PROCESSOR FEATURES The audio processor provides many features to support a variety of audio applications. The follows is a overview of the features of the audio processor with an explanation of any differences between audio systems on different workstations. General Features Independent input and output sample rates Simultaneous input and output of audio data to/from applications Multiple applications sending and receiving audio data Input audio from digital inputs and microphone/line independently Output independently to headphone/loudspeaker, line out, and digital out Onyx2, Octane, and RADical cards support locking audio sample rates to video Input and Output Analog Input: Microphone (mono) or Line level (stereo) Variety of sample rates (from less than 8 kHz to 48 kHz) Controllable input attenutation Digital Input: Stereo Compatible with AES3-1992 and EIAJ CP-340 channel coding Sample rate encoded in incoming stream Onyx2, Octane, and RADical cards have an ADAT(r) 8-channel digital optical input Input sample rate may serve as sample clock source for analog in, analog out and digital outs Analog Output: Line level (stereo) and Speaker(mono)/Headphone(stereo) Controllable output level for speaker and headphone Variety of sample rates (from less than 8 kHz to 48 kHz) Digital Outputs: Stereo Onyx2, Octane, and RADical cards use AES3-1992 channel coding (professional-level configuration). Others use IEC958 channel coding standard (nominally set to consumer-level configuration). Onyx2, Octane, and RADical cards have an ADAT 8-channel digital optical output Electrical Specifications The following describes the electrical characteristics of the audio hardware for Onyx2, O2, Octane, Tezro, and DM8. Legacy audio including Indigo, Indigo2, and Indy are also included for reference. Line-level analog input: Computer-controlled attenuators (trim) adjust for a variety of levels. Each input channel may be adjusted in 1 dB steps on Onyx2/Octane and 1.5 dB steps on all others. Nominal Input Impedance: Onyx2/Octane: 7.5 kOhm Indigo : 5 kOhm Indigo2 : 20 kOhm Indy : 10 kOhm O2 : 100 kOhm Tezro : 50 kOhm DM8 Option : 50 kOhm Amplitude at Full-Scale: Onyx2/Octane: +6.5 dBV Indigo : 1 Vpp to 10 Vpp Indigo2/Indy: 0.63 Vpp to 8.4 Vpp O2 : 0 dBV Tezro : 0 dBV DM8 Option : 0 dBV Microphone input: A monophonic condenser microphone is included with IRIS Audio Processors on the Onyx2 and Octane. Nominal Input Impedance: Onyx2/Octane: 2.2 kOhm Indigo : 2 kOhm Indigo2 : 1.5 kOhm Indy : 2.0 kOhm O2 : 20 kOhm Tezro : 5 kOhm DM8 Option : 5 kOhm The condenser microphone supplied with your audio system obtains a small amount of power from the microphone input connector (5V via the respective input impedance shown above). The microphone input connector on all systems is compatible with other convential microphones besides the one supplied. Serial Digital Interconnect: The Stereo Serial Digital Audio interconnect is a coaxial digital interconnect that makes noiseless audio connections to DAT machines, professional audio gear, CD players, etc. It corresponds to the electrical characteristics of AES-3id-1995, 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, 50kHz sampling. It is based on the following standards: CP-340 : EIAJ AES3-1992 : Audio Engineering Society AES-3id-1995: Audio Engineering Society IEC958 : IEC Line-level analog output: Output Impedance: Onyx2/Octane: 47 kOhms Indigo : 600 Ohms Indigo2/Indy: 600 Ohms O2 : 600 Ohms Tezro : 1 kOhms DM8 Option : 32 Ohms Full-scale amplitude: Onyx2/Octane: +6.5 dBV Indigo : 6 Vpp Indigo2/Indy: 4.7 Vpp O2 : -3 dBV Tezro : 0 dBV DM8 Option : +4 dBV Connectors All audio connectors on the DM8 are 3.5 mm (1/8 inch) stereo mini phone connectors, as are the microphone and headphone/speaker connectors on the O2 and Octane. For those systems which use these connectors, the table below gives the pin assignments. By "input," we mean an input to the computer, and by "output," we mean an output from the computer. Connector Tip Ring Sleeve Microphone Input Not Connected Ground Line In L R Ground Line Out L R Ground For Onyx2, O2, Octane, and Tezro, Line In and Line Out are RCA-style connectors. For Onyx2 and the RADical option card, AES Digital In and Out are on 75 ohm BNC connectors. For Octane, AES Digital In and Out are on RCA-style connectors. CAVEATS AND KNOWN PROBLEMS The following information will allow you to troubleshoot audio problems and also explain some behavior you may observe when using the audio system. Audio CPU Usage Each audio port that remains open consumes a small but relatively constant amount of the MIPS CPU. Output ports tend to be more expensive than input ports, since they require the kernel to perform mixing on behalf of the application. Rate Settings It is meaningless to set the input source to digital input and the output rate to match the input rate while no digital input signal is connected to the workstation. This condition places the hardware in an indeterminate state, and the behavior of the audio system is unpredictable. Output Rates When monitoring digital input streams, the output rate should be set to match the input rate (i.e., ``Input Rate'' on apanel) as opposed to a rate numerically equal to the input rate. This way, the output stream is clocked by the same source as the input stream instead of a numerically equal but independent clock source. When using the ``Digital'' rates (see apanel(1)) for input or output, audio applications compiled before IRIX 5.1 may set the sampling rates to a fraction of the digital input rate. SUGGESTED PRACTICES You can get the best performance out of your IRIS audio processor by following some basic audio connection and recording practices: Digital Transfers The best quality recording is made by transferring audio material entirely in the digital domain. If the source of your recording has a serial digital output, use that connection to the workstation's serial digital audio input port to get noise-and-artifact-free audio data as your recorded sound. For example, if you are using the output of one IRIS audio processor as the input to another, using a serial digital audio transfers the audio information exactly, bit-for-bit, with no loss of dynamic range, added noise or distortion. If you use the analog line-out of the first (playing) workstation as the line-in to the recording workstation, you can still make very good recordings, but not as precise as the digital transfer. Note that serial digital transfers can be made for any sampling rate between 30kHz and 50kHz. If you want to use other sampling rates, you must use the analog interconnect. Equipment and Connections Avoid using mono plugs in the stereo connectors, or cables that short some of the signals together. Some cables have attenuation built into them; these should be avoided for most applications. Setting Input Levels The IRIS Audio Processor provides easy ways to make great analog recordings without a great deal of complicated equipment or specialized knowledge. There are, however, a few guidelines to follow which significantly improve the signal quality. Minimizing Input Gain/Maximizing Input Signal The analog input circuitry for microphone and line-in recording has a software-controlled gain setting. The first step in the process of making a good analog recording is to get a ``hot'' (loud, big) enough signal to the input connector BEFORE it gets to the input gain circuit. The input circuitry accommodates signal levels of up to 10Vpp (peak-to-peak) at the line-in connector. This is in excess of the levels produced at most modern consumer equipment line-out connectors. If the source machine for your recording has an output level control, turn it up as far as you can, but not to exceed the 10Vpp level which would introduce clipping distortion. Clipping distortion of this kind can be fairly easily detected by careful listening. 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 ALMOST light up the entire level meter. A low setting of the input gain control with a signal that registers nearly the entire span of the meter gives the best results. Minimizing Output-to-Input Crosstalk The headphone output amplifiers in your workstation are very powerful devices relative to the extremely sensitive circuits used for analog-to-digital conversion in the analog recording process. You can minimize distortion and crosstalk effects between these systems by reducing the headphone volume level as much as possible during critical analog recording. This does not affect the line-out levels. Avoiding Ground Loops The high-quality analog audio instrumentation circuits in your workstation 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. Most modern consumer equipment utilizes only two-prong power plugs and remains ground-isolated from the power lines, so this type of equipment does not typically cause a problem. The IRIS workstation chassis is intended to be electrically grounded. It is equipped with a three-wire saftey grounding plug-a plug that has a third (grounding) pin. Do not defeat the purpose of the grounding plug! Take care when connecting an IRIS workstation with other professional- type audio equipment (such as another IRIS workstation), which connects its chassis ground to the safety grounding conductor. In a professional environment, these problems are typically addressed by signal isolation transformers and other techniques. Note that the serial digital audio ports on your IRIS workstation are already isolated by transformers within the machine, so that ground- reference problems do not typically apply to serial digital audio connections. Other Cautions and Advice 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 your workstation. Be sure to use the line-out port of the workstation with the line-in port of your monitoring equipment; it performs better than the headphone output as a line-level source. Page 8