audio(1)audio(1)NAMEaudio - 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
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audio(1)audio(1)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)
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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.
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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.
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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
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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
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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.
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