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audio - Introduction to audio facilities
An IRIS Audio Processor is included with the IRIS Indigo2, Power Indigo2,
Indy, Indigo, Onyx2 and O2 systems. It is an optional upgrade for the
Onyx and Power Onyx systems (using an Audio/Serial 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 IRIS Development Option.
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
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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 [Toc] [Back]
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 workstations support locking audio sample rates to video
Input and Output [Toc] [Back]
Analog Input:
Microphone (mono or stereo) or Line level (stereo)
Variety of sample rates (from less than 8 kHz to 48 kHz)
Controllable input attenutation
Digital Input:
Stereo
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Compatible with AES3-1992 and EIAJ CP-340 channel coding
Sample rate encoded in incoming stream
Onyx2 has 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 uses AES3-1992 channel coding (professional-level
configuration). Others use IEC958 channel coding standard
(nominally set to consumer-level configuration)
Onyx2 has an ADAT 8-channel digital optical output
Electrical Specifications [Toc] [Back]
The following describes the electrical characteristics of the audio
hardware for Onyx2, Indigo and Indigo2/Indy.
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 and 1.5 dB steps on Indigo and Indigo2/Indy.
Nominal Input Impedance:
Onyx2: 7.5 kOhm
Indigo: 5 kOhm
Indigo2: 20 kOhm
Indy: 10 kOhm
Amplitude at Full-Scale:
Onyx2: +6.5 dBV
Indigo: 1 Vpp to 10 Vpp
Indigo2/Indy: 0.63 Vpp to 8.4 Vpp
Microphone input: A monophonic condenser microphone is included with the
IRIS Audio Processor.
Nominal Input Impedance:
Onyx2: 2.2 kOhm
Indigo: 2 kOhm
Indigo2: 1.5 kOhm
Indy: 2.0 kOhm
The condenser microphone supplied with your audio system obtains a small
amount of power from the microphone input connector (5V on Onyx2, 3V on
Indigo2/Indy and Indigo via the respective input impedance shown above).
The microphone input connector is compatible with other convential
microphones beside the one supplied.
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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: 47 kOhms [Onyx2]; 600 Ohms [Indigo and
Indigo2/Indy]
Full-scale amplitude: 6.5 dBV [Onyx2]; 6 Vpp [Indigo]; 4.7 Vpp
[Indigo2/Indy]
Headphone Output: The stereo headphone output includes separate volume
controls for each channel. Volume for each channel may be adjusted to one
of 256 linear steps.
Output Level Maximum, each channel Indigo Indigo2/Indy
64 Ohm load 180 mW 36 mW
16 Ohm load 280 mW 74 mW
The headphone volume also controls the speaker volume on all platforms.
[Note: Onyx2 has no built-in loudspeaker]. When the headphones are
plugged in, the speaker circuit is disabled.
Connectors [Toc] [Back]
All audio connectors for Indigo and Indigo2/Indy audio are 3.5 mm (1/8
inch) stereo mini phone connectors, as are microphone and
headphone/speaker connectors for Onyx. 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 (Indigo, Onyx2) Input Not Connected Ground
Microphone (Indigo2/Indy) L R Ground
Line In L R Ground
Line Out L R Ground
Headphone Out L R Ground
Digital I/O Output Input Ground
(for this
circuit alone)
For Onyx2, Line In and Line Out are RCA-style connectors, and AES Digital
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In and Out are on 75 ohm BNC connectors.
Differences Between Indigo2/Indy and Indigo Audio
The Indigo2/Indy audio system represents the second generation of SGI
digital audio. The Audio/Serial Option for Onyx/Challenge is, for the
most part, identical to the Indigo2/Indy audio system. These systems
deviate from an exact replica of the Indigo audio system in the following
ways:
Digital Signal Processor [Toc] [Back]
The original Indigo contains a Motorola 56001 Digital Signal
Processor (DSP). This processor is used to maintain real-time flow
of audio data to and from the MIPS processor, and to perform mixing
operations between audio applications. Indigo2 and Indy do not
contain a dedicated DSP chip; instead, the operation of the audio
system is split between the HAL2 ASIC, HPC3 ASIC, and software
running on the MIPS CPU.
Input Attenuation (Trim) [Toc] [Back]
As in the original Indigo, normal line input levels may be digitized
as full range signals. The input level attenuation control which
sets this trim has a larger range of attenuation in Indigo than
Indigo2 and Indy. You might have become accustomed to using this
trim control (which is mapped to apanel) as an input volume control
whose range extends to full input attenuation (zero signal). The
Indigo2 and Indy input attenuator does not attenuate the input
signal completely, but it does support the full specified range for
input digitizations.
Headphone Power Output [Toc] [Back]
For the same digital audio signal, the headphone driver in Indigo
provides noticeably more power (volume) for a given output volume
setting than Indigo2 and Indy.
Internal Speaker [Toc] [Back]
For the same digital audio signal, the internal speaker in Indigo
provides noticeably more volume for a given output volume setting
than Indigo2 or Indy. Indigo2 and Indy have substantially greater
fan noise than Indigo, which increases your perception of this
effect. The distance between the listener and the chassis
loudspeaker, and the direction of the loudspeaker with respect to
the listener, both strongly influence the perceived loudness.
Support for Four-Channel Audio [Toc] [Back]
On Indigo2 and Indy workstations, the audio processor is capable of
four channels of analog line-level input and four channels of analog
line-level output. The Audio Control Panel apanel performs the
channel mode switch with a toggling menu item. Application programs
enable the four-channel capability with an Audio Library procedure
call.
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On workstations without the four-channel capability (e.g., Indigo),
applications created to input or output four channels of audio can
be executed and tested but monitored only in two-channels.
For further information regarding four-channel audio on Indigo2 and
Indy workstations see the section below entitled "Four-Channel Audio
Support."
Support for Stereo Microphones
Indigo2 and Indy workstations support up to two channels of
microphone-level signals. The Audio Control Panel apanel specifies
the microphone channel mode with a toggling menu item. Application
programs specify the microphone mode with an Audio Library procedure
call.
When a stereo microphone is connected and the stereo microphone
software is not enabled, the left channel of the microphone is used
as the input to the audio system.
Four-Channel Audio Support [Toc] [Back] System Modes
Indigo2 and Indy systems support four simultaneous analog audio input and
output channels. To support this additional functionality while
maintaining functional compatibility with the Indigo systems, the Indigo2
and Indy provides two modes of operation: four-channel mode and Indigo
mode. By default, the system is in Indigo mode, and remains in it unless
you explicitly change to four-channel mode, via apanel. Switching between
the modes changes both the electrical properties of the connectors on the
back panel and the behavior of the system audio software. Both stereo
and four-channel audio applications function in both modes, with some
behavioral changes. These electrical and behavioral changes are
documented in the rest of this section.
Sampling Rates in Four-Channel Mode [Toc] [Back]
In four-channel mode, the analog input and analog output sampling rates
must be the same. Changing the input sampling rate causes the output
sampling rate to become ``Use Input Rate.'' Likewise, changing the
output sampling rate causes the input sampling rate to match the output
rate. When you switch into four-channel mode, the system must change
either the input or the output sampling rate to match the other. To
minimally interfere with already running applications, it decides which
rate to change based upon whether or not input or output is in use. Note
that this rate restriction applies only to the analog I/O channels; the
digital input and analog output may still perform simultaneous I/O at
different sampling rates.
Input selection in Four-Channel Mode [Toc] [Back]
In four-channel mode, four channels of analog input are available. The
first two channels represent the analog signal at the line input
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connector; the next two channels represent the analog signal at the
microphone input connector. The line input connector always accepts a
line-level signal. However, the electrical properties of the microphone
connector are configurable. If you select the line-level source from
apanel, the microphone connector functions as a line-level input. If you
select the microphone source from apanel, the microphone connector
accepts either a stereo or mono microphone.
If you select digital input in four-channel mode, the digital input is
used. Since the digital input is only a two-channel source, it provides
only two channels of data to four-channel applications. However, those
applications still function properly, as discussed below.
Stereo Applications in Four-Channel Mode [Toc] [Back]
Applications that perform stereo audio output function in four-channel
mode. Because they provide only two channels of audio data, their output
appears only at the line output connector, since the headphone output
connector represents the second pair of line-level output.
Applications that perform stereo audio input function in four-channel
mode. If you select an analog input, four channels of data are
theoretically available. A stereo input application gets the sum of the
two left-channel inputs as its left channel, and the sum of the two
right-channel inputs as its right channel.
Four-Channel Applications in Indigo Mode
Four-channel output applications function in Indigo mode or on Indigo,
though there are only two physical output channels. The translation from
four to two channels occurs as follows. The application's output is
considered as two stereo pairs. The application's two left channels are
summed and sent to the left output channel, and its two right channels
are summed and sent to the right channel.
Four-channel input applications function in Indigo mode, or on Indigo,
though there are only two physical input channels. The translation from
two to four channels occurs as follows. The application's input is
considered as two stereo pairs. The system's stereo input comes into the
application's first stereo pair, and the application's second stereo pair
contains zeros.
Digital Output in Four-Channel Mode [Toc] [Back]
The digital output on any IRIS workstation is a stereo output. In fourchannel
mode, it transmits the first stereo pair of output.
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.
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Audio CPU Usage [Toc] [Back]
As noted above, the Indigo2 and Indy no longer contain a dedicated DSP
chip. Thus, the impact of some Audio Library functions upon system
performance has changed. In particular, 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 [Toc] [Back]
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
5.1 may set the sampling rates to a fraction of the digital input rate.
You can get the best performance out of your IRIS audio processor by
following some basic audio connection and recording practices:
Digital Transfers [Toc] [Back]
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.
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Equipment and Connections [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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.
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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 professionaltype
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 groundreference
problems do not typically apply to serial digital audio
connections.
Other Cautions and Advice [Toc] [Back]
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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