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audio(1)							      audio(1)

NAME    [Toc]    [Back]

     audio - Introduction to audio facilities

DESCRIPTION    [Toc]    [Back]

     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.

     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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audio(1)							      audio(1)

     aifc2aiff(1)	 converts an AIFF-C file to an AIFF file

     aifccompress(1)	 converts an AIFF or AIFF-C file to a compressed

     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

	  displays status of all open audio ports on system

	  graphically displays status of audio ports

     The audio processor provides many features	to support a variety of	audio

     The follows is a overview of the features of the audio processor with an
     explanation of any	differences between audio systems on different

   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
	  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:

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audio(1)							      audio(1)

	  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:
	  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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audio(1)							      audio(1)

     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
	  Full-scale amplitude:	6.5 dBV	[Onyx2]; 6 Vpp [Indigo]; 4.7 Vpp

     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

									Page 4

audio(1)							      audio(1)

     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

     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

									Page 5

audio(1)							      audio(1)

	  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 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

	  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

									Page 6

audio(1)							      audio(1)

     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.

     The following information will allow you to troubleshoot audio problems
     and also explain some behavior you	may observe when using the audio

									Page 7

audio(1)							      audio(1)

   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

   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

									Page 8

audio(1)							      audio(1)

   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

	  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

   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.

									Page 9

audio(1)							      audio(1)

     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

   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.

								       PPPPaaaaggggeeee 11110000
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