vat - remote audio conferencing tool

vat [ -aAcdEjJklLMnnsSv ] [ -F device ] [ -f audiofmt ]

[ -g geometry ] [ -N sessionname ] [ -n ] [ -C conferencename ]

[ -I channel ] -K key ] [ -P priority ] [ -R ] [ -t ttl ]

[ -U socket ] [ -u script ] [ -X resource=value ] dest/port/[/ttl/format]

Vat allows users to conduct host-to-host or multihost audio teleconferences over an internet (multihost conferences require that the kernel support IP multicast). On most systems, no hardware other than a microphone is required - sound I/O is via the built-in audio hardware. On DEC systems, an AudioFile server must be running.

The conference is carried out on the address specified by dest. Point-to-point conferences are initiated by supplying a standard IP address, while multiparty conferences use a Class D group address. Port specifies the UDP ports to use, and ttl specifies the IP time-to-live (see below). Port specifies two consecutive ports, one for data and one for control. The data port is set to the greatest even integer less than or equal to port. The control port is one greater than the data port.

(Note that all the parameters set by the following flags can also be controlled by X resources (which all have `reasonable' defaults) so one should not need to give vat any flags in the usual case. The flags only exist to temporarily override some resource.)

-a

Enable automatic gain control on the output (speaker).

-A

Enable automatic gain control on the input (mike).

-c

Start up in `conference mode' (see description below). (This flag is the opposite of -l.)

-C

Use conference as the title of this vat window. If no -C flag is given, the destination address and port are used as the window title.

-d

Start up with the VU meters disabled (this can be changed using the `Disable Meters' checkbox on the auxiliary controls panel).

-E

Include a checkbox in the auxiliary controls panel for specifying that echo cancellation is being performed externally (i.e., in hardware). This option can also be effected by setting the X resource Vat.externalEchoCancel to ``true''.

-f

Sets the output audio packet format to fmt. (Note that it not necessary to set an input format since vat will accept any packet format it knows about.) Currently available audio formats are:

	pcm	78Kb/s 8-bit mu-law encoded 8KHz PCM (20ms frames)
	pcm2	71Kb/s 8-bit mu-law encoded 8KHz PCM (40ms frames)
	pcm4	68Kb/s 8-bit mu-law encoded 8KHz PCM (80ms frames)
	dvi	46Kb/s Intel DVI ADPCM (20ms frames)
	dvi2	39Kb/s Intel DVI ADPCM (40ms frames)
	dvi4	36Kb/s Intel DVI ADPCM (80ms frames)
	gsm	17Kb/s GSM (80ms frames)
	lpc4	9Kb/s Linear Predictive Coder (80ms frames)

(The dvi encoding was contributed by Jack Jansen of CWI. The gsm coder was contributed by Carsten Bormann of the Technische Universitaet Berlin. The lpc coder was contributed by Ron Frederick of Xerox PARC.) The default audio format can be set with the audioFormat X resource. It defaults to pcm.

-g

Override the default window geometry with geometry, which should be a standard X geometry string.

-I

Use the ``LBL Conference Bus'' to interact with other multimedia conferencing tools. The small integer channel, which must be non-zero, is used as the channel identifier for group interprocess communication on the local host. This value should be consistent across each group of applications that belong to a single conference, and should be unique across conferences. The session directory tool (sd) will allocate appropriate values. (Vic and vat currently use this mechanism to coordinate voice-activated video switching as well as other conference control abstractions.)

-j

Start up with audio output to the external audio jack. This flag can be defaulted by setting the X resource Vat.outputPort to Jack.

-J

Start up with audio output muted.

-k

Keep all sites in the `Conference Hosts' panel. Normally sites that exit are deleted from the panel. With -k, sites that exit are grayed-out but not deleted.

-K

Use key as the encryption key for this vat session.

-l

Start up in `lecture mode' (see description below). This flag can be defaulted by setting the X resource Vat.lectureMode to true.

-M

Start up with audio input unmuted.

-N

Use session, in lieu of your user name and local host, to identify you to other sites. If no -N flag is given, the X resource Vat.sessionName is used.

-n

Use a packet format that is compatible with the obsolete vat protocol, used by version 3 and earlier of vat.

-P

Use priority as this vat window's priority for obtaining the audio device. All vat windows have a priority which is typically set by the X resource Vat.defaultPriority (defaults to 100) but this can be overridden by a -P flag. If a window requests the audio (because new network data arrived or the mike has been unmuted) and the window currently holding the audio is either lower priority, the audio holder immediately gives it up. Otherwise the new window's request is ignored.

-R

Disable the mike controls in the user-interface. This prevents the user from enabling the mike and transmitting audio to the session.

-s

Start up with audio output to the internal speaker. (This flag is the opposite of -j.)

-S

Make new sites come up `suppressed' (the check box next to the sitename will be checked and you will have to click on it to hear the site speak). This flag is intended for something like meeting audiocasts where a moderator wants to have control over who is able to speak. This flag can also be set by the Vat.muteNewSites X resource.

-t

Set the multicast ttl (time-to-live) to ttl. (The ttl is ignored if the destination address is not an IP multicast address.) If no -t flag is given, the value of the X resource Vat.defaultTTL is used.

-U

Use the unix-domain stream socket specified by socket for audio I/O. Some process should bind to and listen on this socket before vat is run. The data is raw 8khz mulaw samples. If socket is a number, then AudioFile is used. The number indicates the corresponding AudioFile device.

-u

Source script, in addition to the compiled-in script, to build the user interface. You can provide addtional tcl code that is loaded at startup in $HOME/.vat.tcl.

-X

Override the X resource Vat.resource with value.

Note: In addition to invoking the ``quit'' button, typing `q', `Q', ctrl-C or ctrl-D anywhere in the window will terminate vat.

The vat window is divided into two parts: the right has controls for the local audio and the left has a status display of the remote users participating in the current conference. The audio controls consist of two sliders that control the mike and playback gain, a button to toggle output between the built in speaker and the headphone jack, buttons to enable/disable either the mike or speaker, and buttons to control acquistion of the audio hardware. Just to the left of each slider is a VU meter. A rule of thumb is to adjust the mike and speaker gain sliders so the peak readings on the meter are about 80% of full scale.

To change the audio output line (i.e., speaker, headphone, lineout, etc.) click on the speaker icon (it should change to a headphone icon). Additional clicks will round-robin among the available lines. If there is only one option, the button will be disabled. Similarly, click on the mike icon to select among the input lines. By default, vat starts with the mike muted and the speaker enabled. The mike is active when the ``talk'' button is selected, while output can be muted by de-selecting the ``listen'' button.

The Conference Hosts window lists all the remote users participating in the conference. Each user's name is displayed in a box that is highlighted whenever that user speaks and grayed-out if a `session' message from that user hasn't been received for at least 30 seconds (vat sends `session' message every so often) - this usually indicates that the site has lost connectivity or that vat has been aborted or stopped. There is a checkbox to the left of each participant name. Clicking on the box will cause audio from that participant to be discarded instead of played (for example, this might be used to suppress a site that is generating echoes).

One host can be running an arbitrary number of vat sessions (presumably with different destination addresses). However, since most workstations have only one set of audio hardware, only one of those sessions will be able to access the mike and speaker. For the most part, the vat sessions will automatically follow the action. If you select the ``talk'' button or press ``Keep Audio'' button, the audio device will be acquired by that session and the session that previously held the audio will relinquish it. Vat displays it's title bar in an oblique font when the audio is not being held.

A vat session will also acquire the audio if there is input from the network. But to prevent a background vat session from stealing the audio from the foreground session, you can toggle the ``Keep Audio'' button. When the ``Keep Audio'' button is selected, vat will reliquish the audio only if there is a user demand in another window (i.e., unmuting the mike or selecting the ``Keep Audio'' button).

Participants in a multi-site conference often want to have `side conversations' that don't bother the rest of the conference participants. Vat has some support for establishing side conversations: If you middle-click on the name of some site in the conference hosts window, a new vat window will be created that talks only to that participant (it sends unicast datagrams rather than multicast). If that other participant also middle-clicks on your site, you can have a private conversation between just your two sites using the newly created vat windows. Note: due to a `bug' in the way most systems implement multicast, if you create a new window aimed at a particular participant but they haven't created a window aimed at you, they will hear you speaking in the main conference window and may not realize that your audio is being sent only to them and not multicast. One can view this either as a feature (it provides a semi-private channel you can use to ask someone to set up a side conversation) or a bug (it often leads to strange, one-sided conversations where one side multicasts and the other doesn't).

Clicking on the ``Menu'' label at the bottom of the vat window will cause a panel of auxiliary controls to open.

The Audio Tests buttons will enable some audio test modes. These should not be selected during a conference. The loopback mike button will cause input from the mike to be sent to the local speaker/jack. This might be useful for checking levels and debugging cable problems but the 20ms delay from input to output makes talking in this mode almost impossible. The three tone buttons will generate one of three reference tones through the local speaker. Level setting should generally be done with the -6dBm tone.

The Output Mode buttons control what vat will do to avoid feedback/echo from the mike to the speaker. In mike mutes net mode, vat will mute the speaker whenever it thinks that you are talking, while in net mutes mike mode, vat will mute the mike whenever input from the network arrives. In full duplex mode, vat will assume that feedback can't happen and do nothing to avoid it. In echo cancel mode, vat will attempt to eliminate echoes by doing some fancy signal processing. (EchoCancel requires the BSD sound driver - it is disabled when running vat under Sun OS because the Sun driver does not provide any mechanism to time correlate audio output and input.) The internal speaker should only be used in `speakerphone' or `echo cancel' mode - selecting `headphone' mode for it will result in your site injecting a lot of unpleasant echoes into the conference. The headphone jack should be set to `FullDuplex' mode if you have headphones plugged into it and `MikeMutesNet' or `EchoCancel' mode if you have an external amp and speaker plugged into it.

There are two type-in boxes (see below) at the bottom of the Auxiliary Controls panel. The one labeled `Name' can be used to change the session name announced to other sites. The one labelled `Key' can be used to specify an encryption key (see next section).

(N.B.: Because of U.S. export controls, the standard distribution of vat from LBL does not support encryption. In this case, the ``Key'' type-in box will be disabled.)

Since vat conversations are typically conducted over open IP networks there is no way to prevent eavesdropping, particularly for multicast conferences. To add some measure of privacy, vat allows the audio packet streams to be DES encrypted. Presumably only sites sharing the same key will be able to decrypt and listen to the encrypted audio.

Encryption is enabled by entering an arbitrary string in the key box (this string is the previously agreed upon encryption key for the conference - note that key distribution should be done by mechanisms totally separate from vat). Encryption can be turned off by entering a null string (just a carriage return or any string starting with a blank) in the key box.

The following are the names and default values of X resources used by vat. This list is incomplete. Consult the tcl code in ui-resource.tcl from the vat source distribution for the complete set.

Vat.defaultTTL:	127
Vat.audioFormat:	pcm
Vat.lectureMode:	false
Vat.inputPort:	Mike
Vat.outputPort:	Speaker
Vat.speakerMode:	Speakerphone
Vat.jackMode:	Headphone
Vat.mikeGain:	32
Vat.speakerGain:	180
Vat.jackGain:	180
Vat.mikeAGC:	false
Vat.mikeAGCLevel:	0
Vat.speakerAGC:	false
Vat.speakerAGCLevel:	0
Vat.maxPlayout:	6
Vat.defaultPriority:	100
Vat.idleDropTime:	20
Vat.autoRaise:	true
Vat.pushToTalk:	false
Vat.keepSites:	false
Vat.key:
Vat.muteNewSites:	false
Vat.siteDropTime:	30
! fonts
Vat.titleFont:	-*-helvetica-bold-r-normal--*-140-75-75-*-*-*-*
Vat.audioFont:	-*-helvetica-medium-r-normal--*-100-75-75-*-*-*-*
Vat.helpFont:	-*-times-medium-r-normal--*-140-75-75-*-*-*-*
Vat.ctrlFont:	-*-helvetica-bold-r-normal--*-100-75-75-*-*-*-*
Vat.ctrlTitleFont:	-*-helvetica-bold-r-normal--*-120-75-75-*-*-*-*
Vat.entryFont:	-*-helvetica-medium-r-normal--*-100-75-75-*-*-*-*

Vat.maxPlayout is the maximum `play out' delay, in seconds, that can be tolerated. I.e., vat dynamically adapts to delays introduced in the network by delaying the play out of a remote site's audio packets. The range of adaptation is limited by the size of a buffer in vat and this parameter essentially sets the size of that buffer. Setting maxPlayout larger than 10 seconds will probably result poor vat and system behavior.

Vat has two different modes of adapting the playout delay, one more suitable for an interactive, multi-party discussion or conference and the other more suitable for listening to a speech or lecture. The two modes differ in how quickly they `forget' the delay vat introduces to adapt to transient network congestion: In Conference mode vat attempts to minimize the delay (since large delays make interactive conversations difficult) but this usually results in more lost packets when the delay becomes too short handle the next congestion event. In Lecture mode vat attempts to minimize lost packets by reducing delays very slowly. This results in the clearest playback but interactivity may suffer.

Conference mode is the default when vat starts up unless the -l flag is given or the X resource lectureMode is set to true. There are radio buttons in the network section of the Auxiliary Controls panel to switch between Conference and Lecture

Clicking on a name with the left mouse button will bring up a small window of identification information for that user. The window includes several of the RTP identification descriptors, the audio format in use, and the times of reception of the last data and control packets. A packet statistics window can be brought up from the ``Stats...'' pulldown menu by selecting ``RTP''. There are three columns of numbers. The last column is the aggregate statistics since vat started, while the middle column is the difference between the last update time and the current time. The first column is an smoother version of the middle column, using an exponentially weighted average with gain Vat.statTimeConst.

The statistics are updated every second or so while the window is mapped. Any of values can be plotted with a stripchart by clicking on the name of the desired statistic.

audio(4)

Van Jacobson (van@ee.lbl.gov) and Steven McCanne (mccanne@ee.lbl.gov), both of Lawrence Berkeley Laboratory, University of California, Berkeley, CA. Jack Jansen (Jack.Jansen@cwi.nl) of Stichting Mathematisch Centrum, Amsterdam, the Netherlands, contributed the Intel DVI ADPCM codec. Ron Frederick (frederic@parc.xerox.com) of Xerox PARC, Palo Alto, CA, contributed the LPC codec which is based on an implementation done by Ron Zuckerman (ronzu@isu.comm.mot.com) of Motorola which was posted to the Usenet group comp.dsp on 26 June 1992. Carsten Bormann (cabo@cs.tu-berlin.de) and Jutta Degener (jutta@cs.tu-berlin.de) of the Communications and Operating Systems Research Group (KBS) at the Technische Universitaet Berlin contributed the GSM codec. Steve Casner (casner@isi.edu) of ISI, Los Angeles, CA, and Steve Deering (deering@parc.xerox.com) of Xerox PARC have invested tremendous effort in making vat work on a scale far beyond the authors' wildest expectations and have contributed greatly to vat's development, both directly (via careful analysis of bugs and useful suggestions) and indirectly (via setting up several global conferences that severely pushed the envelope of vat's capabilities).

Speakerphone mode is difficult to get right - use a headset if you can (or run BSD instead of Sun-OS to get a kernel audio driver that can support echo cancellation). If you have to use speakerphone mode, try to position the mike as far as possible from the speaker (the speaker in a sparcstation is on bottom of the machine in the front right corner near the LED). If there's a problem with echo (i.e., you transmit whenever other people start speaking), try reducing the mike gain or mute the mike when you're not speaking.

In speakerphone mode vat assumes that if there is audio data from the net being sent to the speaker at least part of the signal from the mike is pickup from the speaker. So, unless the mike signal is `large' compared to the signal from the net, vat assumes it is echo and suppresses it. This means that if you want to interrupt someone who is talking, you may have to talk a bit louder than usual at the start (you can tell if you succeed because your site's name box will light and the speaker will mute).