Beginner's Guide to the Visual Audio Tool (Vat)

Listening to a session advertised in sdr

Adding vat to your path: The first thing you need to do is to make sure that the vat binary is in your path. In Unix you can check if vat is in your path, by using the which command (which vat). If this command returns a valid vat executable then vat is in your path. If it does not then you need to edit your path variable and add the directory where you have placed the vat executable to your path. In Windows, the path variable is set in the autoexec.bat file. It is generally recommended in Windows 95 or NT that the vat binary be placed in a directory called C:\MBONE and then add C:\MBONE to your path.
Launching vat from sdr: If the session was advertised as having audio then when you click on the session in sdr you should get a window that lists the various media included in the session and one or more of them should be audio. If you click on either the join or the audio button then the vat session will pop up with the parameters set appropriately for receiving the advertised session. If vat does not pop up, the first thing to check is that vat is in your path.
Rat or vat??: At this point, a window might appear asking you whether you would like to use vat or rat. Both these tools are capable of receiving audio but they have slightly different properties. The rat tool allows you to send the audio using a redundant encoding. The vat tool does not understand this redundant encoding. Unless the session announcement specifically stated that the session is using rat, I would recommend choosing vat.
Getting rid of the session information sub-window: If you want to get rid of the sdr window containing the session information, simply click on dismiss.

Launching vat from the command line

You can launch vat from the command line in Unix and in Windows 95/NT. In windows you will need to open a DOS window and issue the command from there. In Unix you will need to issue the command in an X-terminal.

Launching a multicast session: There are several things you need to know about the session you are trying to join in order to launch vat from the command line. You will need to know the multicast address and port that you wish to send and receive vat transmissions on. The multicast address will normally look like an IP address but the first number will be probably be 224 (a class D address). The port will normally be a 4-5 digit number. You will also need to specify a time-to-live (ttl) for the session. The ttl corresponds to how many hops through the network the packets in the session can make. A ttl of 1 will keep the packets on the local subnet and a ttl of 127 will allow the packets to go throughout the world (something between these two values is probably what you want). The command to start vat is then
vat -r -t ttl multicastaddr/port
so an example command might be
vat -r -t 16 224.35.36.37/4567
The above command would launch vat on the multicast address 224.35.36.37 and port 4567 (the ones used for the audio medium) with a ttl of 16. The ttl of 16 is normally used to keep a multicast confined to a site. The -r tells vat to use RTPv2 to transmit packets which is what most people will set vat to use and the only way that you will interoperate is if you also launch using RTPv2.

Launching a unicast session
When launching vat as a unicast session you need to specify the other host participating in the unicast session on the command line. There can be no more than two participating machines. The port specified is the same on both machines and ttl is not needed since it only applies to multicast. If you are trying to start a unicast between tweety.lbl.gov and george.mit.edu on port 12345 you would use the following commands.

on tweety.lbl.gov
vat -r george.mit.edu/12345
and on george.mit.edu the command would be
vat -r tweety.lbl.gov/12345
Launching a unicast session: When launching vat as a unicast session you need to specify the other host participating in the unicast session on the command line. There can be no more than two participating machines. The port specified is the same on both machines and ttl is not needed since it only applies to multicast. If you are trying to start a unicast between tweety.lbl.gov and george.mit.edu on port 12345 you would use the following commands.

on tweety.lbl.gov
vat -r george.mit.edu/12345
and on george.mit.edu the command would be
vat -r tweety.lbl.gov/12345

Navigating the vat window

Hooking up your audio equipment: If you are intending to use an internal speaker and mike or already attached audio equipment then go on to the next section. In order for your audio to work, you need to have audio capabilities on the computer. On a PC this means you need at least a Soundblaster compatible card or better. On a workstation, audio capabilities usually come built-in and there are jacks for plugging in your equipment in the back. The jacks will normally be mini stereo, like the headphone jack on a walkman. These jacks will be labeled with symbols or words. If they are symbols, the mic will probably be a circle with a vertical line next to it, the headphone jack will have a picture of headphones, line-in in will have a circle with an arrow pointing into it and line-out will have a circle with an arrow pointing out.
Testing your audio equipment
The first thing you need to do is test your audio equipment to make sure that you can hear audio and transmit audio. The first thing you need to do is to click on the "Menu" button at the bottom of the vat window. This should pop up another window that contains all the vat controls.

Audio output device
You want to look at the "Pri/Test" or "Audio Tests" section of the menu. Click on the option for -6dBm tone. If you do not hear a tone out your speakers, the first thing to check is which output vat is using. The current output is indicated by the symbol shown in the box directly under the word listen near the upper right corner of the vat window. The symbol should correspond to your what jack is your current output device. The speaker symbol indicates an internal speaker, the headphone symbol indicates a headphone or speakers plugged into the headphone jack, and the line-out symbol corresponds to plugging into line-out. To change output devices, click on the symbol until it shows the desired symbol. If you still do not hear a tone, try increasing the gain on the output slider of vat and check the connection, power and settings of the audio output device you are using. If you are still not getting audio out, try turning on the "Keep Audio" button in the lower right corner of the vat window. If you still aren't getting audio, try plugging your output device into other jacks. Once you have finished testing your audio output device, put the audio test setting back to "none". NOTE: Be sure that the tone you hear actually comes from the output device you want to use ( and not from the internal speaker ! )
Audio Input device
The first thing to check is that the symbol under the word "talk" in the upper right hand corner of the main vat window corresponds to the input your microphone is plugged into. The choices should be microphone and line-in. Now go to the "Pri/Test" or "Audio Tests" section of the vat menu. Click on the option for loopback. Now tap on your microphone and see if you hear it out the speakers. You may need to adjust the gain sliders on the main vat window. In loopback mode vat will feed whatever goes in the microphone directly to the output device. NOTE: If you have a hardware echo canceller, this test may not work since the echo canceller will keep the audio from playing out. You can also test your microphone by turning on the talk button in the upper right corner of the main vat window and talking into the microphone. If the level meter next to the slider moves up and down with your speech then you are transmitting. Keep in mind that when your talk button is on, you are transmitting to the session!

Gain settings
The pair of vertical sliders on the right side of the main vat window allow you to adjust the microphone and speaker gain (volume). The slider to the left is the microphone gain and the one on the right is the speaker gain. You want to adjust your microphone gain until it usually stays out of the red when you speak.

Transmitting audio
To transmit audio in a push-to-talk mode, simply move the mouse into the vat main window and hold down the right mouse button. Any time you are holding down the right mouse button in the vat window your mic is on and you can talk. When you release the button your microphone will turn off. For information on how to use hands-free modes of talking, refer to the Vat Advanced Techniques
Testing your audio equipment: The first thing you need to do is test your audio equipment to make sure that you can hear audio and transmit audio. The first thing you need to do is to click on the "Menu" button at the bottom of the vat window. This should pop up another window that contains all the vat controls.

Audio output device
You want to look at the "Pri/Test" or "Audio Tests" section of the menu. Click on the option for -6dBm tone. If you do not hear a tone out your speakers, the first thing to check is which output vat is using. The current output is indicated by the symbol shown in the box directly under the word listen near the upper right corner of the vat window. The symbol should correspond to your what jack is your current output device. The speaker symbol indicates an internal speaker, the headphone symbol indicates a headphone or speakers plugged into the headphone jack, and the line-out symbol corresponds to plugging into line-out. To change output devices, click on the symbol until it shows the desired symbol. If you still do not hear a tone, try increasing the gain on the output slider of vat and check the connection, power and settings of the audio output device you are using. If you are still not getting audio out, try turning on the "Keep Audio" button in the lower right corner of the vat window. If you still aren't getting audio, try plugging your output device into other jacks. Once you have finished testing your audio output device, put the audio test setting back to "none". NOTE: Be sure that the tone you hear actually comes from the output device you want to use ( and not from the internal speaker ! )
Audio Input device
The first thing to check is that the symbol under the word "talk" in the upper right hand corner of the main vat window corresponds to the input your microphone is plugged into. The choices should be microphone and line-in. Now go to the "Pri/Test" or "Audio Tests" section of the vat menu. Click on the option for loopback. Now tap on your microphone and see if you hear it out the speakers. You may need to adjust the gain sliders on the main vat window. In loopback mode vat will feed whatever goes in the microphone directly to the output device. NOTE: If you have a hardware echo canceller, this test may not work since the echo canceller will keep the audio from playing out. You can also test your microphone by turning on the talk button in the upper right corner of the main vat window and talking into the microphone. If the level meter next to the slider moves up and down with your speech then you are transmitting. Keep in mind that when your talk button is on, you are transmitting to the session!
Gain settings: The pair of vertical sliders on the right side of the main vat window allow you to adjust the microphone and speaker gain (volume). The slider to the left is the microphone gain and the one on the right is the speaker gain. You want to adjust your microphone gain until it usually stays out of the red when you speak.
Transmitting audio: To transmit audio in a push-to-talk mode, simply move the mouse into the vat main window and hold down the right mouse button. Any time you are holding down the right mouse button in the vat window your mic is on and you can talk. When you release the button your microphone will turn off. For information on how to use hands-free modes of talking, refer to the Vat Advanced Techniques

Debugging vat

The output meter is moving but no sound is coming out: There are several things that could be wrong here. Try going back through the "Audio output device" section of "Testing your audio equipment" above.
When I speak into the mike the right hand meter doesn't change: Try increasing the microphone gain (right hand slider in the main vat window) or moving the microphone closer to your mouth. If you still do not register, make sure that the test mode has been returned to "none" and that the talk button on the main vat window is red.

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