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Setting Audio Levels

Setting IRLP Audio Levels
Without a Service Monitor
"Poor Man's Method"


PURPOSE:

To help node owners get the audio levels of their nodes set according to a standard, eliminating or greatly reducing the audio level variations from node to node; thus, providing a more homogonous sounding network.

I'm putting this page together to help provide information about how to properly set the levels on an IRLP node until the "official" standards are accepted and published. The rationale behind these standards is to get node owners to send a standardized level to the IRLP network. Once all node owners have performed these procedures, all nodes should have the same audio level and variances between nodes will be nil to nonexistent.

While it is recommended that a service monitor be used when making these adjustments, an AC volt meter or even a good ear will suffice.

Once the procedures on this page have been completed, you can be confident that your audio levels are correct or are very close. If someone who is connected to your node from another node sounds too loud or too soft, OR they say you sound too loud or too soft, it is most likely due to improper settings on their node, not yours. Do not make any adjustments to your node, get them to adjust their levels. Better yet, get them to run these procedures, so that their system will be in balance with the rest of the world.

Items required to get your node levels set:

  1. Radio with DTMF pad;
  2. Another radio or scanner to listen to the various signals;
  3. DVM or a good ear.
  4. audiotest.wav file (a standard file found in all node software distributions)

Use of the audiotest.wav file is imperative as it provides the standard audio levels to which all nodes should be adjusted. If you do not use this file to make your initial adjustments, you might as well throw everything else out, because you will only be guessing. If you would like a better understanding of what goes on inside the Sound Blaster card, please check the Sound Blaster links under the Tech Stuff heading on this web site.

The Preliminaries

This procedure assumes that you will be working from the node console where there is a monitor and keyboard directly connected to the node computer. If you are logging in from a remote computer, you can open two windows for the two screen mentioned below.

We will be utilizing two screens, one where you will be logged in as repeater and the other where you will be logged in as root. The reasons for this is that user repeater is the only user who is allowed to perform node operations. User root is the only user who is allowed to set and save aumix settings.

Press Alt-F1 on the keyboard and log in as repeater.

Press Alt-F2 on the keyboard and log in as root.

Setting Node to Radio Levels

This part of the audio setup procedure makes sure that the audio that comes out of your Sound Blaster card is at the proper level for your radio or repeater. Basically, in this procedure, we are adjusting the level coming from the Digital to Analog Converter (DAC).

  1. Press Alt-F2 to switch to the "root" screen and start the aumix utility. Move the Pcm control setting to it's maximum setting (as high as it will go) and set the Vol, Bass and Trebl controls to their mid positions. Move all other controls to their minimum setting.

    I'm not sure about the IGain and OGain settings. My Sound card does not use these so I just leave them set to their minimum settings. If my diagrams in the Tech Stuff section are correct, these controls may help if you do not have enough range in VOL (OGain) or Line (IGain) settings.


    Figure 1
    Initial aumix settings
  2. Press Alt-F1 to return to the "repeater" window and play the audiotest.wav file using the following command:

    bash-2.05a$ $SCRIPT/audiotest

    NOTE: Your prompt (bash-2.05a$ in the above example) may be different.

  3. Switch back to the window where aumix is running (press Alt-F2), adjust the Vol control until the audio being transmitted by the node radio or repeater sounds to be the same level as the audio being transmitted by someone talking on their radio. You should use the same radio to do the listening, with the volume turned down and held close to your ear. Have someone else transmit on the same frequency as the node radio or repeater so that you can make the comparison. You want to be comparing the level of VE7LTDs voice with the level of your friends voice.
    N.B.: NEVER ADJUST THE Pcm CONTROL!!!
    IT MUST ALWAYS BE SET AT MAXIMUM.
  4. Repeat steps 2 and 3 until you are sure of your settings.

    Figure 2
    Node 4494 aumix TX Level Setting
  5. Return to the aumix window by pressing Alt-F2, then press the "S" key to save your settings.

This concludes the IRLP node TX level setting procedure. Now that this is done, the audio being transmitted by your node should be at or close to the proper level.

Setting the Radio to Node Levels

What we are striving for is the node to have input to output audio levels with a ratio of 1:1. Since we have set the output to have a proper level using a known reference, we must now set the input so that it too will produce the proper output level. Basically, we are adjusting the signal levels going to the Analog to Digital Converter (ADC).

This will require connecting to an echo reflector or using a local echo script. What we will be doing in this portion of the setup procedure is making sure that the audio level from your radio or repeater is adjusted so that the audio being sent to the Internet is at the proper level.

This procedure also assumes that you are using the LINE IN connector on your Sound Blaster card. In general, it is not recommended that the MIC IN be used because of several issues: noise, distortion and AGC. (Note: The commercial OSS drivers may give you control over the AGC.)

  1. If you are continuing from the above procedure, proceed to the next step. Otherwise, press Alt-F2, login as root and run the aumix program. The levels that were set in the above procedure should be where you left them
  2. Set the aumix Line control to it's mid position as a starting point.

    Figure 3
    Initial aumix Setting For RX Level Adjustment
  3. Press Alt-F1 to switch to the "repeater" window and connect to the Echo Reflector (9990) by entering on the console keyboard:
    bash-2.05a$ decode 9990
    Then switch back to the aumix window by pressing Alt-F2.
  4. While listening to the input frequency of the node radio or repeater, have someone transmit a DTMF "D" on the input frequency of your node for a few seconds, then unkey. After a few seconds delay, the Echo Reflector will send the DTMF "D" back to your node which will then send it out the node radio or repeater. Note the level of audio coming back as compared to that which was originally sent. An AC voltmeter comes in real handy for doing this; however, a good ear will get you close. (The reason for using the DTMF "D" key is that the IRLP node will pass the tones for this key, it is not muted like the other DTMF keys.)
  5. Adjust the aumix Line control as indicated below:
    • If the audio level was too high, move the aumix Line control to a lower setting.
    • If the audio level was too low, move the aumix Line control to a higher setting.
    N.B.: DO NOT adjust the Vol or Pcm controls as these were set previously!!!
  6. Repeat steps 4 and 5 until the audio coming back from the Echo Reflector is at the same level as that from the originating radio.

    Figure 4
    Node 4494 Final aumix Settings
  7. Press the "S" key on the aumix widow to save the settings.

That's all there is to getting your levels set properly. This procedure eliminates the question of why some settings seem to affect other settings. It helps to ensure that the audio level you send out to the world via IRLP is correct and it ensures that (assuming everyone else is sending the proper levels) the audio being transmitted by your node is proper.

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Copyright © 2004
Permission to copy and use portions or all of the material contained on this site
is granted providing proper credit is given.
Randy Hammock KC6HUR
SFI Communications