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Setting Audio Levels

Setting IRLP Audio Levels
Using a Service Monitor


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 audio 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.

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. A service monitor;
  2. 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.

Figure 1
Reference levels provided by audiotest.wav
Image courtesy Lu Venci KA4EPS

The Preliminaries

This procedure assumes that you will be working from node console where there is a monitor and keyboard directly connected to the node computer.

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.

  1. Press Alt-F2 and start the aumix utility and move the Pcm control setting to it's maximum setting (as high as it will go) and set the Vol, Bass and Trebl control to their mid positions. Move all other controls to their minimum settings. (The Up and Down arrows are used to move the highlight to the desire control. The Left and Right arrows are used to move the selected slider.)

    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 2
    Initial aumix settings
  2. Press Alt-F1 to switch 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. Press Alt-F2 tio switch back to the aumix window and adjust the Vol control until the audio being transmitted by the node radio reads 3.5 KHz deviation while the tone is being sent. During the voice portion of the test audio, there should be peaks at 5 KHz. (Figure 1 shows what the wave forms should look like.) You should not see any clipping of the audio waveform. If you do see clipping, check your limiter setting. It should be noted here that the Vol adjustment occurs in 2 dB steps; therefore, you might not be able to get the level set exactly unless you have an analog pot somewhere between the Sound Blaster and your transmitter where you can perform a final tweak.
  4. Repeat steps 2 and 3 until you are sure of your settings.

    Figure 3
    Node 4494 aumix TX Level Setting
  5. 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 sent by your node is at or close to the proper level.

Setting the Radio to Node Levels

This will require connecting to an echo reflector or using one of the local echo scripts. 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 auido sent to the Internet is 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.

  1. If you are still logged in as root from the above procedure and aumix is still running, procede to the next step. Otherwise, login as root and run the aumix program. The levels that were set in the above procedure should be where you left them.

    Figure 4
    Initial aumix Setting For RX Level Adjustment
  2. Press Alt-F1 and connect to the Echo Reflector (9990).
    bash-2.05a$ decode 9990
    (NOTE: your prompt may be different than bash-2.05a$)
  3. With the service monitor still monitoring the output of your node radio or repeater, use the serivice monitor to transmit a 4.5 KHz dev test signal to the input of your node for a few seconds, then unkey. After a few seconds, the Echo Reflector will will sent the test signal back to your node which will then send it out the node radio or repeater. Note the level of deviation.
  4. Adjust the aumix Line control as indicated below:
    • If the deviation was too high, move the aumix Line control to a lower setting.
    • If the deviation 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!!!
  5. Repeat steps 3 thru 4 until the audio coming back from the Echo Reflector is being transmitted at 4.5 KHz dev by your node radio or repeater.

    Figure 5
    Node 4494 Final aumix 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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is granted providing proper credit is given.
Randy Hammock KC6HUR
SFI Communications