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Disable Input Monitoring on Your Sound Card

  • By Mike Phillips
  • Thursday January 24, 2013
Filed Under:
Audio Broadcasting

Mix-minus is a relatively simple concept that can get very complicated very quickly. Since most Internet broadcasters use Skype for co-hosts and guest interviews, let’s use Skype as an example of an application for mix-minus. This article illustrates an often-overlooked problem that can give you a real headache when you’re setting up your studio.

The sound you hear from your mixer, whether you’re listening on speakers or headphones, is called the “mix”. When you send the main output of your mixer to a content delivery system, such as Mixlr or Ustream, you’re sending your mix. You have to send audio to the Skype caller so that he can hear what’s going on. The best solution is to use a “mix-minus” feed, which is normally sent from an auxiliary output on the mixer to the line input jack on the Skype computer. The Skype caller does not need to hear himself. In fact, if you send the audio from the Skype computer through the mixer and back to the Skype computer, you will create an echo or feedback. (Skype does a good job of canceling echoes, but it does not work nearly as well as a mix-minus feed.) Therefore, the mix-minus feed includes the “mix” audio from the mixer “minus” the audio from the Skype computer. Now you’re an expert on mix-minus.

Sometimes things are not so simple. It’s possible to properly set up a mix-minus feed and still have problems. There is a gremlin that creeps into some systems. Most sound cards, whether on board, add-on, or USB, include the ability to monitor the input audio being fed to the input jack. This feature is by design. If you are using the on board sound card, as an example, the easiest way to tell if your sound card has input monitoring enabled is to feed continuous audio into the input jack. Grab an MP3 player and set it to play music. Connect a pair of headphones to the headphone jack to make sure you can hear music from the player. Now, disconnect the headphones, and connect the headphone jack of the MP3 player to the line input connector on the sound card using a 1/8” to 1/8” stereo (TRS) cable. Connect the headphones to the speaker jack of the sound card. Do you hear the music that you are sending to the input jack from the MP3 player? If so, input monitoring is enabled.

The audio feed from the Skype computer should consist only of the audio from the Skype caller. When you bring up the fader on your mixer for the Skype computer, the only audio you want to hear is the audio you’re receiving from Skype. If input monitoring is enabled, the audio feed will include the audio you’re sending to the card. The audio looping through the sound card and then being mixed back in with itself can grunge up the sound, particularly if the looped audio is out of phase with the main mix. If you increase the volume of your Skype caller and notice that your mic level goes up at the same time, making it hard to balance your voice with the caller’s voice, you may have an input monitoring issue. In some configurations, the looped audio can cause an echo. The character and duration of the echo vary based on your configuration, but it will be obvious.

Unfortunately, there is not a single solution for every sound card to disable input monitoring. The feature is usually controlled by the installed drivers. One trick I learned the hard way is that if you build your computers from scratch and like to install as few drivers as possible, you may not have the ability to turn off input monitoring. For example, Realtek sound cards, which are used on millions of motherboards, usually have two driver installation packages available for downloading and installing. One package is much smaller than the other and is often labeled “drivers only”. Unfortunately, that smaller package does not allow you to disable input monitoring. Install the larger package, which will provide you with a control panel that gives you access to several features that are not otherwise available. (Many thanks to Vance Willis for discovering the differences between the capabilities of these installation packages.) With a USB sound card, such as the Roland UA-1G or the Behringer UCA202, there is a DIP switch that controls input monitoring. If you’re using Windows 7, go to the tab in the Sound control panel, click on the input device, click on Properties, click on the Listen tab. Make sure that “Listen to this device” is NOT checked.

Take a few moments to do a simple test to see if you have input monitoring enabled on your sound card. If you do, and if you’re using that computer for Skype calls, you’ll be surprised at how much additional control you’ll have over your levels when you disable it.

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Mike Phillips is formerly a radio broadcast engineer, as well as a disk jockey, sales manager, and general manager. His technical experiences include building and maintaining radio stations and transmitter sites. A production console he built for a radio station in the early eighties is still in use on a daily basis. He also has been an electrical design engineer for a manufacturing company developing analog and digital industrial transducers. His undergraduate degree is in Electrical Engineering.

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