Calibrating Your Audio

Audio Levels Calibration
One of the unsettling aspects of controlling audio in a VTC is the fact that the engineer at one site can neither hear the sound nor see the audio levels at the other site. This inability to compare audio levels between sites often leads to situations where one site believes they are sending the audio signal at an appropriate level, yet the other site claims the level is either too high, or to low. The Audio Calibration tool allows both sites to adjust their input levels so that the meter readings are calibrated to the same level between the two sites. This provides a degree of certainty that both sites are receiving audio at the same level it is being sent. To calibrate audio levels, both sites must first enable audio processing. Open the Audio Calibration tool and follow these steps:

• Decide and select your MODE by pressing the appropriate button.
  • Tx (Transmit) mode means you are the site initiating the test.
  • Rx (Receive) mode means you are receiving the test from the other site.

• Press the Tx button to enable Transmit mode.
• Press the TONE button to transmit a 1kHz sine wave tone to the remote site at -12dBfs.
• The meters on the CODEC OUT channels in the Mixer will illuminate the first yellow LED (meters turn yellow at -12dBfs).
• Once the Receive site has finished adjusting their input levels, click the TONE button again to stop transmitting the sound.

Audio Calibration in Transmit (Tx) mode TONE button is pressed

CODEC OUT meters just turn yellow

• Press the Rx button to enable Receive mode.
• Do not press the TONE button.
• When you begin receiving the tone from the Transmit site, adjust the input trims on your hardware audio interface until the LED meters on channels 3 & 4 just trigger the first yellow LED (-12dBfs).
  • You will also hear a small amount of the tone in your loudspeakers to confirm that it is present.

Audio Calibration in Receive (Rx) mode TONE button is not pressed

Adjust input trims on your interface until channel 3 & 4 meters just turn yellow

• Once the levels are set in one direction, reverse the audio calibration process so the meters are calibrated in both directions.

Calculating Round-Trip Latency
In order for the 2-Channel Ducker to work properly, ECHODamp needs to know the entire round-trip latency of the VTC, including all audio and network components. The Latency Detector sends a brief—and loud—beep through the entire system and calculates the amount of time it takes for that beep to be transmitted and returned through all the componentry and network. For this tool to work properly, both sites must place a microphone approximately six inches (15cm) directly in front of their left (Ch. 1) loudspeaker (see diagram).

Once microphones and loudspeakers are properly positioned, enable audio processing in ECHODamp at both sites, and follow these steps:

Note: To prevent feedback, the microphone channels on the Mixer are muted during this test. Thus, you will not be able to communicate through the audio channels of the VTC. Agree on your test procedure before you begin the test and/or arrange for some sort of "back-channel" communication such as a telephone or a chat client. Once the test is complete, the audio channels on the Mixer will be restored to the state they were in when you began the test.

• Decide and select your MODE by pressing the appropriate button.
  • Tx (Transmit) mode means you are the site initiating the test.
  • Rx (Receive) mode means you are receiving the test from the other site.

• Select a microphone INPUT to use for the test.
• Press the Tx button to enable Transmit mode (you must select a microphone INPUT before pressing this button).
• Press the ARM TIMER button to arm the Latency Detector timer (button illuminates yellow and button text changes to ARMED).
• Press the BEEP button.
  • WARNING: the BEEP button emits a loud 1kHz tone from the left loudspeaker. Please use caution when pressing this button to prevent hearing damage to those in proximity to the loudspeaker.
  • A brief 1kHz tone will be emitted by the left loudspeaker, picked up by the microphone in front of the speaker, and transmitted to the Receive site.
  • The Local Beep LED will flash to indicate that the beep was detected and that the timer has started (ARM TIMER button illuminates green and button text changes to TIMING).
  • The tone will return from the Receive site in the left loudspeaker where it will, again, be picked up by the microphone in front of the speaker, stopping the timer.
  • The Remote Beep LED will flash to indicate that the beep was detected and that the timer has stopped and calculated the complete round-trip latency (ARM TIMER button returns to its original state).
       • If either the Local Beep or Remote Beep LED fails to flash, adjust the SENSITIVITY dial upward until both beeps are detected.
       • You may need to temporarily boost the output loudness of the loudspeakers at one, or both, sites in order to ensure the accuracy of the test.
  • The round-trip latency will be displayed in milliseconds (ms) in the Elapsed Time display.
• To compensate for the fact that some VTC systems experience a latency drift over the course of a session, you may want to subtract a bit of time with the LATENCY OFFSET knob. Most people will find an offset of about 50ms will be sufficient. If the latency in a VTC increases by a small amount during a session, it is usually not a problem for the echo. However, if it decreases during a session, your sound can start echoing back to you before the 2-Channel Ducker has had a chance to work.
• The Elapsed Time minus any LATENCY OFFSET will be reported to the Sidechain Delay in the main Mixer.

Ready to calculate round-trip latency Input microphone selected Transmit Mode selected Timer Armed		After pressing the BEEP button Timer button indicates "Timing" Beep LED indicators flash with each received beep

After the Remote Beep is received, the timer stops. The round-trip latency is displayed in milliseconds in the Elapsed Time window. In this example, 688ms of Elapsed Time minus 50ms of LATENCY OFFSET is automatically reported to the Sidechain Delay in the main Mixer

• Select a microphone INPUT to use for the test.
• Press the Rx button to enable Receive mode (you must select a microphone INPUT before pressing this button).
• When the Transmit site begins the test, you will hear a loud beep come from your left loudspeaker. It will be picked up by the microphone in front of the speaker and sent back to the Transmit site.
• Once the test is complete, press the Rx button again, to turn off Receive mode, and prevent any audio loopback.
• If desired, you can do the test in the reverse direction. However, since the entire round-trip path was included in the test, it will be considerably easier to just get the latency calculation from the Transmit site and enter that value in the Sidechain Delay (along with any desired LATENCY OFFSET) in the main Mixer.
• By default, the SIDECHAIN DELAY knob moves in increments of 100ms. To move the knob by 1ms increments, hold down the shift key while moving the knob.

• When these two audio calibration tasks have been completed, you can close the Calibration Tools window, because you are now ready to use ECHODamp to mix your audio and control your echo!

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