Optimizing Audio Levels

The MVC 121 Plus uses fixed point DSP technology, processing data using a combination of 32- and 64-bit algorithms. The ADCs (analog to digital converters) and DACs (digital to analog converters) sample at 48 kHz, with 24-bit resolution.

With fixed point DSP, it is possible to clip the audio signal after the ADC and before the DAC (within the DSP audio signal chain). It is important that the audio signal is not clipped at the input ADC, or at any point within the DSP. Clipping gives audibly undesirable results, and when the audio is clipped at the input or within the DSP, there is no remedy further down the signal chain. If audio clipping occurs within the DSP (before the output DAC) and it is not a result of clipping at the input ADC, the only way to address it is by lowering the signal within the DSP signal chain at the mix-point gain control.

The meters in DSP Configurator indicate clipping at a user-definable point, with the default setting at -1 dB. This means that the meter indicates clipping when it reaches -1 dBFS, which is 1 dB below actual clipping. Setting the clipping meter below actual clipping provides a safety net to let you know to pull back on input gain before clipping occurs.

About Setting Gain Structure

On the MVC 121 Plus, adjusting the level at the output volume block changes the listening level for the mix of microphone and program signals. As long as optimal levels are achieved at the line input gain stage, and levels at the mix-point do not rise level to where it clips at the output, the MVC 121 Plus gain structure will be set up for the maximum signal-to-noise ratio.

It is recommended to use a soft limit on the Mix-point gain controls that prevents the gain from rising above
0 dB, which will help to prevent clipping within the DSP.

Setting Line Input Gain

It is important to optimize the audio by setting the input level to as close to 0 dBFS (shown on the input meters) as possible. This maintains the bit resolution at 24-bit.

You can set the input gain using:

The intended input source device and typical source material
Pink noise (when source material is not available). Pink noise can be obtained either from a prerecorded track of pink noise (played through a device such as a DVD player) or a pink noise signal generator.

If you are using program material:

Set the input level so that the meters reach approximately -15 dBFS to -12 dBFS, with peaks at approximately -6 dBFS. This setting provides enough headroom to accommodate transients or unanticipated loud events in the program material in order to avoid possible clipping.

If you are using pink noise:

Pink noise should be recorded at -20 dBFS. Whichever pink noise source is being used, set the input gain in DSP Configurator so that the input meter reads -20 dBFS.
If the player has an output level setting control, set the output of the player to its maximum, or 0 dB of attenuation.
If the maximum output setting provides gain, adjust the gain slightly away from the maximum setting.
If you are using a signal generator, set it to output -10 dBu.

Setting Output Levels

In order to set up a gain structure, you may need to listen as you proceed. At some point in setting output gain structure, volume may be excessive — so either clear the room of other listeners or give fair warning.

Set mix-point #3 to 0 dB, unmuted. Set the amplifier input sensitivity to fully attenuated. Set the output volume to 100 percent.
Start playback of the program source device.
Raise the amplifier input sensitivity until the amplifier clip LED begins to light, then lower the input sensitivity slightly (approximately 3 dB). If this is an intolerable listening level for the room, set the amplifier input sensitivity lower so that the source material plays at a volume level that is reasonably loud but tolerable.
Adjust the pre-mixer gain or volume control as necessary to create a reasonable working level for continuing setup.

Setting Mic Input and Mix Levels

The default level for the microphone input is 0 dB, muted. Having the input muted before plugging in a wireless microphone and especially before turning on phantom power is recommended.

In this example, the mic/line input 1 signal is sent to output 1.

Double-click the mic mix-point for mic1/output 1 to open the Mix-point dialog box for that mix-point.
Unmute the mix-point to place the mic signal into the mix. The default level is unity gain (0 dB).
Open the Mic/Line Gain dialog box for mix input 1 (see the Mic/Line Input Gain section).
With the desired microphone attached to mic/line input 1 and the mic gain set to 0 dB (turn on phantom power if the mic requires it), unmute the channel.
While testing the mic, raise the fader level until the mic is clearly audible. The amount of gain and meter level reading will vary at this point. However, as a general guideline, you are likely to find the input gain level at approximately 40 dB to 50 dB and the meter averaging -20 dBFS to -15 dBFS.
Repeat steps 1 through 5 for additional line inputs and associated mix points.

Voice levels at microphone inputs can vary significantly. By having the meters average -20 dBFS to -15 dBFS, there is enough headroom to accommodate sudden changes to voice levels. Further adjustment may be necessary.

Adjusting Overall Gain Structure

Depending on how you have set up input and mix-point levels and the actual signals present at the input, the output volume level may clip when set to 100 percent (or less). If this occurs, you will need to overcome clipping by lowering the mix-point gain. Also, check the line input gain setting to ensure that clipping is not occurring at the input stage. Although you may prevent the output volume control from being raised to the clipping level, if clipping is observed at the output stage, it signifies that clipping is occurring before the output stage. Clipping any digital device is audible, so it will be necessary to lower gain prior to the output stage.

To prevent user controls from changing the volume to a level where clipping occurs at the amplifier, use soft limits on the output volume control.