Optimizing Audio Levels

The MPX Plus 866 A uses floating 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 floating point DSP, it is difficult 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. Clipping gives audibly undesirable results and when the audio is clipped at the input, there is no remedy further down the signal chain. If audio clipping occurs at the output DAC and is not a result of clipping at the input ADC, there are ways that clipping can be addressed in the DSP audio signal chain.

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.

To increase or decrease the clipping meter indicator:

1. From the Tools menu, select Options. The Options dialog box opens.

2. In the Processor Defaults section, click the button beside the Defaults field  to expand the list of settings.

3. Click the button beside the Meter Clipping field.

4. To the right of the Clip Threshold field, enter a value between 0 dB and -20 dB. When the meter clip indicator is set to 0 dB, clipping is indicated when actual clipping occurs.

Meters within DSP Configurator are peak-type meters, referenced to full scale, or 0 dBFS. For the
MPX Plus 866 A, 0 dBFS corresponds to +21 dBu, which is the maximum output level of the device. Input meters are post-ADC, while output meters are pre-DAC.

About Setting Gain Structure

There are three approaches that the system designer can follow when setting up gain structure, depending on where output volume will ultimately be controlled. The output volume of the MPX Plus 866 A can be controlled by either of the two output gain blocks:

• Volume - Adjusting  the level at the output volume block will change the listening level for the mix of microphone and program signals.

• Post-matrix gain - Adjusting the level at the post-matrix gain block will change the listening level of the program material independent of the microphone levels.

• Microphone levels - When adjusting mic levels independent of program material, mic levels should be adjusted at the mix-point. This can be done by adjusting mic levels at individual mix-points, or by configuring a group master to control multiple mix-points.

In the instructions that follow, the setup is described for either output volume control point when appropriate.

Setting Line Input Gain

Floating point DSP technology is internally more flexible than fixed point. However, an ADC and DAC always run as fixed point, so it is important to optimize the audio by setting the input level to as close to 0 dBFS as possible. This maintains the bit resolution at 24-bit. Within the DSP, it is not critical to maintain audio levels at 0 dBFS in order to secure 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.

• Extron building blocks

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 to include signal processing, 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. Keep in mind that after inserting processors in the output signal chain, output gain structure may need to be adjusted.

1. Make an audio tie from the source to the speakers that will carry program material in the room that you are setting up. Set the amplifier input sensitivity to fully attenuated. Set the post-matrix gain to 0 dB and the output volume to 100 percent.

2. Start playback of the program source device.

3. 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.

4. Adjust the post-matrix 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 microphone and especially before turning on phantom power is recommended.

A convenient way to set mic input gain is to use the building blocks feature. This will give you a gain setting for a specific microphone type, brand and model, designed to bring the mic to a nominal level, turning on phantom power if the mic requires it. Additionally, a building block can load other processing options such as bass and treble tone control and compression for level normalization and system protection. See the Building Blocks section for more information.

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

1. Double-click the mic mix-point for mic 1/output 1 to open the Mix-point dialog box for that mix-point.

2. Unmute the mix-point to place the mic signal into the mix. The default level is unity gain (0 dB).

3. Open the Mic/Line Gain dialog box for mic input 1 (see the Mic/Line Input Gain section).

4. 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.

5. 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.

6. Repeat steps 1 through 5 for additional mic 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 Pre-matrix Trim

After you set the input gain for all line level sources, add any processors that you want to use into the input signal chain (see the Inserting and Deleting a Processor section). You can use the pre-matrix trim control to compensate for any level changes that result from added processing.

• Adding a compressor generally reduces the signal level.

• A filter may boost or cut the overall signal level. If you make changes to filter settings after you set the dynamic processors, recheck the levels in the dynamics processors to ensure that they are still valid.

To start adjusting the pre-matrix trim:

1. Open the line input Gain dialog box (see the Line Input Gain section), the output Volume dialog box (see the Output Volume section), and the pre-matrix Trim dialog box (see the Pre-matrix Trim section).

2. With program material (or pink noise) present at the input, adjust the pre-matrix trim so that the meter level in the input Gain dialog box matches the meter level in the output Volume dialog box. This maintains the audio at an optimal level in the input signal chain.

3. To make further adjustments, after you set up the microphone input gain and mix-point levels, output processing, and post-matrix gain levels:

4. If you need more headroom to prevent clipping at the outputs, you can return to the pre-matrix trim controls and lower each one by (for instance) 5 dB or 10 dB.

5. You may also find that further minor adjustments to the pre-matrix trim controls will help balance the perceived audio levels of the different inputs.

Inserting Output Channel Processing

Add any processors that you want to use in the output signal chain. A convenient way to add output channel processing is to use the building blocks feature. This will add room EQ for a number of commonly used Extron speakers, plus a limiter for system protection. See the Building Blocks section for more information.

• A filter may boost or cut the overall signal level.

• Adding a compressor generally reduces the signal level. However, since a limiter is the most likely choice for output processing and only reduces the signal to prevent overload, you do not need to consider the reduction of signal level.

• Loudness boosts the overall signal level, but mostly at lower volume settings (see the Loudness Calibration section).

After you add processors to the output signal chain, the output volume level may clip when set to 100 percent (or less). Floating point DSP allows you to overcome clipping by lowering either the post-matrix gain or the output volume setting.

Unless you prevent user controls from changing the volume setting to 100 percent (or to a position where clipping occurs), it is best to:

• Adjust the pre-matrix trim or post-matrix gain control to set levels.

• Prevent any possible clipping as described in the Adjusting Post-matrix Gain subsection below.

To prevent user controls from changing the volume to a level where clipping occurs, use soft limits on a group master (see the Group Masters section).

Alternatively, you can use the post-matrix gain controls to adjust the output volume. Post-matrix gain controls provide 12 dB of gain, so it is advised that you use a group master with soft limits to control the level, with an upper limit of 0 dB or less. Mix levels will also contribute to possible clipping at the outputs and may need to be lowered to maintain the balance between program material (line outputs) and voice.

Adjusting Post-matrix Gain

At this point, setting up gain structure may become a bit of a balancing act. You may need to go back and forth to set levels optimally.

For example, you can control the output level and maintain the level below clipping by using a compressor or limiter in the output DYN (dynamics) block. However, adjusting the post-matrix gain affects how the compressor or limiter works.

1. Open the output Volume dialog box (see the Output Volume section) and the post-matrix Gain dialog box (see the Post-matrix Gain and Mono Sum section).

2. Set the output volume to 100 percent (mute if necessary).

3. With program material (or pink noise) present at the input, adjust the post-matrix gain until the meter level in the output Volume dialog box is below clipping (or, ideally, matches the level in the input gain meter). This maintains the audio at an optimal level in the output signal chain and prevents clipping at the output.

When using the post-matrix gain for output volume control, the procedure may be reversed.

1. Set the post-matrix gain to 0 dB (mute if necessary).

2. With program material (or pink noise) present at the input, adjust the output volume until the optimal level in the output Volume dialog box is below clipping (or, ideally, matches the level in the input gain meter).