Optimizing Audio Levels

The MLS 608 D Series 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
MLS 608 D Series, 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

In the MLS 608 D Series, the default control point for program volume is the post-switcher gain, while the default control point for microphone volume is the pre-mixer gain.  For both program volume and mic volume, a group master control has been preconfigured to create a single point of control for the post-switcher gain points and the pre-mixer gain points, respectively. The front panel volume controls are tied to these group master controls. The front panel volume rotary encoders are linked to these preconfigured group master controls, such that adjusting the position of the front panel encoders will adjust the level of the corresponding group master.

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 without clipping. This maintains the bit resolution at 24-bit, and therefore the full dynamic range of the device. Within the DSP, it is not critical to maintain audio levels at 0 dBFS in order to secure the bit resolution at 24-bit.

A convenient way to set input gain is to use the building blocks feature. This will give you a gain setting for a particular line level device, designed to give you a nominal operating level. 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.

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 (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 is typically recorded at -20 dBFS. Whether using a pre-recorded pink noise source or signal generator, 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 (Non-amplified Outputs)

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 amplifier input sensitivity to fully attenuated. Set the post-matrix gain to 0 dB and the output volume to 100 percent.

2. Start the playback of program source device.

3. Raise amplifier input sensitivity until the amplifier clip LED begins to light, then lower 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, unmuted. Muting the input 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 set 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-switcher 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-switcher 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-switcher 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-switcher Trim dialog box (see the Pre-switcher Trim section).

2. With program material (or pink noise) present at the input, adjust the pre-switcher 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:

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

• You may also find that further minor adjustments to the pre-switcher 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-switcher 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-switcher trim or post-switcher gain control to set levels.

• Prevent any possible clipping as described in the Adjusting Post-switcher 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-switcher gain controls to adjust the output volume. Post-switcher 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-switcher Gain

At this point, setting up gain structure may become a bit of a balancing act. This subsection and the Setting Volume Control for the Amplifier Stage subsection below provide some guidelines, but 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-switcher 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-switcher 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-switcher gain for output volume control, the procedure may be reversed.

1. Set the post-switcher 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).

Setting Volume Control for the Amplifier Stage

The amplifiers inputs on the MLS 608 D MA and SA models are set to accept maximum output from the audio signal path. This means that you can set the output Volume control to 100 percent output, with the signal approaching 0 dBFS on the output meter.

If 100 percent output volume results in a volume level that is too high for your application, it is recommended that a soft limit is placed on the volume control. A soft limit can be used to prevent the volume control from going above (or below) a selected point.

Soft limits are a component of the group master controls. The default group masters for program volume and mic volume are described in the Group Masters section, as well as instructions for applying soft limits.