Optimizing Audio Levels

The DMP 44 LC 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.

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 DMP 44 LC,
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 two approaches that the system designer can follow when setting up gain structure, depending on where output volume will ultimately be controlled. Is it suggested that the output volume of a DMP 44 LC device be controlled by either of these two gain blocks:

• Volume - Adjusting the level at the output volume block will change the listening level for the mix of all signals sent to the output.

• Pre-mixer gain - Adjusting the level at the pre-mixer gain block will change the listening level of different sources independent of each other.

It is recommended to use a group master control for grouping gain blocks, then use a single command string to control multiple gain blocks. When grouping pre-mixer gain controls, a soft limit can be imposed that prevents the gain from rising above 0 dB, which helps to prevent clipping within the DSP. In the instructions that follow, the setup is described for either volume control when appropriate.

Setting Line Input Gain

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.

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. Unmute mix-points to mute audio 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 pre-mixer 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 pre-mixer gain or volume control as necessary to create a reasonable working level for continuing setup.

Setting Line Input and Mix Levels for Wireless Mics

The DMP 44 LC line inputs can accept line level signals from wireless microphones. The default level for the line input is 0 dB, muted. Having the input muted before plugging in a wireless microphone is recommended.

A convenient way to set wireless microphone input gain is to use the building blocks feature. 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 mic1/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 Line Input Gain dialog box for mix input 1 (see the Line Input Gain section).

4. With the desired microphone attached to line input 1 and the mic gain set to 0 dB, 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 0 dB to +12 dB and the meter averaging -20 dBFS to -15 dBFS.

6. 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 Pre-mixer Gain (for Normalizing Channels)

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-mixer gain 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, re-check the levels in the dynamics processors to ensure that they are still valid.

To start adjusting the pre-mixer gain:

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-mixer Gain dialog box (see the Pre-mixer Gain section).

2. With program material (or pink noise) present at the input, adjust the pre-mixer gain 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-mixer trim levels:

4. If you need more headroom to prevent clipping at the outputs, you can return to the pre-mixer gain 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-mixer gain 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.

• Since a limiter only reduces the signal to prevent overload, you may not need to consider the reduction of signal level, depending on the threshold setting.

After you add processors to the output signal chain, 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 post-mixer trim. Lowering mix-point gain or pre-mixer gain will work as well. 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 clipping level, if clipping is observed at the output stage, it signifies that clipping is occurring before the output stage. Clipping will be 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, use soft limits on a group master (see the Group Masters section).

Adjusting Post-mixer Trim

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 limiter in the output DYN (dynamics) block. However, adjusting the post-mixer trim affects how the limiter works.

1. Open the output Volume dialog box (see the Output Volume section) and the post-mixer Trim dialog box (see the Post-mixer Trim 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-mixer trim 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.