Optimizing Audio Levels

The DMP 64 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, reference to full scale, or 0 dBFS. For the DMP 64,
0 dBFS corresponds to +21 dBu, which is the maximum output level of the device. The maximum input level is
+24 dBu. Gain from -3 dB to +80 dB is applied in the analog domain, while attenuation from -3 dB to -18 dB is applied in the digital domain. The input meters are post-ADC, while the 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. The output volume of the DMP 64 can be controlled by either of the two output gain blocks:

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:

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:

NOTE:  A convenient way to set input gain is to use the building blocks feature. This will set the gain 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.

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 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.

NOTE:  If you are using the volume control for this purpose, set the post-mixer trim to 0 dB. If you use
post-matrix trim for this purpose, set the volume to 0 dB (100 percent).

Setting Mix 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/line 1/output1 to open the dialog box for that mix-point.
  2. Unmute the mix-point to place the signal into the mix. The default level is unity gain (0 dB).
  3. Open the Mic/Line Gain dialog box for mix 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-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.

NOTE:  This procedure is valid only if no processing is active in the output signal path and if the trim is set to unity gain (0 dB). If you inserted processors in the output signal path, open the processors and engage Bypass in order to temporarily remove the processors from the signal path.

To start adjusting the pre-mixer gain:

  1. Open the line input Gain dialog box (see the Mic/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:

Adjusting Pre-mixer Gain (for Controlling Output Volume)

When using the pre-mixer gain for output volume control, you may reverse the procedure.

To do so:

  1. Set the pre-mixer gain to 0 dB (mute if necessary).
  2. With program material (or pink noise) present at the input, adjust the output volume until the meter level in the output Volume dialog box is below clipping (or, ideally, matches the level at the input gain meter).

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.

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 pre-mixer 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:

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-mixer trim controls to adjust the output volume. Pre-mixer 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. Mic 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-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 compressor or limiter in the output DYN (dynamics) block. However, adjusting the post-mixer trim affects how the compressor or 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.