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 ADC (analog to digital converters) and DAC (digital to analog converters) sample at 48kHz, with 24-bit resolution.

With floating point DSP it is extremely difficult to clip the audio signal after the ADC and before the DAC, i.e., within the DSP audio signal chain. What is extremely important, however, is that the audio signal is not clipped at the input ADC. Clipping gives audibly undesirable results, and once the audio is clipped at the input there is no remedy further down the signal chain. If audio clipping occurs at the output DAC that is not a result of clipping at the input ADC, there are ways that this can be addressed in the DSP audio signal chain.

The meters in the DSP Configurator indicate clipping at a user-definable point, with the default setting at -1dB. This means that the meter indicates clipping when it reaches -1dBFS, which is 1dB below actual clipping. Setting the clipping meter below actual clipping provides a bit of a safety net, letting you know to pull back on input gain before clipping actually occurs. You may increase or decrease this "safety net" by going to Tools | Options | Gain Defaults, and setting the Meter Clip Indicator to a number between 0dB and -20dB. When the Meter Clip Indicator is set to 0dB, clipping is indicated when actual clipping occurs.

Meters within the DSP Configurator are peak-type meters, referenced to "full scale," or 0dBFS. For the MPX Plus 866 A, 0dBFS corresponds to 22dBu, which is the maximum output level of the device. The input meters are post-ADC, while the output meters are pre-DAC.

Setting Input Gain

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

Input gain can be set using the intended input source device and typical source material. It can also be set using pink noise when source material is not available, obtained either from a pre-recorded track of pink noise (played through, e.g., a DVD player) or a pink noise signal generator.

For program material, set the input level so that the meters reach approximately −15 to -12dBFS, with peaks at approximately −5 to -3dBFS. This setting provides enough headroom to accommodate transients or unanticipated loud events in the program material in order to avoid possible clipping.

The pink noise should be recorded at −20dBFS. If  the player has an output level setting control, set the output of the player to its maximum, or 0dB of attenuation (if the maximum output setting provides gain, then back off slightly from the maximum setting). If using a signal generator, set it to output −10dBu. Whichever pink noise source you're using, set the input gain in the DSP Configurator so that the input meter reads −20dBFS.

Setting a Nominal Output Level

In order to set up a gain structure, you may need to listen as you proceed. Make an audio tie from the source to the speakers that will carry program material in the room that you are setting up. With the output channel Volume set to −20dB, set the external amplifier so that the source material plays at a volume level that is reasonably loud but very tolerable. Verify that the amplifier is not clipping by observing the amplifier clip indicator. This will set the amplification/volume nominal level of the system. Adjust or mute the Volume control as necessary. Also see the section below, Setting Volume Control for the Amplifier Stage.

Adjusting Pre-matrix Trim

After you have set your input gain, add any processors that you wish to use into the input signal chain (see, Inserting a Processor). The Pre-matrix Trim control may be used to compensate for any level changes due to processing. Adding a compressor generally reduces the signal level, while a filter may boost or cut the overall signal level. If you make changes to filter settings after you have set your dynamics processors, re-check the levels in your dynamics processors to make sure that they are still valid.

Note: This procedure is valid only if no processing is active in the output signal path, and if Post-matrix Trim value is set to 0dB/unity gain. If you have processors inserted in the output signal path, open them and engage bypass in order to temporarily remove them from the signal path.

Open the line input Gain dialog, the output Volume dialog, and the Pre-matrix Trim dialog. Set output Volume to 100% (mute if necessary). With program material (or pink noise) present at the input, adjust the Pre-matrix trim so that the meter level on the input Gain dialog matches the meter level the output Volume dialog. This will maintain the audio at an optimal level in the input signal chain.

This is a good starting point. If, after you've set up your microphone input Gain and Mix-Point levels, output processing, and Post-matrix Trim levels, you find that you need more headroom to prevent clipping at the outputs, you can come back to the Pre-matrix Trim controls and lower each one by the same amount, e.g., 5dB, 10dB, etc. You may also find that further minor adjustments to the Pre-matrix Trim controls will help to balance out the perceived audio levels of the different inputs.

Setting Output Gain Structure

Add any processors that you wish to use into the output signal chain. Adding a compressor generally reduces the signal level, while a filter may boost or cut the overall signal level. Loudness will boost the overall signal level, but mostly at lower volume settings (see Loudness Calibration for more information).

After adding processors to the output signal chain, the output Volume level may be clipping when set to 100% (or less). Floating point DSP allows you to overcome the problem of clipping by lowering the output Volume setting. However, unless you prevent the user controls from changing the Volume setting to 100% (or to any position where clipping occurs), it is best to adjust the Post-matrix Trim control to prevent any possible clipping (described in the section, Adjusting Post-matrix Trim). Mic levels will also contribute to possible clipping at the outputs, and they may need to lowered as well to maintain the balance between program material (line outputs) and voice.

Setting Mic Input and Mix Levels

In this example, we will send the Mic/Line Input 1 signal to Output 1. Double-click the mic mix-point for Mic 1/Output 1 to open the dialog for that mix-point. Unmute the mix-point to place that mic signal into the mix. The default level is 0dB, or unity gain.

Open the Mic Input 1 Gain dialog. With your chosen microphone attached to Mic/Line Input 1 (turn on phantom power if your 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 the meter level reading will vary at this point, but as a general guideline you're likely to find the input gain level at approximately 40 to 50dB, and the meter averaging somewhere around -20dBFS. Certainly you want to optimize audio here, but voice levels at microphone inputs can vary greatly. Having the meters average around -20dBFS gives enough headroom to accommodate sudden changes to voice levels. Further adjustment may be necessary.

Adjusting Post-matrix Trim

This is where setting up gain structure becomes more of a balancing act. These last several sections will give you some guidelines, but it may take a bit of going back and forth to set levels just right. For example, output level can be controlled and kept below clipping using a compressor or limiter in the output dynamics block. However, adjusting the Post-matrix Trim will affect how the compressor or limiter work.

Open the output Volume dialog and the Post-matrix Trim dialog. Set output Volume to 100% (mute if necessary). With program material (or pink noise) present at the input, adjust the Post-matrix trim until the meter level in the output Volume dialog is below clipping (or ideally, matches the level at the input Gain meter). This will maintain the audio at an optimal level in the output signal chain, and prevent clipping at the output.

Setting Volume Control for the Amplifier Stage

The maximum output of the MPX Plus 866 A is +22dBu. Let's assume that the maximum input level of the power amp is +4dBu when the input attenuator is fully open. If you wish to control volume levels using the output Volume controls of the MPX Plus and ensure that clipping does not occur at the amplifier, you will need to turn down the input attenuator of the power amp the equivalent of 18dB (22 - 4 = 18), which would put the amp's input level at -14dB (+4 - 18 = -14). If, when using the system, you find that the amp setting (when the output Volume controls of the MPX Plus are at 100%) is too loud for the room, you may turn it down further. If you find that it's not loud enough for the room, you'll need a more powerful amplifier.

On the other hand, if you wish to control volume levels using the power amplifiers input attenuation control, (using the same power amp max input level) you can set the output Volume control in the MPX Plus to -18dB. This is another way that clip points of the two devices will be matched, ensuring that clipping will not occur.

However, we recommend using the output Volume control on the MPX Plus 866 A for controlling output volume. If you are using Loudness processing on the unit, it will only work in conjunction with the output Volume control.