Maintaining A Clean Signal When Adding Processing

The Music Telegraph | Text 2020/11/02 [12:29]

Maintaining A Clean Signal When Adding Processing

The Music Telegraph| 입력 : 2020/11/02 [12:29]


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Maintaining A Clean Signal When Adding Processing


You know how to get as loud an undistorted signal as possible coming into the mixer and exiting to a storage device. (refer to this article) However, life in professional audio involves processing audio signal with things like equalizers, effects programs, dynamic level controllers, etc. When we use these devices (both outboard and built into the mixer) they can end up adding more level to your (already) maximized signal, sending into overload distortion. And as before, once a signal is distorted, no amount of processing futher down the path can remove it. So we need to become aware of what processors do to a signal and how to use them and still have an undistorted signal.



Adding an Equalizer to the Signal Path

When you process a signal with an equalizer, you are adding level in specific frequency ranges that correspond to the frequency center and Q of the settings. If the signal was already very high before you eqaulize, then the additional level could send the equalized signal into overload.


In the following operation you will process a signal with the mixer's equalizer section and develop techniques to avoid adding distortion from equalization.


1) Make sure the mixer controls are set up as follows.

Control Setting
CTR Room / SubMix Unity Gain
Main Mix Unity Gain
Channel 1 Gain Unity Gain
Channel 2 Gain Unity Gain


2) Play your mix repeatedly.


3) Increase the Channel 1 input TRIM control for maximum signal before distortion.

To do this, play the signal from your mix and listen to it while adjusting the TRIM control until you just begin to hear and see distortion, then back off the control a little. When you are satisfied the signal is as loud as it can with no distortion, go on.


4) Adjust the low band equalization control to maximum boost (+15 dB).

Set the low band frequency equalizer on your mixer to a shelving type EQ with a corner frequency of 80 Hz. As you boost, you will undoubtedly hear more bottom end in the signal. Continue to boost the signal until you get to the maximum or +15 dB then answer the next question.


5) Listen to the signal on your headphone. Does it sound distorted?

Answer 'Yes' or 'No'.


What's going on here is that you have taken a signal that was very close to the upper limit of the amplifier chain and added yet more level to it. Something had to give. You sent the signal into overload, but in which amplifier? Clearly the input amplifier is not distorting because it comes before the equalizer, so it must be somewhere after that. Let's try the approach from the last operation, namely start lowering level from the output end backwards until you find which amp is distorting.


6) Lower the Main Mix control.


7) Listen to the signal on your headphone. Does it sound distorted?

Answer 'Yes' or 'No'.


Lowering the Main Mix buss level didn't solve the problem. All it did was make the distorted sound have a lower output level, not cleaner. You could still hear the distortion on your headphone although at a lower level. The wave amplitude got smaller but the distortion remained. So the Main Mix buss wasn't causing the overload. The problem must be somewhere before this point. The mix buss gets its signal from the buss combining amp which has no controls, so we must look further up the chain. The combining amp gets its signal from the channel level amp, so try that next.


8) Reset the Main Mix control to the "Unity Gain" position.


9) Listen to the signal on your headphone. Does it sound less distorted?

Answer 'Yes' or 'No'.


10) Reset the Channel 1 Gain control to the "Unity Gain" position.


11) Set the low band equalizer control back to the "Unity Gain" position.


12) Listen to the signal on your headphone. Does it sound less distorted?

Answer 'Yes' or 'No'.


Now the signal does not sound distorted. So it seems like the addition of 15 dB of boost at 80 Hz caused the entire chain to process the distortion. So don't use equalization, ever.



But I need the boost at 80 Hz. What am I to do?

If you must have the equalizer boost so drastically that it overloads the circuits further along the path, then you must take that amount of level change into account before you send the signal into the equalizer. In other words, reduce the amount of gain at the amp just upstream of the equalizer, which in the mixer is the input TRIM control. Do it now. 


13) Reset the low band eqaulizer control back to +15 dB.

You should hear the distortion again.


14) Reduce the level of the line input Trim control until the siganl is no longer distorted.


15) Listen to the signal on your headphone. Does it sound less distorted?

Answer 'Yes' or 'No'.


So, now you have an equalized signal that has the amount of boost you need without distortion.



What have we done here?

These two operations dealt with setting gain structures in a signal path without regard to the type of music or signal you are using. It doesn't matter. It almost doesn't matter if you listen to the signal as you do this set up. Whatever your signal is, you can follow these steps to assure yourself that regardless of the quality of the what is going through it, at least the signal path is clean and not adding any coloration to the sound.



The 4-Step Plan to A Cleaner Signal Path:


1. Set up the master mix buss to unity gain.

2. Set up the channel gain to unity gain.

3. Set up your equalization or effects to your desired settings.

4. Slowly adjust the input gain until distortion is just heard then back off till it disappears.






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