When we did our last video for In The Studio we discussed the basics of microphones. Part of that discussion was that microphones were one of the entry points for inputting and capturing sound in your system. Today I thought we’d do a post discussing two of the next steps of ‘the chain’ (really two possible options for the next step of sound capture). My usual next step for capturing sound from a microphone would be to run them in to an outboard microphone pre-amp and then in to an outboard compressor. This is my personal choice as my mixing boards have inboard mic-pre-amps and compressors that could also be used. Again, we’re going to discuss this in relatively non-technical terms so anyone can have a basic idea of what the equipment does. I’m also not recommending specific brands or models. They all have their strengths and weaknesses and that’s a whole separate discussion.
Let’s start with the mic pre-amps. The simple explanation of a mic pre-amp is that it increases the signal from the microphone so that you have enough signal volume to get a good sounding recording. Let’s look at the pre-amp below:
This is a two channel unit so we’re only looking at one side. First, it has a button to activate phantom power. As we discussed with microphones, condenser mics put out less voltage than dynamic mics, so we need to use phantom power to add that voltage. If you use a separate phantom power unit in line before the pre-amp, you won’t need to activate this. The pre-amp has a dial where you will determine the volume added to the signal going in to the unit (Input) and a dial for adding volume to the signal leaving the unit (Output). The amount of power added to the input is measured by the LED lights at the bottom right and the output power is measured by the needle display in the upper right. You also have a button labeled ‘Gain’ where you can boost the input signal by another 20 decibels. You have a button to reverse the signal phase (if you want to know about signal phase, dig in our archives for the video on recording a snare and phase cancellation). The ‘HPF’ button is a high pass filter. You can use this to ‘roll off’ at a certain frequency – we’ll save a deeper explanation for another time.
This mic pre-amp is a little more high end and allows you to be a bit more specific in adding signal to certain frequencies. These are the toggle switches and dials to the right in the photo. Other than that, the mic-pre works in the same fashion.
After the mic pre-amp, the signal is sent to a compressor. The basic function of a compressor is to regulate the volume peaks of the signal before it moves to the next step. This can keep the signal from ‘clipping’ and adding audible ‘pops’ to the track if a volume peak is too loud. Let’s take a look at one and go through it’s options.
Going left to right. ‘Threshold’ determines at what volume the compressor takes hold of the signal. The ‘Ratio’ determines what the unit does with the volume once it reaches the Threshold. If you select a Ratio of 4:1 it means that once the volume level hits the Threshold, the compressor will allow the output to increase by 1 decibel for every 4 decibels of actual volume. The ‘Attack’ determines how quickly the compressor applies the Ratio and ‘Release’ determines how quickly it stops applying the Ratio when the signal drops below the Threshold. When you apply compression to a signal, you will reduce the overall top volume. The Output dial allows you to make up for this by increasing the overall output from the unit. Finally, this unit also has a ‘Gate’. The Gate allows you to select how much volume is needed to allow the signal to pass through. For instance, when you are recording a vocal, you don’t want anything from the mic to pass through when the vocalist is not singing. You would be picking up unwanted background noise. The Gate shuts off the signal when there is not enough volume going in to the mic. The ‘Rate’ dial allows you to adjust how quickly the gate takes effect.
The compressor in this photo also has a limiter (far right), which means that beyond applying the Ratio, you can set the compressor to cut off the output signal totally at a certain level.
Above and beyond controlling the signal level, mic pre-amps and compressors are used to add color, depth and tone to the signal you are putting through them. Mic pre-amps can use tubes or solid state circuits to accomplish their goal. They can run from affordable to incredibly expensive. Each type will add their own flavor to the sound you are recording. Compressors have been used as an effect to give certain sonic qualities to an instrument – you can find lots of info about using heavy compression on snare and kick drums to achieve a specific sound. Compression can be added at many stages of the recording process. You can add compression to tracks already recorded in the software. You will usually add compression to the final mix to even out the overall volume of the music.
We’ll continue to move through the recording chain in future posts. As always, let us know if you have questions or comments.