Mastering Signal Flow – Here it Comes and There it Goes

Growing up in a one hundred year-old home, I learned how to fix plumbing problems.  One lesson was tracing all the pipes in the basement so I could tell which pipes had fresh water and which ones carried sewage.  You might say it was my first exposure to signal flow.

Signal flow is the flow of the audio signal from the sources of input to the places of output, from sound-to-speaker, if you will.  Learning how to set up the stage, you saw how the signal flows from an electric guitar to a pedal board to a DI box and then into a stage jack.  Now you will see what happens to that signal once it gets to the sound booth.

Asking for directions

An audio signal travels from the source towards some sort of output.  For example, a singer’s voice is picked up by the microphone which, through a series of components, makes its way out to the house speakers.  Consider this as the general directionality of the audio signal – from a source to a destination.

The sound booth is like a giant airport where signals are coming in and going out, from multiple sources to multiple destinations.  Destinations can be house speakers, floor monitors, recording software, and even church nursery speakers.  The primary component that takes care of all these transfers is the mixer.

Coming into the mixer, are a variety of sound sources which will all be assigned to the channels on the mixer.  Naturally, you have your sources from the stage but you also have sources such as a computer, tape deck, CD player, and even audio feeds from video devices.

Once the signal is going to a channel, it tends to follow a general path that can vary slightly from one mixer to the next.  From the signal input, it usually follows as such;

1. Gain control (controls how much of the signal you are letting into the system)
2. Insert loop (plugs in back of mixer for auxiliary effects)
3. High-pass filter (used to cut out frequencies below a fixed point)
4. Equalizer
5. Channel on/off or mute switch
6. Fader
7. Pan control (for stereo panning)
8.  Out to groups (for control over multiple channels from one group channel)
9. Out to main fader control

Looking at the signal flow on the mixer channel, most extra on-board controls like compression and padding occur before the signal goes to the equalizer.  Also, signal to the auxiliary controls for the channel’s audio sends, such as for monitors, will be either before or after the fader depending on it the auxiliary control is set to send as pre-fade or post-fade.

Out of the Mixer

The signal can travel out of the mixer in a variety of ways;

• Auxiliary sends.  This could be for monitors, hallways speakers, or however your system is set up.
• Tape Deck.  Yes, some mixers have separate send controls for tape decks.  Seriously! I know we’ve moved on to other methods of recording, such as straight to a USB drive. Bottom line is there can be an output specific for a recording device.
• Insert loops.  These are at the channel level and are used to send the signal out to a processing unit like a reverb unit, and then return that sound back into the channel at the same point.
• Group ins/outs. You can send the specific group signal to a separate out.  For example, a group could be used for additional signal processing like a compressor and therefore you can route the signal out to the unit and back in.  Or, route it out for some other use.
• House speakers.  You gotta fill the room with sound!

The output signal can go directly to another processing device, like a reverb or compressor as I mentioned above.  Or, it could be routed for your house speakers.  It’s here where you need to dive behind your components and start following some cables.

Audio signals can be routed a variety of ways.  I’ve seen systems where the main out was first routed through a tape deck before going to the amplifier.  Such routing is understandable considering the fact that you commonly want to record a service.  However, for more finite control, they could have routed the recording device to an auxiliary channel where the mix for the recorded media could have been different.

Note that the signal strength between components is considered to be at line level.

You should expect the mixer’s main output to go out to the house EQ, through any house compressor or limiter, and then to the amplifier and speakers.

What to do when the volume stops

Knowing how sound travels through the audio system, you can quickly find the source of audio problems.  Take for instance an electric guitarist who has started playing during the sound check but you don’t hear them in the house mix.  By tracking the audio signal from its source, you can investigate where the problem might be located.  For example, if you are getting a green light on the channel, you know you are getting a signal from them.  Perhaps the gain is too low.  Maybe you aren’t getting any signal light.  Start at their guitar and make sure the signal is taking the right path to the sound booth.  It could be a mistake as simple as improper cabling into a direct input box.

When the sound is bad

Every time an audio signal goes from one component to another, you can pick up unwanted noise.  This is called line noise.  Regarding the signal flow coming from the stage, we need to look at something called the signal-to-noise ratio.  In short, this ratio explains the quality of your sound.  For example, let’s say you have an acoustic guitar with an on-board amplifier.  This gives the musician the ability to control the level of the signal from their guitar.  If they don’t raise the on-board amp’s volume high enough, you could hear a large amount of noise in their signal.  By increasing the volume on their guitar amp, they are improving / increasing the signal-to-noise ratio.  This means when the signal is amplified, there is very little noise heard because the strongest signal is coming from the guitar.

Summary

The flow of the audio signals through your system can take different paths.  There is an order to the mass of cables running to and fro.  You can have a great grasp of how your system works, when you know how your audio signals are routed.