Growing up in a 100-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 the other stuff. You might say it was my first exposure to signal flow.
In every sound system, signal flow is the flow of the audio signal from the sources of input to the places of output, from source to loudspeaker. Learning how to set up the stage, we see how the signal flows from an electric guitar to a pedal board to a direct (DI) box and then into a stage jack. Then we 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 loudspeakers, floor monitors, recording software, and even church nursery loudspeakers. The primary component that takes care of all these transfers is the mixer.
Coming into the mixer are a variety of sound sources that will all be assigned to the channels on the mixer. Naturally, you have sources from the stage, but there are also have sources such as a computer, tablet, CD player, and 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:
- Gain control (controls how much of the signal you are letting into the system)
- Insert loop
- High-pass filter (used to cut out frequencies below a fixed point)
- Channel on/off or mute switch
- Pan control (for stereo panning)
- Out to groups (for control over multiple channels from one group channel)
- Out to main fader control
Looking at the signal flow on the mixer channel, most extra onboard 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, hallway loudspeakers, or however your system is set up.
• 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 loudspeakers. You gotta fill the room with sound!
The output signal can go directly to another processing device, like a reverb or compressor as mentioned above – or – it could be routed for the house loudspeakers. It’s here where you need to dive behind the 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 recording before going to the amplifier. Such routing is understandable considering that you commonly want to record services. However, for more finite control, the recorder could be routed to an auxiliary bus 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. Expect the mixer’s main output to go out to the house EQ through any house compressor or limiter.
Knowing how sound travels through the audio system, you can quickly find the source of problems. Take, for instance, an electric guitarist who’s 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’re getting a green light on the channel, you know you’re getting a signal from them. Perhaps the gain is too low. Maybe you aren’t getting any signal light. Start at the 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 It Sounds Bad
Every time an audio signal goes from one component to another, it 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 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’s very little noise heard because the strongest signal is coming from the guitar.
The flow of the audio signals through a system can take different paths. There’s an order to the mass of cables running to and fro. You can have a better grasp of how the system works when you know how the audio signals are routed.