A Musician's Intro to Live Sound, Pt. 3: Transmission

This is part 3 of a 4-post series. For the rest of the story, read the first and second posts in the series.

When I first started setting up sound for my band, I thought cables were cables. I even bought a bunch of converters so that I could connect an XLR cable to a ¼” jack, and vice versa. Fortunately, I didn’t ruin any equipment, but my fundamental lack of understanding did make for some pretty long sound checks just guessing at what needed to be connected to what.

The purpose of the most of the equipment on stage – cables, DI boxes, mixing boards – is to bring different signals into one place, convert them to the same type of signal, and send one unified signal to the speakers. All the components of the live sound equipment are designed with a specific signal type in mind, and the biggest cause of frustration is when you don’t understand what types of signals each equipment is using.

Cables

Electrical signals are transmitted in cables. Although a cable looks like one wire, there are typically two, three or more separate wires, separated by insulation, inside the cable.

Let’s start with a few ways to classify cables:

Cable types (the inside)

Shielded versus unshielded

Cables act like an antenna and pick up noise (hum) from radio waves, fluorescent lights, cell phones, other cables, etc. For long distances at speaker level, it's no big deal: the noise is too tiny. For short distances at line and even instrument level, it's also ok. At mic level, or instrument level over long distances, this causes problems.

Shielding is the first way to protect your signal. Instead of having two wires inside the cable, one wire is instead a braided shield that creates a tube. The second wire passes in the center of the braided shield.

Mono verse stereo

Mono cables carry one signal, using two wires (to create a loop). Stereo cables carry two signals, using three wires (to create two loops).

Balanced vs. unbalanced signals

When you have mic level inputs, or when you have line level inputs over a long distance, shielding isn’t enough. You need balancing. (For a reminder how that works, check out the first post in the series.)

Technically, it’s the signal that is balanced, not the cable. Balanced signals need two loops, so the inside of the cable is the same as a stereo cable, with three wires inside. You’ll still hear the cables described as "balanced cables," though. 

Connector types (the outside) 

• An XLR cable has a round plug with 3 prongs or holes and is balanced. Usually used for microphones.
• A ¼” instrument cable (TS cable) has a prong on both ends, one small black stripe in the prong, and is unbalanced. Usually used for instrument level inputs. Keep these as short as is feasible.
• A ¼” speaker cable looks like an instrument cable but is constructed differently. It carries speaker level inputs. Using speaker cable for your instrument creates noise; using instrument cable for your speakers can damage your amp or speakers and can create a fire hazard. (You’re probably fine at low volume, but use caution.)
• A ¼” balanced or stereo cable (TRS cable) has two black bands in the tip. It can be used for balanced mono, or for unbalanced stereo sound. This can be used in some pro setups, but typically you won’t need to worry about it. 
• A 1/8” cable is usually TRS (two black bands) and typically carries stereo signal to/from an audio player or headphones at line level.
• An RCA cable has a prong and sheath plug, and is unbalanced. Usually used for line inputs. 

What you need to know

Ok, lots of cable types – so what? There are a few key places that cause the most problems.

Shielding and balance

Most of the time, the choice of cable is determined by what type of signal the equipment produces, and for the most part you can get away with just using the cable that fits. Mic level signals always need balance and shielding, so they use an XLR plug. Instrument and line level signals don’t need balancing when they are traveling a short distance, but the signal is still small, so you’ll need a shielded cable. If you need to take a line or instrument signal a long distance, you’ll need to balance it first. Speakers don’t need shielded cables typically, but you do want larger gauge wire. Sometimes, though, you can’t just plug it in where it fits...

¼” cables

If only they’d put different connectors on the different types of ¼” cables! The "guts" of a ¼” cable can be either stereo or mono, and shielded or unshielded. 

First off, pay attention to the number of black bands on the connector. Two bands is TRS, which is stereo – good for headphones, bad for your guitar. 1 band is mono, which is just what your guitar needs. 

The second challenge is shielded, which is harder to tell apart. Normally the only indication is some tiny, illegible text on the cable itself that says "instrument" or "speaker." My recommendation is to mark your cables clearly when you buy them.

If you plug your guitar into your amp and the sound is super noisy and muddy, you might be using a speaker cable by mistake. Also, ever wonder why you can’t find guitar cables longer than 25 feet? Beyond that and the shielding can’t block the noise, so if you need to take signal farther you’ll need to convert to a balanced signal (using a DI box). 

Also, for instruments, the shorter cable the better for preventing noise, so use a short guitar cable if you can get away with it.

Converting signals

So you picked the right cable to deliver the right type of signal based on what your instrument or other equipment produces. Where does the other end of it go?

Remember, at the end of the whole system we want one, unified signal to send to the speakers. Right now, all the equipment has signals at different levels. At some point, we need to convert those signals and combine them together. That’s the main job of the rest of the equipment.

• A preamp boosts the signal from mic or instrument level up to line level.
  If your instrument’s output level is low, you might need a pre-amp to get it up to line level. 

• An amplifier converts from instrument or line level up to speaker level; instrument amps have a speaker built in, while the amp for the sound system might connect to separate speakers.
  If you are using an amp (say, for an electric guitar), you can either use the amp sound directly if it’s loud enough, put a microphone on the amp, or some amps also include a line level output.
• A DI box converts from an instrument or line level down to mic level, and sends out a balanced signal. This is a good article explaining DI boxes.
  You use a DI box when you need to send an instrument level signal over a long distance, such as when you connect an acoustic guitar pickup or keyboard line out to the mixing board. A DI box also blocks the phantom power sent from the board, so beware if you need that phantom power! 

• A mixing board takes multiple inputs of various types, mixes both their levels and their frequencies (i.e., an equalizer), combines these signals, and sends the combined signal out to different speakers. It can send phantom power out to devices that need them. Sometimes it includes an amplifier for the speakers built in (meaning it sends speaker level outputs directly to the speaker). Or, sometimes the amp must be separate (it sends line level outputs). Sometimes different channels ("main" output versus "monitor" output) have different output types; for instance, the monitor might have line level output and the main might have speaker level output. A digital mixing board typically operates the same as an analog one, but it can save setting combinations and can often also output each track separately to a computer interface. 

• Gain and volume knobs can be found on instruments, amplifiers and mixing boards. Think of volume as the squeeze handle on the end of the hose, and gain as the faucet knob where the hose comes out of the house. Both need to be adjusted correctly.

What you need to know

Most of the transmission equipment is completely in the domain of the sound engineer, meaning you, as a musician, hopefully don’t need to deal with the intricacies of it. But, musicians usually have two things they do need to pay attention to. First, you’ll need to talk intelligently to your sound engineer about where the signal from your instrument is going. Second, you’ll need to help diagnose problems when they occur.

Tell the engineer what he needs to know

Take a look back at the previous article and figure out what level of signal your equipment produces (mic, instrument, line). Then, figure out how you’ll need to get to the sound board. The most common answers are:

• Your instrument has a pickup and you have your own amp – tell the engineer if you have a line out they should use, or if the amp should be mic’d. 
• Your instrument has a pickup and you don’t have an amp (acoustic guitar, violin, etc.). Normally, you’ll need to go into a DI box that then goes to the sound board. If the instrument level on your pickup doesn’t provide enough volume, you may want to add a preamp to boost your signal – they’re not expensive. 
• If you have a keyboard, pay attention to what output type you have (XLR vs. ¼” and stereo vs. mono). Usually you’ll go into a DI box, but you also might go through an amp. 

Help fix problems

On stage, a musician can help with three of the most common problems you encounter in live sound.

Feedback occurs when a microphone hears itself in one of the speakers. Often this happens when someone on stage moves the microphone somewhere bad. You can read more about fighting feedback in this past blog post, but when you’re on stage and you hear feedback, you can:

• Cover the microphone closest to you with your hand (block some sound)
• Look at where it’s pointing. Is it pointing at a speaker? Did you move past the line of where the speakers are in front of the stage? Adjust. 

No sound happens when there is a break anywhere in the chain from the input to the speaker. Sometimes it’s on purpose (sound engineers often mute channels when they’re not needed). If the channel is needed, you can help diagnose the problem by:

• Making noise continuously, for instance, by talking into the mic. You won’t know when you fix it if it isn’t making sound. 
• Start at the source and check every connection, volume knob and power switch. Pay attention to ones located on your instrument in particular. 

Noise is usually due to bad connections, the wrong cables or grounding issues. Wiggle all the connections to make sure they’re sure, and only use instrument cables for instruments!

Transmission is the biggest section of a live sound setup, so it’s naturally the most complicated. Next up, we’ll briefly cover outputs, the final step in the process.