Tech Tuesdays is our weekly geek-out, with a new geeky topic every Tuesday.

 

This is the first in a series of Tech Tuesday posts about different kinds of signals.  In the audio world, there are lots of different kinds of cables and jacks, not to mention a myriad of signal types that flow on through these cables.  Not surprisingly, different manufacturers and models use different kinds of cables, jacks and signals to connect gear together, and they don’t often offer a lot of explanation.  So what’s the deal with all these formats?

Cable Types:

Probably the easiest thing to start with is the two most common Analog Audio Cable Types.  (Analog Audio refers to a signal that is AC voltage representing an audio waveform – and NOT digital information.)

1) Balanced Cable - A balanced cable is an audio cable with two “signal” leads and one “ground” lead.  The more common kinds of cables you’ll see his in are XLR microphone cables and TRS 1/4 Inch Phone Cables (think of a regular guitar cable with an extra lead).  These cables are used to transmit “balanced signals,” and are used in pro audio situations for better noise rejection on cables.    They are also used for carrying stereo signals that aren’t technically “balanced.”  Confused yet?

2) Unbalanced Cable – An unbalanced cable is an audio cable with one “signal” lead and one “ground” lead.  These are most commonly TS 1/4 Inch Phone cables (standard guitar cable).  Also included in this bunch are cables like RCA cables and other Coax cables.  These cables are used to carry “unbalanced signals,” and are more susceptible to noise entering your audio through the cable.  

3) Multi-pin Cables –  These are the various kinds of ginormous cables you might encounter in pro/semi-pro audio situations.  They carry multiple signals in one cable and often look kind of like giant VGA cables (the cable you use to hook a computer to a monitor).  These are generally pretty specific in pinouts and sizing, and I don’t really have space to go into them here.  If you do happen to encounter one or a piece of gear that requires one, you’ll probably have to do some extra research to figure out exactly what you need (even the pros have to too). 

Analog Audio Signal Types

This get’s us into some funky territory because there are two common ways of classifying audio signals.  You can talk about relative dB values which gives you Mic, Line, Instrument, and Speaker level.  Or you can talk about how the signal is transmitted which gives you the categories: Unbalanced and Balanced Signals (related to but different than balanced/unbalanced cables).

Lets tackle relative levels first:

1) Line Level - Line level is the general operating level of most audio processors.  The outputs of your mixer, rack gear, and recording interface are line level. Roughly speaking the output of your iPod’s headphone jack is line level.  Its the ubiquitous standard level for analog audio signals, and the most common level you’ll be working with.  

It’s also used as the standard reference for comparing other signals too.  Line level is generally denoted as +4 dB.  That’s a relative value, which means if we consider line level to be +4 dB, we can assign dB values to other kinds of signals that relate to it.  This will make more sense later, I promise.

2) Mic Level –  This a low volume signal, usually denoted as between -40 and -60 dB.  This is a relative volume level, meaning that compared to line level signals, you’ll find mic level to be 40 to 60 dB lower.  Thats a considerable difference in volume as dB is a logarithmic scale (fancy math meaning that it scales with multiplication, not addition).  Roughly speaking adding 3dB to an audio signal is like multiplying the power (Watts) by 10. 

In order to take mic level signals and increase them to the level of most audio processors, we use microphone preamps.  This should help explain why preamps are such a big deal to audio heads.  They play a big part in the tone of your sound, and are often the more expensive part of your mixer.  Because Mic level is so low in amplitude, most pro mics require balanced cables (XLR) for better noise rejection on the cable.  

3) Instrument Level - This is the level that describes the kind of signal put out by guitar pickups, and is often described as -10 dB.  That means about 14 dB lower than your standard line level signal.

 Electric guitars, passive basses, and acoustics with pickups tend to get lumped together as having instrument level, because their pickups have a high impedance and slightly lower level than standard line level.  This often requires the use of a direct box, which converts instrument level signal to mic level signal (with lower impedance).  That mic level signal is then sent to a mic preamp to boost it to line level.  

Another place you’ll encounter instrument level signals in your guitar stomp boxes and other pedals.  They are built to accept and spit out the high impedance signals that guitars produce and guitar amps accept.  (Incidentally this sheds some light on why keyboards run through guitar pedals often attain results that are a lot different than a guitar run through the same pedal.  Keyboards output line level signals.)

Lastly, you might encounter instrument level signals when re-amping.  This is the process by which you take an already recorded track and play it out of your recording device (at line level) to a guitar amplifier or effects pedal to “re-amp” the sound.  This can be done without a re-amp device that converts line level signal to instrument level (with a higher impedance), but it often results in extra distortions of your sound.  

4) Speaker Level –  This is the level spit out of an Amplifier to speakers.  Most people encounter this only if they have to wire speaker wire from their tv’s or stereo recievers to their speakers.  (Think of wires that you strip the ends of and screw down to the contacts of amps and speakers.)  

This is a high amplitude, low impedance signal and you really only have to know to use proper gauge wire and match speaker resistances to amp outputs.  (Consult manufacturers on this kind of info, as using the wrong stuff can damage speakers/amps and void warranties.)

Last but not Least – Balanced/Unbalanced Signals!

1) Balanced Signals - Are audio signals that are often denoted as “pro” level or +4dB.  These signals require a balanced cable, because the signal is actually a copied across to leads in the cable.  One of the signals is flipped entirely out of phase (180 degrees) which means that they would mathematically sum to zero.  

If this is confusing, try to imagine a digging a whole exactly 1 cubic foot in size and then adding exactly 1 cubic foot of dirt to the hole.  What are you left with? Nothing – the two actions cancel each other out.  In the same way, the peaks of one signal will cancel the troughs of the other.  

These signals travel down the cable to their destination where the out-of-phase signal is flipped back in phase with the other and the two are summed.  Instead of canceling, the two, phase-corrected signals double the volume of the signal.  This also allows for common mode rejection of noise that may have been picked up at as the audio travels through the cable.  (kind of a complicated process, but basically it it zaps any signal that wasn’t present at the beginning of the cable). 

Its also important to note that balanced cables are also used to transmit stereo signals.  These stereo signals are not technically balanced because they two signal leads are not copies of each other.  A common place to find a balanced cable that is NOT carrying a balanced signal is the cable in your head phones.  To sum up – balanced cables can transmit signals other than balanced signals, but a balanced signal requires a balanced cable.

2) Unbalanced Signal – This kind of signal does not have all the fancy phase reversing and recombining, and as-such does not have the added noise-rejection of balanced signals.  This kind of signal is commonly referred to as “semi-pro” or -10 dB signals.  This is often found in cheaper recording equipment or as an alternative to balanced outputs on higher-end gear.  An example of an unbalanced signal is the signal coming from your guitar.  

Wait a minute… isn’t that instrument level?  Yes it is, but it’s also an unbalanced signal because it is just an AC audio signal and ground wire – no second signal lead.  This is why discussion of different signals gets confusing.  Line level signals can be transmitted as unbalanced signals, and so can mic level signals – The definition really hinges on how the audio is transmitted, not how loud it is.  

Practically Speaking

If you’ve got gear that says it has balanced outputs/inputs, use balanced cables if you can.  You can use unbalanced cables, but you may encounter noise/lower  volume problems.  If your gear has unbalanced connections, many pros still suggest using balanced cables to connect them, although balanced cables will work.

Well I hope that helps clear up some of the confusion about signal types.  At the very least I hope it doesn’t confuse you even more.  More to come next week in Part 2 – Digital Signals, and Digital Cables.

 

 

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