What is the difference between Line Level and Mic Level?
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Imagine looking at the sea. The level at which the water tends to settle down on a calm day is the sea level.
It’s sort of a given. It’s what the sea wants to do.
If you’ve read the chapter on RMS and the Decibel from Driving Miss Digital, you’ll know that sound is basically the effect pressure has on our ears. To record and transmit this sound, we have to convert it into audio; which means sound pressure is converted into electrical pulses, or signals.
Electrical signals have voltages. Most of the time the voltage fluctuates, which looks somewhat like this on a graph:
The ‘value’ of the voltage of a signal is called its voltage level. There are many ways to calculate this voltage, and the method most widely used is the RMS method (Root Mean Square).
The takeaway from this is, every audio signal has a standard voltage level. Unlike the sea, this level isn’t arbitrary. It is predetermined by the design of the device(s) used. This is very similar to the electrical voltage standard, which is around 120V or 240V, depending on which country you’re in. A standard helps everybody stay sane.
The Reference Level
Now, levels always need a reference point. E.g., the sea has a surface (sea level), but also a bottom (reference).
For better or for worse, the reference levels for consumer and professional equipment are different:
- For consumer equipment – 1 Volt
- For professional equipment – 0.7746 Volts (Notice how this voltage is similar to the composite video signal?)
In audio, it has traditionally been the case that values are represented by dB (decibels), instead of voltages or whatever. Again, there are two systems:
- dBV (decibel volts) – Used for consumer audio equipment, like CD players, etc.
- dBu (decibel unloaded) – Used for professional audio equipment, like mixers, etc.
So, what are the dBV and dBu versions of the reference level?
- 0 dBV for consumer equipment (1 Volt)
- 0 dBu for professional equipment (0.7746 Volts)
Yeah, I know. I could have just said 0 dBV or 0 dBu from the start. But where’s the fun in that?
The voltage or strength an audio signal must have (the standard) is called its Line Level (sometimes people also call it the Nominal Level, but that’s rare). The two standard line levels are:
- -10 dBV for consumer equipment (0.316 V RMS)
- +4 dBu for professional equipment (1.23 V RMS)
No matter what gear you’re making, you must design your gear such that the voltage is pegged at either of these line levels (thereby also screaming to the whole world which ‘camp’ your gear falls in!).
There are always exceptions and complications, but let’s not go there.
Notice, the first number is negative because the voltage falls under the reference mark (1 Volt). In the second case, the voltage is higher than the reference (0.7746 V), which makes it positive.
Now, what happens if you want to connect a consumer-grade device to a professional device? You’ll need to amplify the voltage level before passing it on. This is why amplifiers are a big deal in audio. You’re always connecting consumer devices to professional devices when you’re producing music, audio or whatever.
Microphone Level (Mic Level)
Microphones are transducers, which means they convert pressure to electrical signals. The signals are weak, which is another way of saying their voltage levels are lower than the line level (Sometimes -60 dBV).
Hey, it can even be higher than the line level. These standards are self-imposed. But most of the time, it’s lower. This is why you need an amplifier (called the Preamp) to increase its value to the line level.
The voltage of the signal that a microphone produces is the Microphone Level.
Similarly, headphones and loudspeakers have their own ratings, depending on their usage. Since the ‘Wattage’ of speakers are much higher than the circuitry, the current (amperes) and voltage need to be jacked up. The amplifier that does this is called a Power Amplifier.
This is what it all looks like:
The Microphone produces a signal with a voltage that’s called the microphone or mic level. It is amplified by the Preamp and the resulting signal is the line level.
This signal is manipulated, blah blah, and when it’s time to playback, is passed through another amplifier (if necessary) called the Power Amplifier. The resulting level (no standard, because it depends) drives the speaker or headphone.
Want to see an example of the whole thing in reverse? Power is generated by a power plant and transferred over vast distances to a sub-station, which has a transformer (power amplifier in reverse). From the sub-station, power is routed to your area, which will have a step-down transformer (preamp in reverse). This transformer brings down the voltage to 120/240V and sends electricity to your home.
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March 16, 2013