Mic, Line and Instrument Level – What’s the Difference?
When sound hits the diaphragm of a microphone very low voltage signals are produced. On a dynamic microphone these signals are typically around 1.5 millivolts. For a more sensitive microphone the voltage would be more like 70 millivolts.
These signals are weak and can be affected by electrical interference quite easily. If you plug a mic directly into an active speaker for example, you probably won’t get any sound. As this signal is so weak a pre-amp is required to boost the signal to line level. The outputs of the pre-amp can then be connected to the inputs of a mixing desk or audio interface for example.
Microphone impedance affects the efficiency of how the signal is transferred to the input of the pre-amp. Mic level is -56 to -40 dbm. High quality microphones have low impedances and are best suited to low impedance inputs. Low impedance means the current is stronger therefore giving a better signal to noise ratio. Consumer microphones are often high impedance (Hi-Z) and should be connected to high impedance inputs. High impedance signals are more prone to interference and noise.
Some modern condenser mics have quite a high output. When using them with a loud sound source such as drums or vocals with phantom power (+48v) enabled, they can be plugged directly into a compressor or mixing desk without the need for a pre-amp.
Line level signals have a much higher voltage output than mic or instrument level, usually somewhere around 750 millivolts. As this signal is stronger it can be carried over a longer distance. 750 millivolts is the industry standard and allows for the interconnection of different devices from different manufacturers.
There are two types of line level:
- consumer line level is at -10dBV
- professional line level is at +4dBu (or dBm)
RCA (phono) connectors are usually -10dBV. XLR connectors are usually +4dBu (Mic Level). TRS quarter inch jack plugs can be either -10dBV or +4dBu
Although keyboards and guitars do not output at the same level they are usally somewhere between mic and line level. To take the signal of an instrument to line level an active DI (Direct Injection or Direct Input) Box is required. To take the signal of an instrument to mic level an active or passive DI Box can be used.
DI (Direct Injection or Direct Input) Box
DI Boxes convert unbalanced high impedance signals from guitar pickups and contact microphones, into balanced low impedance signals required by mixing desks.
Cheap SPY software, hardware?
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I want to set up a camera and microphone to record a noisy neighbor who has been deliberately slamming his door in my apartment building. Camera and microphone will be mounted somewhere just outside my door. If cables are required, they'd have to be thin enough to fit between a door and a doorframe, very snug space.
The neighbor is maybe 12 feet away from my door, the hall is reasonably well-lit. Ideal situation would be to record it all with a time and date stamp, on my computer.
However, I'd like to do it all (mike, camera, software) for no more than $100. Links?