Re: gEDA-user: Line and mic signal characteristics?

[al davis <ad151@xxxxxxxxxxxxxxxx>]

On Tuesday 16 October 2007, Robert Butts wrote:
> I've been trying to find info on line and mic level signal
> characteristics. I checked ieee for an audio signal, computer
> io... standards and anything else I thought would lead me to
> a definition of each and came up with nothing.

It depends what you want to do.  I don't think there is an 
official standard.  There is established practice.

For line level, most consumer equipment runs at a level of 
about -10 dBs, nominal, driving a load of 10k or higher.  The 
actual source impedance is low.  Pro equipment tends to run at 
a higher level, likely about +4, capable of driving 600 ohms, 
but is rarely loaded like that.  Typical loads are "bridging", 
about 10k.  Headroom in decent consumer products is about what 
you get with typical op-amps, with +/- 12 supply.  Headroom in 
cheap consumer products is much less, sometimes a single +12 
supply or +/- 5 volts, or even +5 only.  Pro products will have 
at least that, often equivalent of a +/- 24 supply to the 
op-amps.  (meaning the typical +/- 15 volt IC op-amps don't 
make it)

Microphones vary a lot.  The level and impedances depend on the 
design of the microphone.  Typically, dynamic mikes have an 
impedance of about 150 ohms, and are designed for a load of 2k 
or so.  A pro preamp typically has adjustable gain up to 60 dB.

Other types of microphones usually have some sort of preamp 
built in.  In a pro setting, power is supplied "phantom" on the 
audio lines.  As I recall, the standard supply was 48 volts, 
through a pair of resistors, balanced, to both signal lines.  
The shield is the DC ground.  The design is such that if the 
power is not needed, it doesn't hurt to have it anyway.


"dBs" is dB relative to .775 volts, a hack derived from the old 
days when "dBm" was commonly used to mean dB relative to 1 
milliwatt, quietly assuming the load is 600 ohms, hence .775 
volts.




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