Posted by
Jim Brown-10 on
Jul 02, 2004; 4:32am
URL: http://elecraft.85.s1.nabble.com/OT-Line-level-tp368401p368412.html
Line level for "pro" equipment is generally considered to be a nominal (average) level of +4 dBu,
where 0 dBu = 0.78 volts rms. For typical, uncompressed speech, the rms value of a sine wave equal
to the peaks is usually about 10 dB higher than the average level. Compression or limiting will
generally reduce that difference to about 6 dB. Translating that to volts, +4 dBu is about 1.23 volts, 6
dB more is 2.5 volts, 10 dB above +4 is 3.9 volts. The key number there is the peak value of the
waveform at clip, which is 1.414 x 3.9 volts = 5.5 volts. So an output stage (or input stage) that handles
line level would expect to see a peak to peak swing of 11 volts with a nominal line level signal. But VU
meters often swing against the peg, so we need some "headroom" -- another 6-10 dB "just in case."
6 dB more is double the voltage, so that's 22 volts peak to peak.
For consumer (home) equipment, the "nominal" level is roughly 300 mV, with peaks of 1-2 volts. The
same math applies, except that virtually all pre-recorded music has already been subjected to
considerable peak limiting and compression, so "headroom" is less important.
BTW -- forget all that ancient stuff about 600 ohms. Pro audio hasn't used a 600 ohm reference for at
least four decades. 600 ohms came from the characteristic impedance of telegraph lines between
cities (spaced at a foot or so) where lines were long enough that they needed to be treated as
transmission lines. Audio lines are almost never that long (4000 ft or more). Besides, the
characteristic impedance of audio cable is on the order of 60-80 ohms (do the math on conductor size
and spacing), so if termination WERE used, that's the value that would be required! Modern audio
gear has a low (50-100 ohm) output impedance and a high (10K typical) input impedance. Mic input
stages have a much lower input impedance (1K typical) to maximize signal to noise.
Jim Brown K9YC
http:audiosystemsgroup.com
On Thu, 1 Jul 2004 19:44:45 EDT,
[hidden email] wrote:
>When an input to, or output from an audio device is specified as being "line
>level", what exactly does that mean? Is it a specific RMS voltage perhaps?
>Its a term that I've heard bandied about over the years but either never knew,
>or have forgotten the precise meaning.
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