Meaning of dBm measurement wrt compressors

Dave,
dBm is a measurement of power level relative to 1 milliwatt.
dBu is a measurement of voltage relative to the voltage that would produce 1 milliwatt into a 600 ohm load (=.7746V).

So if we are always expecting a 600 ohm load then dBu is equivalent to dBm. Most often, you will not actually have a 600 ohm load (it will probably be a higher impedance). What that means is, if the specs you quoted are correct, the unit will still be able to output a maximum of +21 dBu, but the output power needed will be considerably less than the maximum the unit is cable of (+24 dBm).

I would guess the reason for the difference in the max dBm and max dBu values given is that they are saying that the output drivers are capable of producing a power level of +24 dBm, but as you increase the input signal level, something else internally will cause the unit to clip when the output reaches +21 dBu and above.

BTW, +21 dBu is plenty of output level to drive any DAW input.

Back on the forum this morning and looked at this thread again. Just realized I misread the OP -- he was talking about 2 different compressors. Don't know how I missed that. So, the Elysia unit, which only quotes its output in dBu (+21 dBu), is not saying anything about the power handling capability of its output drivers based on the specs listed by the OP (although maybe Elysia might have other specs that state max load on output).

One refinement to what Mike said above "1dBm = 1mw into a 600 ohm load which also = 0.7746vAC" -- There is nothing about the unit dBm that includes or requires a 600 ohm load. However, here must be some load for power to be delivered-to in order to get a measureable amount of power. If the output is measured with it unconnected to anything else, then the output power in dBm would be -infinity, whereas the output in dBu would still presumably be 24 dBu max. In the case of the dbx 160A specs they did say that their 24 dBm spec was with a 600 ohm load and this is important information when specifying output power handling capability (ie it probably coudn't drive an 8 ohm load nor would you ever want to hook it up to an 8 ohm load). Seems like I remember dbx compressors being popular in the broadcast industry and it could be that input impedances to some of their gear may be as low as 600 ohm or 1Kohm, etc, so the dBm spec on the output of the compressor would be more important in that application.

Here's some follow up... I found a interesting quote on the web:

from: http://recording.org/diy-...mpedance-question.html


Bill Whitlock wrote:

"Thought you might be interested to know a little history behind the venerable "600 ohms". The first telephone lines that linked cities miles apart were actually existing telegraph lines. The wise engineers at Bell knew transmission line theory and realized that, even at audio frequencies, the lines were long enough to be true transmission lines (a pair of conductors behave as a transmission line when their physical length becomes more than about 1/10 wavelength at the highest frequency of interest). Therefore, they needed to know the characteristic impedance of the existing telegraph line pairs. They were typically #6 or #8 AWG wires spaced about 1 foot apart. If you do the calculations, an average value is about 600 ohms! To eliminate reflections (echoes), any transmission line must be driven from and loaded by a resistance equal to its characteristic impedance. So all telephone filters, transformers, etc. were designed for a 600 ohm system. And this hardware found its way into the first radio stations, and later into the first recording studios. Today, it is rarely necessary (or desirable) to "terminate" an audio cable unless it is a vintage passive filter or tube gear with transformers. Incidentally, audio cables begin to exhibit slight transmission line effects only when they are over about 4,000 feet long!"


best regards,
mike