The 0 dBm spec is to make 1 mw into a 600 ohm load. in amplifiers without feedback the amplifiers would only make that level reliably when the source and load impedances matched. Come along the 1960's and integrated circuit amplifiers came into common use and they were almost always operational amplifiers. Early work with the use of solid state operational amplifiers in professional audio being pioneered by Bell Losmandy of OpAmp Labs in Hollywood.
Operational amplifiers maintain their output voltage as a function of their input voltage and their feedback network design over a wide range of load impedances. Now it is common practice to spec 0 dBm (or +4) at as a voltage value equivalent to that power level when presented to a 600 ohm load. 0 dB means nothing without a reference point. dB values are logarithmic but the values for different measurements use different log functions (i.e. voltage referenced values are log20 and power referenced values are log10). 0dBFS is always a constant in any digital audio system. It's the highest level that the system can produce. It's the only thing you can count on and that makes it important. It's probably the only value anyone needs to talk about until bits are converted to the analog domain. At our station we don't do that til the audio gets inside a DSP at the transmitter so we don't worry much about all the transformations. The only place it's represented for anyone to look at is at the console VU meters and they are set to read 0 dBm when a constant level single tone sine wave is present at -20 dBFS on the program bus. Bill On 12/12/12 7:16 AM, Cowboy wrote: > On Tuesday 11 December 2012 08:47:51 pm VE4PER/ Andy wrote: >> RMS is 0.775 and average is closer to 0.5 so be sure to compare >> apples to apples and oranges to oranges > That one is telephone standard. > Actually .7745966692... volts RMS, and that is in 600 ohms, so > that is the original zero dbm. > > The apples to apples thing is SO important in this regard. > > Most professional audio gear these days has an output > impedance that is very low, on the order of 45 ohms, > and most pro gear has an input that is greater than 10K ohms, > and while there is good reason for that, it also immediately > means that our arbitrary db reference needs to be defined. > Is it an apple, or is it an orange ? > ( the actual output Z of the amps is closer to zero ohms, and > the build-out resistors are there to avoid blowing amps ) > (( and if you wire it wrong, that output Z can actually > go negative !! )) > > .775V RMS in 600 ohms is exactly the same voltage as > .775V RMS in 45 ohms, or 10K ohms, but the power is > substantially different, and quite possibly the characteristics > of the audio through it. > > 0db is not the same in 45 ohms as in 10K ohms. > Or, is it ? > > This has little to do with program automation, but can > have quite a bit to do with how it sounds. > Of course, it's irrelevant when you play MP3's. > _______________________________________________ Rivendell-dev mailing list [email protected] http://lists.rivendellaudio.org/mailman/listinfo/rivendell-dev
