On Tue, Dec 01, 2009 at 08:53:55PM -0500, Rick Green wrote: > Fons, thank you for your clear explanation! I got quite an education from > that one post alone. Makes me want to go out and measure all the preamps > in my environment. > So how do I do it? > > Just off the top of my head, I'm thinking of something as simple as > placing a 150ohm resistor across the input of the preamp, turning the gain > all the way up, and measurnig the p-p voltage at the output. Comparing > that to the '0dB' reference level would give the EIN. Am I even close? No... Do you want to measure a preamp with an analog output or a soundcard which has mic inputs ? > On a related note, what is the maximum dynamic range of a 24-bit a/d > converter? Each bit adds 6 dB, but you don't start with 0. The RMS noise level, assuming a uniformly distributed quantisation error, is 1 LSB / sqrt(12). The maximum RMS value of a sine wave is 3 dB below peak. One bit is used as a sign bit. Combining all this the S/N ratio for N bits is 6 * N + 1.8 dB. For a 24-bit converter this would give 145.8 dB, but that is pure theory. First, the lower bits will not be very accurate. Second, the digital S/N can't be better than the analog one. For an ADC with a range of +/- 1 V, and using top quality analog circuits, the S/N will be around 125 dB. Very few converters reach this figure. > I don't even know if audio is represented by signed integers, > or unsigned integers... It doesn't matter for S/N ratio. 'VU' refers to an analog level, in principle it has no meaning at all for digital. If you use a 'real' (with the correct speed etc) VU on a digital signal, the 0 VU mark should correspond to something in between -10 and 20 dB below digital peak. There's no standard, and the best value depends on the type of signal. Ciao, -- FA Wie der Mond heute Nacht aussieht ! Ist es nicht ein seltsames Bild ? _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
