>From G3PLX: Chris: I wondered about dithering. If I understand the idea, they introduce some deliberate noise in the ANALOGUE part of the ADC in order to spread the spectrum of any quantisation noise. Without it, I can imagine a tiny sinewave input of, say, one bit amplitude, looking to the software like a one-bit squarewave, with 3rd, 5th, ... harmonics. Dithering the analogue input would add a little noise to the fundamental sinewave but push the harmonics down.
But, as you say, surely they would only introduce about one bit's worth of dither noise, not eight? 6dB not 48dB? I just did some proper measurements on the Firebox. On runs of 1200 samples at 48kHz, the r.m.s. noise level on the line input is about 400. It's about 450 on the Mike input. I did this by squaring each sample, summing these, dividing by 1200, and taking the square root. At the same time I calculated the DC level, just to make sure that was a lot smaller and didn't confuse things. This is about 9 bits of noise, or more if you suppose the peaks are higher. On the built-in 16-bit card in this laptop the DC level is 15 and I am not sure how to allow for this in the r.m.s calculation, but only the lsb toggles randomly. The MP3+ is the same, unless I select the mike input when the rms noise level is about 10. These figures are what I would have expected intuitively. One would expect the analogue noise-level to be roughly the same as the quantisation noise, in the same way that one would optimise any receiver so that all sources of noise contribute more-or-less the same in order to maximise the dynamic range. Intuitively I would have expected a 24-bit card to do the same, but maybe I am being naive. These measurements, and the figure quoted by Alberto, tells me that the "24-bit" figure in the spec. is to some extent a marketting ploy. I can imagine a scenario where the designer of a new soundcard measured that it only gave 18 bits of useful data, so he decides to mask the data to 18-bits and sell it as an 18-bit card. The marketting manager of the company would surely override him, especially if the competition were all actively marketting so-called 24bit cards! 73 Peter _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
