>From G3PLX: I was talking to a physicist friend about the 24-bit Firebox and the dissapointing performance that I had seen, and he said he had an M-audio Delta 1010LT and would plug it in and see what it was like. I don't know anything about it, but he seemed to think it would be pretty good. His employers probably paid a lot of money for it!
He reported slightly lower noise levels than I did, but noticed something odd about the statistics of this noise. Although all 24-bits were 'busy', there was a very high probability (almost 100%) that the difference between two consecutive samples was a multiple of 16. He speculated that the hardware was a 20-bit DAC, and there was some kind of added low-frequency noise or dither added in at the 24-bit resolution. I went back to the Firebox with this in mind. The first thing I found - and I should have seen this much earlier - was that the bottom 2 bits of the 24-bit data from the Firebox were solidly fixed at zero! But on going deeper into the statistics, I found exactly the same characteristic as the Delta 1010. It's a basic 20-bit ADC, but with an extra 2 bits of low-frequency dither rather than 4. I wonder what to make of all this. The discussion on this reflector about 24-bit soundcards revealed that there was a good reason to add-in a bit of dither noise to mask the quantisation effect, but surely that should be at the full sampling bandwidth, not a very low-frequency dither. I estimate it's only a few tens of Hz on the Firebox. Perhaps this isn't a dither mechanism but some kind of DC offset cancellation? In that case it isn't adding LF noise (1/f noise?) but removing it. Whatever, it looks like the Firebox says 24-bits on the box, has 22 bits inside, of which 20 are derived from the ADC, of which 17-18 are useful. That's two weeks in a row I have learnt something new. 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
