Hi Peter: What you're seeing is likely the age old salesmanship game of A-D converters. There is a vast difference between hardware bits of resolution, in this case 24, and effective bits of resolution which you indicate you believe to be 18. Depending upon how the converter box has its signal chain & gain structure implemented and the design of the actual delta-sigma ADC chip, you can see vast differences between effective bits and actual hardware bits. One can in some sense equivocate this point to a comparative discussion of sensitivity based upon the SNR performance between two competing devices, but it is meaningless without some mention of the noise floor. BTW, I hear that discussion all the time.
Lee Pedlow NG6B San Diego, CA -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Peter Martinez Sent: Thursday, May 25, 2006 3:42 AM To: Flex Reflector Subject: [Flexradio] 24 bit soundcards? >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 _______________________________________________ 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
