DaveWr;503411 Wrote: > Back to definitions: > > Cliveb is right, his definition of oversampling, is the original, and > if using a straightforward old 14 bit or 16 bit current source DAC is > true. > > Virtually all 24 bit DACs including the Transporter AK4396 use some > form of Delta Sigma DAC. In the AK4396 it is still multi-bit but not 24 > bit! > > In the ultimate case many DACs can be one bit, this guarantees > fantastic easy linearity. > > However the sampling rate required to use these low bit DACs would be > astronomic if using a basic oversampling technique alone. This is where > all the DAC black magic starts - first there is simple oversampling to > get higher sample rate, then a digital filter, usually multiple order, > is used to achieve noise shaping. This moves the DAC conversion noise > that is massive with low bit DACs, up to a high frequency band, that is > easily removed post DAC. This is up-sampling, it has filtering > coefficients, dither etc. not just basic stuffing of new values. > > It is not just BS. You would need a sample rate of many MHz to achieve > 24bit DAC resolutions without it. The clever bit the manufacturers > don't seem to brag about is that once you have done your digital magic > upfront, the DAC being 1 bit 4 bit or 8 bit is a easy and cheap, and > similarly the post DAC recovery filtering is relatively easy and cheap. >
At the time I was working on Transporter I was in touch with a very knowledgeable FAE at AKM who was most helpful in explaining the internal architecture of the AK4396 and how to get the most out of it. You're absolutely correct on the above, and the AK4396 in particular focuses on putting the conversion noise way up above 100Khz where it is easily dealt with. IIRC it is a 4-bit conversion at the end. This and other features such as direct voltage outputs also makes it less sensitive to board layout, process variance, component tolerances and such, which I found was a significant challenge with other DACs we looked at. The specs were realistic, not best case - I had no trouble matching them on the first board spin. > > This moves all the design and cost issues to clocks, power and good > analogue design practice. > Indeed.... the 4396 performs very well even with barebones external circuitry - a key selling point for a DAC is not just the price of the chip but the total system cost. Since that was just not a limiting factor, I goosed it quite a bit further from the reference design by upgrading passives, using super regulators and good clocks. No big secrets there, although other designs will have varying degrees of headroom for such improvement. -- seanadams ------------------------------------------------------------------------ seanadams's Profile: http://forums.slimdevices.com/member.php?userid=3 View this thread: http://forums.slimdevices.com/showthread.php?t=69145 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/audiophiles
