I'd appreciate anyones thoughts on why the digital stream would be better out of an iPad vs an iTouch.Hi Harry,
While I don't think that it is possible to predict which would be better, aside from experience (yours or others), it is certainly conceivable to me that there could be differences.
The likeliest reason would be differences in the amount of jitter (timing fluctuations) on the signal that is received by the DAC. A less likely but still conceivable reason would be differences in radiated RFI that could couple into the DAC or other parts of the system.
Also, I suppose it's possible that the codecs (coder-decoder programs) used in the two devices to decompress the audio data are not the same, resulting in differences in the bits that are output, but I have no particular knowledge of whether or not that may be the case.
A great many design-dependent factors in the iPod or iPad could affect the amount of jitter on the signal that is output from the dock, including electrical noise that will be generated by digital integrated circuits or other circuit devices, waveform distortion caused by impedance mismatches, rapid fluctuations in power consumption that will occur as processing requirements vary from instant to instant, etc. The amount of jitter resulting from causes such as those would depend on signal risetimes and falltimes at various circuit points, "crosstalk" between different circuit points, the mechanical layout of the circuits, and other similarly unpredictable factors.
The sonic effects resulting from the iPod vs. iPad differences can be expected to vary depending on the design of the dock, the jitter rejection capability of the DAC, and on what kind of interface is being used between the dock and the DAC (e.g., S/PDIF or AES/EBU).