I will chime in with my personal tests and experience.
Stereophile thread about another player but discuss the Ipod truncating - http://www.stereophile.com/content/astellampkern-ak100-portable-media-player
I'm testing with Ipod classic v5 from the mid 2000's. USB with no mods and using itunes.
The author of that article is right - but he just scratched the surface. I did sonograph tests with Spark XL and Mackie U.420 which has the unique ability of
being able to feed back into the 24bit chip set of the Mackie with whatever is plugged in (ipod to usb on mac) or played internal on the computer with the
push of a button.
What I saw on the 'scope' is when the 24/48 AIFF raw file from a 24/96 aiff source from vinyl with high peaks at 32khz originally and all high peaks
enhanced with an eq before I stepped it down to 24/48 to make a 24/48 ALAC file - so there would be no mistake if the Ipod was playing back at the resolution
intended.
I must add - this would not conclude if it was playing back at 24bits, but it will show clearly if it's capable of playing back to 48hkz.
What it does in this test, with BOTH the AIFF file and the ALAC at 24/48 each is it clearly shapes the sound and truncates. It pushes all the high frequencies down and while
most peaks were at 22khz and above on the files before they were loaded on the ipod, now most hover at 19khz. The weird thing is it shows these odd spikes
at the highest voltage peaks and frequencies and it reads at above 48khz! But everything else is pushed down. I don't know if this is some sort of additive algorithm or what
but it's nonsense because it's sharp spikes but THEY don't show up unless I push the volume above unity gain on the Mackie.
So I wasted some time preparing 24-44 ALAC files of my vinyl to play on my ipod IMO - I started with 24/48 but the ipod had a hard time with some material at that
rate and it seemed to be a buffer issue .
My next test will be to do the same thing with 16/48 files to see if it's doing the same noise shaping or if that's just the process it uses to step down from 24 bits to 16.
Very weird stuff here - and as some other person mentioned in this post - it makes no sense if it was loaded with the chipset that is capable!
(Yes I had the EQ off in itunes and no 'sound enhancement'
If this thread is expanded and people need proof I will do screen shots of the sonograph and the clear shaping.