The compact disc.

I buy CDs, rip to FLAC for convenience but there is an audible difference when compared to MP3, I have some compilation albums in MP3 format (given away as samplers) and I know when listening to my music on shuffle if i've been served the version on the compilation album or the one i've ripped myself, it's just a bit off, it's mostly from hearing both songs a lot.

I used to listen to music via pretty shit earphones (usually Philips sport earphones which have an earclip though i've had all sorts over the years) now I use a pair of AKG Y500 headphones, my headphones, initially, didn't sound much better than my old headphones but going back to my old headphones is like listening via a speaker in a dustbin, all the faults are much more evident.

It's the same with media formats, once upon a time in the dark ages of 256MB MP3 players i ripped to 64K WMA files and they sounded fine, they sounded roughly like the CD did in my portable CD player with it's 1 or 2 watt speakers. However, going back to them after ripping to FLAC was horrendous and I felt embarrassed that i used to tolerate such bollocks.

So while I probably could take a song I was unfamiliar with, do a side by side comparison with and hear the differences immediately I know that I could hear both over a long period of time and start to notice the differences. Usually it's in the rumble of the drums or the wash of a cymbal but it's definitely there even if it's in he fringes of the sound rather than the core.