Thanks to Murph for starting this intriguing thread, ClubNeon for his sample tracks, and to everyone else that has participated so far. This is a really interesting topic, and one that can easily foster heated debates similar to those discussing the efficacy of different cable dielectrics ad nauseum! *cough* The entire topic of what is and what isn't audible when it comes to digital audio is fraught with complexities and involves serious math, signal processing, and communication theory. To be an expert on this topic is something few can claim, and I'm certainly not one of them. What I do know is that the simple task of taking ClubNeon's sample files and playing them back over headphones plugged into your laptop is probably not going to give you the goods. Why? Simply because most computer systems are not designed for bit-perfect audio playback, which is the first requirement of trying to compare audio files at different sample rates or bit depths. Just because you can play the file does not necessarily mean that it isn't being trans-coded to another format before it gets to your headphones. As an example, if you're casually using iTunes for playing back your music files, there is a slim chance that you are hearing them played back correctly. Then there is the issue, as has already been pointed out, that most commercial material in standard and high-rez formats have been mastered differently, or were created from differing original sources. As a die-hard analogue guy, I am intrigued by the world of high-rez digital, but have never taken serious steps towards educating myself and putting my own ears to work. The first path down this road has been to investigate the idea of home music servers and the various playback and software options that this entails. To say I have repeatedly pulled my hair out is an understatement. The next step has been to talk to people with far more knowledge than myself on the topic and attempt to devise a test that can be performed blind, in a similar manner to our loudspeaker listening tests. Interestingly enough, Ian has also been thinking about this very topic, and we have decided to embark on a research project in the new year which will allow us, and anyone else who's interested, to compare various digital formats in a repeatable manner. To that end we are going to setup a state of the art digital playback system, with the assistance of our friends at Bryston, which will ensure that all of the source material is being passed in a bit-perfect manner. For source material we are going to obtain recordings that were originally created at various resolutions and then down-sample, up-sample, or decimate as required.
Feel free to add your own suggestions and stay tuned!