The sound quality of the sample rate conversion in HD (Pro Tools 5.3) has been improved considerably, and optimized for the new higher sample rates. The improvement mainly occurs when using certain pull-up pulldown rates. For example, converting from 44.144kHz to 48kHz. Converting from other more standard rates such as, 44.1k to 48k has also been improved in many cases. Using the "best" or "tweak head" settings you can now do sample rate conversion with pretty close to a 144dB signal noise ratio and less than 0.001 db pass band ripple. Also, there is pretty much no difference in quality when converting from common rate such as 44.1k to 48k or 44.1k to 96k compared to 44.1k to 88.2k or 48k to 96k. Also, the algorithm is on average about 10 times faster than before.