HE AAC Audio Quality & Tuning Options

[GreenSeer]

I am new to the lossy coding world. I recently became a DROID owner and, in an effort to get down to carrying one device, I ripped all of my music to my DROID at 64 kbps. In the environments I often listen in (Airports, Planes, Rental Cars, Taxi's, etc.) this is good enough for me and has the great benefit of using minimal storage space. The efficiency of a codec that provides satisfactory audio quality while using lower bitrates appeals to me.

My question is whether this codec is still being developed and "tuned"? I guess I am curious as to whether HE-AAC is now about as good as it will ever get. For example, is the crossover point from real sound to where the SBR begins as finely tuned as possible? Will we ever get to the point where a 48 kbps rip is perceived qualitatively by a general audience like a 128 kbps AAC file is today? Thanks!

[GreenSeer]

Is there a reference or an explanation somewhere for what frequency SBR begins when using HE AAC? It would seem to me that the LC "cut point" would be higher in frequency the higher the bit rate used, so that 48 kbps HE AAC has more use of SBR, relatively, than 80 kbps? Is this correct?

[saratoga]

Is there a reference or an explanation somewhere for what frequency SBR begins when using HE AAC?

SBR kicks in at 1/4th the sampling rate, so for 44.1khz audio its 11.025khz and above.