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?

[C.R.Helmrich]

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?

Correct. The SBR start (or cross-over) frequency is chosen by the HE-AAC encoder from a table of possible frequencies (or rather, fractions of the sampling rate). The creation of the table is described in the ISO/IEC standard and a bit complicated, so I won't go into detail. Just so much: the minimum start frequency is around 4 kHz, the maximum is half the sampling rate (in "downsampled" SBR mode) or a quarter of the sampling rate ("normal" SBR mode). So what saratoga mentioned is just one possible - and rare - configuration.

Chris

[saratoga]

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?

Correct. The SBR start (or cross-over) frequency is chosen by the HE-AAC encoder from a table of possible frequencies (or rather, fractions of the sampling rate). The creation of the table is described in the ISO/IEC standard and a bit complicated, so I won't go into detail. Just so much: the minimum start frequency is around 4 kHz, the maximum is half the sampling rate (in "downsampled" SBR mode) or a quarter of the sampling rate ("normal" SBR mode). So what saratoga mentioned is just one possible - and rare - configuration.

I didn't realize that. How does that work with LC backwards compatibility? Would you get a badly distorted file when listening on an LC only decoder?

[C.R.Helmrich]

I didn't realize that. How does that work with LC backwards compatibility? Would you get a badly distorted file when listening on an LC only decoder?

Depends on what you consider "badly distorted". It's playable but the frequency bandwidth is limited to that of the AAC LC core coder signal, so it will sound dull/muffled. If the SBR cross-over frequency is 4 kHz, it will probably sound like coming from a telephone on an LC-only decoder.

Chris