Confused by the different versions of AAC (LC, HE, HE v2, SSR, etc.); LC vs HE

2019-10-18 17:04:58

Hi all -- I'm confused by all the different versions of AAC and was hoping you could clear some things up for me. I've read the Wiki, which was helpful, but I still have some questions.

• First, am I correct in understanding that Apple uses AAC LC with iTunes tracks? Does this apply to songs purchased from the iTunes Store, or songs ripped using the iTunes app as the encoder, or both?
• Why would Apple use the Low Complexity format? Wouldn't that translate into lesser audio quality than regular AAC?
• Which yields more transparent audio at the same bitrate -- HE or LC?

Thanks for your help.

Re: Confused by the different versions of AAC (LC, HE, HE v2, SSR, etc.); LC vs HE

Reply #1 – 2019-10-18 19:05:01

2) Low Complexity is regular AAC.
3) At higher bitrates (say, higher than 80 kbps) LC is better than HE.
At lower bitrates, (lower than 80 kbps) HE is better than LC (note that at such bitrates both LC and HE are not transparent, but HE-AAC sounds better than LC-AAC).

Re: Confused by the different versions of AAC (LC, HE, HE v2, SSR, etc.); LC vs HE

Reply #2 – 2019-10-18 20:46:35

AAC-LC is normal AAC. AAC-HE(v2) is a high efficiency mode used for very low bitrates (streaming radio, audiobooks, etc) when quality is less important. The other modes like SSR are defunct.

Re: Confused by the different versions of AAC (LC, HE, HE v2, SSR, etc.); LC vs HE

Reply #3 – 2019-10-19 00:14:01

Hi (please excuse my English).

1. As far as I'm aware, Originally it was 128 kbps, then there was iTunes Plus which had the increased bitrate of 256 kbps, however now all songs purchased from the iTunes Store are 256 kbps VBR AAC-LC (Stereo). Typically you append -LC when making a distinction is needed between other AAC profiles or codecs.

Songs can be ripped in iTunes in different formats:
AAC (LC, HE - also called HE-AAC v1, CBR & VBR, but not all options will work together, iTunes will tell you if you try).
AIFF
Apple Lossless Encoder (also called ALAC, has .m4a extension like AAC files)
MP3 (Various settings)
WAV

Hope that helps.

Re: Confused by the different versions of AAC (LC, HE, HE v2, SSR, etc.); LC vs HE

Reply #4 – 2019-10-20 11:35:21

There are two pictures on the Wikipedia page of AAC, which I think are quite helpful:

https://upload.wikimedia.org/wikipedia/commons/3/37/AAC_profiles.svg

https://upload.wikimedia.org/wikipedia/commons/4/43/HE-AAC_and_HE-AAC_v2.svg

Note that AAC-LC is technically the baseline encoding system, and algorithms are added to that, to increase compression efficiency while making compression and decompression more complex. However, things like Spectral-Band-Replication or Parametric Stereo only yield better results, if the compressed capacity is already quite constrained. Above a certain level, which depends on the audio and the capacity allotted to the compressor, those additional algorithms, fail to produce any increase in perceived in audio quality.

If you want to see some results of those kind of tests, I suggest you search for "MUSHRA HE-AAC test" (I don't want to plagiarize or hard-link them here) and the likes. Results of listening tests like these, show that there's some average bitrate above which things like parametric stereo at best yield no difference to not using it. At worst, it introduces artifacts which worsens the perceived audio quality, compared to not using it at all.