What is ALL and AAC

[Alexx]

sorry i don't know

[Jacques]

Advanced Audio Coding (AAC) is a lossy data compression scheme intended for audio streams.

AAC is the audio codec used/recognized by iPod/iTunes

In April, 2003, Apple Computer brought mainstream attention to AAC by announcing that its iTunes and iPod products would support songs in MPEG-4 AAC format ...

AAC has now become so associated with Apple hardware and software that people are commonly of the mistaken belief that AAC expands to "Apple Audio Codec."

[Bananatree]

I haven't heard of ALL, but ALAC is Apple Lossless Audio Codec, or Advanced Lossless audio Codec.

[Alexx]

Thanks guys.

even with my dreadful spelling you were able to support me

[eriktous]

Doesn't ALL stand for Atrac LossLess?

[alieninhead]

If you want to get technical...

The AAC we talk about today is part of the MPEG-4 Spec. I'm not that smart when it comes to reeeeally technical things but what I can tell the difference of AAC versus MPEG-2, Layer 3 (MP3) is that AAC doesn't have one single compression scheme; it has a wide variety of 'profiles' put together for certain tasks. It works in conjuction with all of the MPEG-4 standard to achieve certain things in the Variable Bit Rate department.

One nice achievement of AAC over MP3 is more sampling frequencies--anywhere between 8hz up to 96khz (MP3 goes from 16hz to 48khz).

It also applies mathmatical compression directly instead of indirectly like MP3-with this fact and a larger window size of blocks it handles wave forms better to achieve richer sound. How? No idea. I wish I did, though. I'm just gleaming some info off of a few reference sites. (math is not my thing..)

AAC was first widely used in the iPod, but the MPEG-4 spec was standardized in 1998. Apple uses AAC to deliver content from the iTunes store but wraps the files in a DRM scheme called FairPlay. Similar to what Sony does to put MP3s from your hard drive onto the minidisc.

The other thing about (HE-)AAC is that it uses spectal band replication, much like MP3Pro. From what I can gather (and I may be wrong on this) is that this broadcasts the lower frequencies of a sound and using handy math SBR uses a noise generator to create the asociated higher frequencies of the sound. Kinda like algebra or something. So now your sound file is less in size because it only has to contain low frequencies.

This works pretty well, but its psycho-acoustic in practice. Our brains perceive higher frequencies to be harmonic with the lower frequencies. So it works well enough a large percentage of the time, but can be off.

My brain hurts. I can barely comprehend what I'm typing..but the key is, I can comprehend it. Which amazes me...

So in more laymans terms..a sound is played/broadcasted/whatever out of the file and into the SBR decoder. It uses information in the file (AAC, MP3Pro, certain ATRAC(all?) encoding) to serve up some guidance info (leveling, ranges, etc..) and then reconstructs what the higher frequencies must have been in the original file based off of sheer mathmatics.

Keep in mind SBR is used in conjunction with an audio CODEC. The audio CODEC merely encodes the lower frequencies of the sound file and then feeds it into the SBR decoder. So you don't have to exactly chop away at the audio sources bit rate, just disregarding its higher frequency information.

I wish I had a knack for words..I'll stop now.

~alieninhead