PAC and AACPlus where not terms I was familar with so I did a bit of googleing.
"aacPlus delivers superior quality at a fraction of the bit rate compared to Layer 3, and is optimized for excellent reproduction of both voice and music. Listen to the audio files in the demo below to hear for yourself why aacPlus has been called "the best low-bit rate codec on Earth." Bit rates were chosen to showcase results at common POTS connection speeds. "
telos-systems.com
"It seems crazy until you try it, but Mostly Classical proves that aacPlus can sound great at 24 kbps. At 48 kbps, it's almost as crisp as a CD. At 128 kbps, it can deliver 5.1 channel surround sound. AacPlus works by combining three technologies, each of which shrinks the size of an audio signal. The first is AAC, the Advanced Audio Coding technique that Apple licensed from Dolby for iTunes. AAC analyzes the sound and throws away any data it knows human ears won't be able to hear, which is a lot. Then, aacPlus adds Spectral Band Replication, which strips out all of the music's high frequencies and replaces them with a tiny bit of analytical data. AacPlus players reconstruct the highs as a mathematical function of what's left. As a final space-saving trick, aacPlus tracks are recorded in parametric stereo. Instead of a left and a right channel, one channel is the sum of the left and right signals (L+R), and the other is their difference (L-R). This takes up less bandwidth, and the player can easily flip the two channels back to the original left and right. (Bonus trivia: This is how FM stereo broadcasts work.)"
slate.com
"XM Drops PAC for CT-aacPlus
Washington, DC - Apr 18, 2002 - At a press conference held in New York City, XM Satellite Radio unveiled its plans to use a customized CT-aacPlus audio encoding algorithm with neural audio optimization. XM states that the new algorithm will provide superior sound quality that is remarkably close to compact disc. "
industryclick.com
"PAC or no?
The selection of CT-aacPlus means that XM will not
use Ibiquity's Perceptual Audio Codec. An XM
spokesman said, "PAC is a fine technology. But we felt
CT-aacPlus, enhanced by the Neural Audio
preprocessing technology, was the best for our
platform." "
72.14.203.104
General if old information about XM
arstechnica.com
And then a comment that agrees AACPlus is superior, but still rates XM's sound quality as poor (while perhaps hinting at Sirius' being even worse)
"
I just got a XM radio (Delphi MyFi) and I'm surprised at just how bad the quality was. When I first heard it at Best Buy I dismissed the poor quality as being a poor install at Best Buy. But it wasn't!
And it's worse than you think Jake!
XM uses aacPlus compression, with Neural Audio broadcast audio processors in front of them. My understanding is that most of their music channels are 40kb average bitrate. aacPlus is optimized for sounding CD-quality at 64kb and there is no reason to use aacPlus at over 64kb (you would just use normal AAC-LC).
In my experience, XM has their high end rolled off, not as part of the codec but apparently as part of the setting in the Neural Audio processors. This is typical for FM broadcast, as FM broadcasts have pre-emphasis and you can't modulate the high frequencies as much as you can the low frequencies because of that. A typical broadcast processor will have much lower limiting thresholds in the high freqs. However, it set up properly for digital use, you wouldn't use those settings. The aacPlus codec even at 40kb can pump out lots of high frequency content.
Sirius uses a variation of PAC, pac V4 I believe, and they use Orban 6200 DAB broadcast audio processors. They run the PAC in varialble bitrate mode and stat mux all the channels together.
It's my understanding that XM uses constant bitrate, 80kb for the most important music channels, 40kb for the music channels and 32kb for the talk channels.
PAC is not as good as a compressor as aacPlus, you'll hear a lot more artifacts in it when the bit rates are reduced.
Personally, I like the sound of Orban processors and aacPlus codecs. The Neural Audio stuff just sounds dull and lifeless to me."
hear2.com |
