# What Does Sirius Broadcast in?



## Rocko62580

I am ready to try Sirius in my car after being told that it sounds much better than the Internet steam. What does Sirius broadcast in for their normal satellite feed? 128k? 160K? I know the Internet stream is 128K, but is their normal satellite feed at a higher bit rate? I had Sirius on my Direct TV for a while, and even that sounded average at best. Hope the direct satellite route will be better for sound quality.


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## rudeney

Sirius uses an algorithm that is not as straight forward as typical MP3 compression. It's something called PAC that varies between an equivalent of 128k and 320k. I'd say the sound quality is decent, falling somewhere between CD and FM radio, and closer to CD. 

I've had two different cars (BMW and Nissan) with factory Sirius tuners. I now have a car with a custom Pioneer AVIC system using an SCC1 tuner module. Needless to say, the Pioneer system sounds much better than the factory systems. I also have a Sirius Sportster radio that I connect to my home stereo system and sometimes use with a portable boombox and I think it sounds better than the factory systems. 

I have used the Sportster and its built-in FM transmitter in another car and the sound was OK, but not very good. Basically, I'd definitely go with a hardwired system and the best case is an aftermarket tuner with the SCC1.


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## Scott in FL

Also, I believe the data rate is not constant. I've read Sirius uses a stat-muxed variable bit rate. A pretty good description of the technology can be seen here: http://en.wikipedia.org/wiki/Sirius_radio


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## Rocko62580

Scott in FL said:


> Also, I believe the data rate is not constant. I've read Sirius uses a stat-muxed variable bit rate. A pretty good description of the technology can be seen here: http://en.wikipedia.org/wiki/Sirius_radio


Thanks. So would you say that the actual direct satellite signal is better than their streaming signal?


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## rudeney

Yes, the native sat signal is far superior. Even their highest bandwidth online listening is just 128k, and that's the "minimum" for the VBR used by PAC.


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## djbrettb

rudeney said:


> Yes, the native sat signal is far superior. Even their highest bandwidth online listening is just 128k, and that's the "minimum" for the VBR used by PAC.


Has this recently changed or something? IIRC, didn't they originally use something awful like 48kbps AAC? Or was that XM that I'm thinking of?


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## BuggyBoyNYC

Actually, the bit-rates used with Lucent PAC are much lower than you would think. I know first-hand from being in the NYC Sirius studios many times.

Sirius uses Statistical Multiplexing (StatMux for short) which is just variable bit-rates that change on-demand as is statistically calculated by the Lucent equipment.. It lets them optimize each stream within a designated range, depending upon the audio need many times per second. For example, a 1 sec sound clip that is very loud with lots of highs would require the most bandwidth.. While silence would put the bit-rate possibly down to near 0. This is a more efficient process (vs. static or set bit-rates) when you are broadcasting many channels within a narrow amount of overall bandwidth. DirecTV and DISH Network use the same StatMux technology with their broadcasts.

The average music channel averages 30-50 Kbps, some as high as 60 Kbps. The allowed averages per channel vary, and are assigned by Sirius's NOC personnel under the guidance of the program directors and types of genres. For instance, Classical music genre's get higher priority over 40's on 4. Talk averages around 25 Kbps, and traffic channels average around 12 Kbps.

These bit-rates are far lower, but they are not equal to the rates used with MP3. They are more equivalent to the bit-rates used with AAC/mp3PRO formats.

From a source audio feed (direct from the Sirius/XM music database) a 50 Kbps average VBR stream in PAC should be about average to a 128-160 Kbps MP3.

Technically, the XM AAC+SBR (Spectral Band Replication) audio codec is more superior. However, the audio quality can change as the broadcasters needs change. When you've got a ton of sports events to carry one week, you may need to pull more overall bandwidth out of the system to supplement, and other/all channels suffer because of it. So just because the XM codec maybe superior, it does not necessarily mean that XM is driving enough bit-rate for each channel to make an audio channel sound better than Sirius.

The attached picture is from the Lucent StatMux interface. Sorry, it is a little bit blurry.


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## djbrettb

Really awesome information, thanks!


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