Audio question related to the fader and XM radio

Gcole

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So I was adjusting my radio settings and noticed something strange if I push the fader control to the to full rear on XM radio only I only get the Subwoofer and no sound from the rear speakers, if I do the same using any other audio source FM, AM, phone and even a IPOD I get rear speakers, they aren't great but at least there are there.

I have turned off Active Sound with Bimercode and have surround off, turn on surround and the speakers fire.

Has anyone experienced this?
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30MilesOffshore

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XM radio uses an variant of AAC HE codec somewhere between 24–48 kbps. The sample rate of the played back audio is likely 32 kHz-44.1 kHz which will yield 16–22 kHz of audio bandwidth. However, the amount of compression is very noticeable, particularly background noises, detail and stereo separation are all compressed.

It helps them cut down on massive data transmission and increases the number of users online at the same time. Plus they are contracted with the car manufacturers on new cars, and they know the car is equipped with Surround Sound so it saves them from having to put up more satellites or increase their data transmission.
 

consultadrone

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XM radio uses an variant of AAC HE codec somewhere between 24–48 kbps. The sample rate of the played back audio is likely 32 kHz-44.1 kHz which will yield 16–22 kHz of audio bandwidth. However, the amount of compression is very noticeable, particularly background noises, detail and stereo separation are all compressed.

It helps them cut down on massive data transmission and increases the number of users online at the same time. Plus they are contracted with the car manufacturers on new cars, and they know the car is equipped with Surround Sound so it saves them from having to put up more satellites or increase their data transmission.
In the end, aren't all basic stereo signals (music anyway) just 2 channel signals...left and right (hence the term 'stereo')? Whatever the CoDec used to process signals, the amplifier input is just 2 channel sound, right? So, if the car has multiple speakers (front, rear, left, right, center, subs, etc), isn't the signal merely processed through a system that takes those 2 input channels, amplifies them and farms them out through the speakers?

I have XM in all my cars. Some have more advanced audio features than others, and the signal is processed out through all the speakers. It seems that someone on the BMW engineering team made the decision to reduce the output of the XM source to the front speakers and subs only when surround was not enabled. And ONLY for the XM source. The 2 channel input signal from FM, Bluetooth, and USB are output to both front and rear speakers (and subs).

Just seems weird that they would choose that source to do that with. I mean, Bluetooth is also heavily compressed on top of whatever mp3 compression rate is being used.

I have built my fair share of car audio systems, but I'm definitely not an expert audio engineer by any means so I could be missing something.
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