Bill N said:
Thanks to all for the help. I can live with the lower volume for now but it's not a factor with the portable XM receiver that I have in my other vehicle. It functions through the cassett deck in my Audi and it relies on the Bose system for volume and balance. XM may put out a lower volume than FM stations but I believe I may still have a problem. I doubt the dealer will agree but I'll find out next time it's in for service. Will let you know the outcome :seeya
From what I've heard the referenced service bulletin just says that the volume difference is normal.
Basically, everything should be referenced to a standard audio level, which in many systems is 0dB. This 0dB level could then correspond to a known voltage level which could then be sent to your amplifier. This way all signals would result in the same output level. Pretty simple...
Unfortunately the real world doesn't work that way. If I have a audio that is really low, like a soft violin piece, I have multiple choices. I can record it at that level and say this is the way it should sound but then also have noise issues. Or, I can move the level up so it is about the same as other recordings. Or, I can move it as loud as possible so it fully takes advantage of the recording's dynamic range and allow the listener to lower the volume if they choose. All of the above are valid but they all produce different volume levels. A claim can also be made that all of the above are properly calibrated at 0dB.
Compounding this is audio compression (not to be confused with bit-level compression). As I reduce the difference between the lowest volume sound and the highest volume sound, the sound becomes more compressed. Your ears perceive highly compressed sound as louder versus large dynamic ranges. So, what appears to be louder (like FM radio - usually very compressed) may not actually be louder than a CD (usually the least amount of compression, if any at all).
One more factor is that for digital transmissions, the worst thing you can do is attempt to go beyond the highest level. Since there is nothing beyond all '1's' you get a flat line which rougly sounds like your speakers are being ripped apart. In the digital world it is much better to be too soft than too loud. You also don't want to send a signal to the amplifier that could cause the amplifier to clip. I don't see that as likely with the XLR since the CD player does fine.
What does it all mean in terms of the XM radio's volume? Well first it means this isn't an easy problem. XM, while audio compressed, is not as highly compressed as FM, so some level difference is to be expected. I also think that the XM level is lower than it should be - I suspect the analog output voltage from the XM receiver to the amplifier is slightly low. However without understanding the system it is hard to say.
Anyway, just thought I'd write a few paragraphs to help explain some of the issues. I've left a lot out such as input voltages to amplifiers and equalization, etc.
Andy
