Originally Posted by sonitm
It's normal. Your mp3 files are at best 128 kbs while the CD files are over 900 that is one of the reasons. Secondly, even with a good quality cable, there is still signal loss, compared to the CD where there is none. So look for good quality mp3 files, 128 or 320 kbs and use a short gold plated cable. EQ on iphone should be neutral. On the car stereo bass should not be max and treble on 70-80 %. Hope this helps.
If you're actually playing 128kb CBR MP3s, that's certainly going to be a major source of quality degradation compared to a CD, but it's pretty unlikely to be the case these days. If you bought the songs from iTunes or ripped them using iTunes, you're talking about higher bitrate AAC, which is going to sound much much better. If you're pirating music, there's no way you're getting less than high quality VBR or 320kbps CBR MP3s, which will sound grate. Unless you're using gnutella or something, but then, come on. If you're using modern music streaming services like Rdio or pandora, if you've got the quality set to high, you're talking about 192kbps AAC, which will sound great.
The cable won't make much difference, as long as you're not using some 15 year old, internally fraying POS. The gold plating won't make a difference in sound quality unless the connection has been sitting there plugged in for months or years, either: gold has far worse conductivity than copper, and only slightly better than aluminum. The point of gold is to prevent corrosion, it's highly non-reactive.
Originally Posted by M0.92
Don't think anyone has mentioned the Aux volume setting on your head unit. If it's set at max, as mine was, it will sound horrible. I turned it down to mid point and set my iPad volume to max volume. Sounds great now.
You're right but then simultaneously wrong. Almost no device is designed to be cranked all the way up without distortion - you always over-design your output power stage so that, in the case of a file that's encoded slightly softer, you can still recover what you think is a normal volume. For properly mastered music, the top few percent of volume range on a device will be overdriving the output and distorting things.
In fact, this is the exact reason the input volume control exists on the headunit. Obviously, you could just turn up the main volume a little. But if you have a quiet MP3 player or other device, the idea is that you set the input volume appropriately to get the same volume when you switch back and forth to radio or CD.
Suffice it to say, cranking the input volume all the way up is usually bad, cranking the main volume all the way up is usually bad, and cranking the iPhone/iPod volume all the way up is always bad (leave it one or two notches down, and just turn the input or main up a tiny bit to compensate). That said, the output of at least the iPhone 5 is better than it used to be. If you don't have a keen ear, you MIGHT not notice the distortion on the top notch. But if you wait for the song to get loud, it's there and you'll notice.
Originally Posted by wrose100
When will bmw come out with an iphone 5 y-cable. That would negate this issue tremendously
it's actually pretty irrelevant. The lightning to 30pin cable plus the current Y cable is exactly what a lightning Y cable will be, just all integrated and wildly more expensive from BMW. As it stands, the 30 pin adapter not only bypasses the internal output amplifier of the phone, but also the DAC - it includes its own in the 30pin housing. So the adapter DAC feeds a built-in preamp which then sends a line out to the Y cable and head unit.
All in all, the most likely source of a noticeable quality difference between the CD and Y cable/aux in is in the analog input stage of the aux in path itself. Seems they may have simply done a better job in the headunit of signal conditioning from the CD player DAC. Or perhaps the DAC they used on the CD player is better than that in the iPhone and in my case lightning adapter.
All that said, I definitely notice a subtle difference between the CD deck and the same song played from my iPhone via Rdio through the adapter and Y cable and aux in.
I'll have to dig more.