DSP Fun

BMW’s DSP system has thrown a wrench into many upgrade plans. While state of the art, perhaps for the time it was introduced, it complicated the aftermarket because instead of easy analog inputs on the CD changer connections, we have a digital signal directly from the CD changer to the amplifier via a SMB connection on a 50ohm cable. The aftermarket has provided some A/D converters specific to this purpose. I’ve always had the intention of playing around with the DSP system to see if we can help you DSP guys out a little better than what’s out there. What is cool is we are working on a hardware revision for our Bluetooth Adapter, and I realized that we can keep a digital stream all the way through the system. No need to convert to analog and then back to digital for the amplifier. I noticed that our BMW CD changer output is 4x oversampled, which helps during the analog reconstruction. According to the Nyquist theorem, we have to sample at twice the highest frequency we want to reproduce, which for a human ear is roughly 20KHz, meaning that CDs contain a sample rate of 44.1KHz. However, aliasing occurs at every 44KHz, so we have to use low pass analog filters after the DSP amplifier’s DAC to cut the aliasing. However these low pass filters have a phase response which distorts the music, and by using 4x upsampling (perhaps one valid sample and 3 zeros), our aliased images now occur every 176KHz, making it easier to build a low pass filter that will not introduce phase distortion. What I need to find out is whether the DSP amplifier always expects 176KHz samples or can deal with 44.1KHz samples. Some people have had success giving the DSP amplifier any sample rate 44.1KHz or higher, specifically 48KHz – a common sampling rate these days, but mentioned that the music through the 48KHz converter never sounded as good as the CD audio from the changer. Whether it was because of the multiple conversions D -> A -> D(48KHz) -> A, the sampling rate or perhaps the DSP amplifier analog reconstruction filters don’t attenuate aliased images below 176KHz very well, I’d like to know so we can get something working great for you. If you have any insights into this, let me know. And for those DSP customers who cannot use our aux input kit right now, we are working to see if we can put together a better converter for you as well.

We also want to mention that if you need help with your aux input kit, please email us and we will help you out.

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