Today, I did a test in a shop. We used the HU output as the input of Audison VRx and LRx. Here is the test procedure
1) HU left front output --> VRx RCA
The VRx support balanced signal and has a special RCA. Audison call this ABS(Audison Balanced Signal). There are 3 wires in this special cable: signal+, signal-, ground. BMW HU has a 2 wires per channel output, in this case, we named it as LF+ and LF-. We connected the LF+ to signal+, LF- to signal-. Not use the the 3rd wire(ground) in ABS cable. The result was VRx worked well as it supposed to be.
2)HU left front output --> LRx RCA
LRx has a normal RCA (2 wires, signal and ground). First, we only connected LF+ to signal wire and no touch RCA ground. The speaker sounds nice. Then we connected the LF- to RCA ground, the sound is nice too and twice aloud. This prove that LRx low level input can accept the balanced signal
3)HU left front output --> LRx Speaker level input
The sound is ok but with enginee noise which did not occur in test 1&2. This means that LRx high level input can accept the balanced signal but no noise cancellation function. Seems that high level input circuit only use one signal and discard the other.
Now, I have a little question. Since LRx support balanced input, what is the benefit of VRx using ABS?
Not test the bit one since I have not got it. Based the test above, I believe its low level input also support balanced signal. Will test it when I get it.