Bit transparency bug in Cubase 10 mixer-engine

I’ve encountered what i think is a bug in the mixing-engine, or at the very least a phenomenon which calls for an explanation.

I was verifying the bit-transparancy of my system (Cubase 10 Pro on Windows) using the test-files files for my RME ADI-2 Pro converter ( When the signal-path is bit-transparent, the converter will display a “Bit test 24bit passed” message in the display.

This works in Cubase 10 as long as the bit test is the only track in the arrangement. As soon as i add an empty track, the bit test no longer passes. Fader position or mute state of the empty track does not make any difference.

However - if i export a mixdown, and import this into a new arrangement, then the bit-test passes again. So - it seems this is a playback only issue!

Anyway … i did a loopback recording of the failing test (using the internal RME mixer) and inspected the files in a hex-editor - and it turns out every other sample in the right channel changes between zero and the smallest possible sample-value in 24bit. Obviously i also did a loopback-recording of a passing test, and this still passes after the loop.

And - for what it’s worth this bug does not exist in Cubase 6.5.5, which is the only other version i had around for testing!

So - to sum up:

It seems cubase is adding 1 bit of noise to the right channel - but only on the playback, not the mixdown. My main suspect is the MixConvert plugin, which will cause the bit-test to fail if inserted on the track - and i know this is used in the output / control-room path … but it still doesn’t explain why adding empty tracks results in added (inaudible) noise!

Can anybody verify this issue?

Rune / FishCorp,

I’m happy as long as the exported audio is perfect, but of course once you find something like this you want to know whats going on:)
Looking forward to plausible explanations. Could it perhaps have something to do with 64 bit processing (at least that wasn’t around in 6.5…)