Simple BPSK Constellation?

Hi Everyone,

I've been trying to make a simple DBPSK transmitter for a random data (bit) source in GRC. My flowgraph just uses:

                        Constellation Object
Random Uniform Source > Constellation Modulator > Throttle > QT GUI Sink

However, in the constellation plot I see clusters around -1,1 and a point at 0,0 instead of the expected tight grouping just around -1,1. I've confirmed the source produces a stream of 0,1 (bytes) so there should be no errand points being mapped to 0 etc. Can someone point me in the right direction?

Thanks,
Ralph