Demodulate a CPFSK modulated file - how to

i’m trying to demodulate a cpfsk modulation implemented through the
block

gr.cpfsk_bc( )

my scheme is :

bytes_source (a byte vector_source with one byte of stuffing inserted
as first element of the vector)
gr.packed_to_unpacked_bb(1, gr./GR_MSB_FIRST/ )
gr.cpfsk_bc(10,1,200)
file_sink (complex type)

then after different un-succeeded attempts to demodulate the modulated
file through the quadrature demodulator and converting the float output
to byte i was wondering a different approach to demodulate the file by
using the block

gr.constellation_decoder_bc( )

and then removing the stuffing byte to obtain the original file. My
question so is which are , if possible, the parameters to pass to the
constellation decoder in order to demodulate the CPFSK.

thx in advance

 Arturo