Apparatus, methods, and computer program products for detecting or locating a viable data carrier in a downstream channel by a cable modem. Potential channels for data carriers can be eliminated quickly by going through a "screening" process, i.e., the first constellation diagram match, thereby preventing those potential channels from going through a significantly more time-consuming second constellation match. The determination of whether the signal in the potential channel is modulated according to the first particular modulation scheme is done rapidly and only potential channels containing a signal likely to be modulated according to the second particular modulation scheme are examined for the second determination step.