Alexandre,As far as I understand it, the .sync attribute is more like an identifier, rather than a true numeric value (that is, 0 says you are syncing frequency with frequency; 1 says that you are syncing phase with frequency; 2 says you are doing true Frequency Modulation). As you are telling that no value means no sound, one can infer that there is no default value for that, and if you are feeding UGen => UGen, telling which is your sync method is mandatory (but I can be mistaken, as usual).