See notes in Bristol bug id 2444745.
The bitone oscillator, due to generating complex waveforms, does not lend itself to be a synchronising source oscillator. The signal contains too many positive leading edges to allow a decent synchronised output. This enhancement requests that the oscillator generate a separate sync signal, a simple wave at the base frequency. The overhead in an already intensive piece of code is not excessive and it is required by 6 emulations.