One question still: Is it needed to be able to distinguish the two half waves? So that two half wave rectifiers are better than one full wave rectifier?

No, with my new design its not needed to distinguish between two half waves. In an earlier design it was needed in order to determine which SCR of thyristor was coducting when the signal arrived. It helped in detecting SCR failures. But in new design my zero cross detector can tell which SCR the incoming signal belongs to. Thus a full wave rectifier can satisfactorily do the job. I am in hunt of some single supply full wave precision rectifier better if I can find one which uses traditional easily available op-Amp such as TL082 etc.

Regards,
Prahlad Purohi