Principles of Communication - Google Book Search
[http://books.google.co.in/books?id=p9f6ZDbiBNUC&pg=PA256&lpg=PA256&dq=polar+quaternary+format+NRZ&source=bl&ots=LQ087CvTuR&sig=gqXmmu7GayHsnr6br_15nslQyGs&hl=en&sa=X&oi=book_result&resnum=9&ct=result#PPA262,M1] - - public:npranothi