It doesn't. Duobinary encoding takes advantage of predictable ISI (Inter Symbol Interference) to recover the original data stream. Any time you transmit N bits per second, the Nyquist demands you use a bit rate of N/2Hz—if you want to avoid ISI. But if you can tolerate some ISI because you can remove it by clever filtering, that limit—while it still exists—doesn't apply.hi
How does duobinary sequence has double spectral efficiency compared to binary data while having a one-to-one correspondence with the simple binary stream of data?