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Am I right
No.
The hiss which Dolby noise reduction mitigates is natural noise inherent in the physical recording medium, due to the heterogeneous nature of the magnetic domains. Dolby compresses the input to move low amplitude signals further above this noise floor, then expands the output to reverse the effect.
The high frequency signal deliberately added to make tape recording work with any kind of acceptable dynamic range is bias. It can be removed from the output with a simple low pass filter, but as it's both inaudible and beyond the reproduction range of most speakers, it doesn't have to be.
gbj_tester
mwollny
Am I right in thinking that the hiss is added in the recording, then filtered out by Dolby B?