In short it is servo on the bias as they explained. Tape bias is non audible high frequency recorded with music that reduces distortions of the tape. Bias also reduces upper bandwidth so usually some compromise is taken. Because high frequencies in music also work as bias, controlling total amount of them at given moment will keep distortions low while extending bandwidth. System that adjusts amount of bias to keep constant bias+musicHF is called servo and it is the best thing since sliced bread (I had Aiwa deck).
Dolby HX PRO Dicrete Circuit.
My AIWA cassette(AD-R707) has this feature. If anyone can explain what this means in practical terms, pls do.
From the manual
DOLBY HX PRO Discrete Circuit
" When a source containing high freq components is recorded, these components function as a bias and the high freq characteristics deteriorate. DOLBY HX PRO is a servo control system which always provides the most suitable value of bias by calculating the recording signal & the recording bias current as a whole. A metal-tape-like performance is attained even with LH tapes, because the freq response is kept flat at all times by stabilising the dynamic freq characteristics. Furthermore the DOLBY HX PRO substantially expands the high freq dynamic range.
This is not a noise reduction system.
The system functions automatically when the deck is in the Rec mode."
From the manual
DOLBY HX PRO Discrete Circuit
" When a source containing high freq components is recorded, these components function as a bias and the high freq characteristics deteriorate. DOLBY HX PRO is a servo control system which always provides the most suitable value of bias by calculating the recording signal & the recording bias current as a whole. A metal-tape-like performance is attained even with LH tapes, because the freq response is kept flat at all times by stabilising the dynamic freq characteristics. Furthermore the DOLBY HX PRO substantially expands the high freq dynamic range.
This is not a noise reduction system.
The system functions automatically when the deck is in the Rec mode."