Philips and Dolby HX-Pro

Inside the Philips FC920 cassette deck from 1993. Recent purchase for a very small amount of money, to explore analog cassette tapes with better sounding equipment.

This machine was equipped with Dolby HX-Pro which improved the overall sound of pre-recorded cassette tapes. It wasn't a noise reduction system like Dolby B or C, but was a dynamic signal bias system, implemented during the tape transfer process.
Standard biasing mixed a high frequency fixed signal  to the source, to make the signal more linear (and better sounding).    HX-Pro (invented by Bang and Olufsen), made the bias dynamic, by reacting to the high frequency components of the music,  inside a feedback loop.

The NEC chip implemented the HX-Pro function in the cassette deck.

The cassette decks also had to be biased for different tape types, Ferric, Chrome or Metal, during production. You can see the adjustment components next to their text.

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