These 2 units provide DOLBY A noise reduction. It has long been recognised that the best sounding analogue recording quality is obtained by recording at 15" inches per second using Dolby A, not only does it sound better than recording at 30ips it also halves the tape costs.
Why is this ? Recording at 30 ips reduces the tape hiss but there is a hump at around 100 Hz, in order to remove the hump machine manufacturers try to flatten the response using EQ but this is not fully satisfactory. Recording at 15 ips you get a much flatter response producing a better bottom end but the hiss increases, using Dolby A reduces the hiss by 10 db - Problem Solved.