Stereo Dual Dubbing Cassette Deck with Dolby B/C NR 
(140-0644)                 Dolby B and C Described    Faxback Doc.# 18023

Your cassette recorder might incorporate a Dolby C-type and B-type noise 
reduction system.  The newer Dolby C-type NR system starts enhancing 
program (and noise) from 100 Hz and rises 20 dB at 1 kHz and above when 
recording, and reduces program (and noise) by the same amount when played 
back.  The standard Dolby B-type NR system does the same from 500 Hz, and
rises 10 dB at 4kHz and above.

With Dolby C NR and a good formula tape such as our MIV Metal, tape noise
will be below any program material you record- even when you play back at
very high levels.  Note that Dolby NR system reduces only the noise during
the recording process, any noise from the original program source cannot be
reduced.

Making an Ordinary Recording

1.  Music

    Music is made of sounds of different loudness separated by intervals
    of silence.  

2.  Noise

    Any recording tape, even the best kind, makes a constant hissing noise
    when played.  At the very slow speeds and narrow track-widths used in
    common tape cassettes, tape noise is much more noticeable than it is 
    professional tape recordings although even there it is in a problem.

3.  Music and Noise

    When a tape recording is played, the noise of the tape conceals the 
    quietest musical sounds and fills the silence when no sound should be
    heard at all.  Only when the music is loud will the noise be masked and
    usually not heard.  However, tape noise is so different from musical
    sounds that it sometimes can be heard even then.

Making a Dolby NR System Recording

1.  What the Dolby NR system does first

    Before the recording is made, the Dolby NR system "listens" to the
    music to find the places where a listener might later be able to hear
    the noise of the tape.  This happens mainly during the quietest parts
    of the music.  When it finds such a place, the Dolby NR system
    automatically increases the volume so that the music is recorded louder
    than it would be normally.

2.  The Recording

    In a Dolby NR system recording the parts of the music that have been
    made louder stand out clearly from the noise.  As a result, Dolby NR
    system recordings sound brilliant and unusually clear even when played
    back without the special Dolby NR system circuit.

3.  What the Dolby NR system does during playback

    When the tapes are played on a high-fidelity tape recorder equipped
    with the Dolby NR system circuit, the loudness is automatically reduced
    in all of the places at which it was increased before recording.  This
    restores the music to its original loudness again.  At the same time, 
    the noise that has been mixed with the music is reduced in loudness
    by the same amount - usually enough to make it inaudible.


(PH 10/31/95)