[net.audio] A short intro to tape noise reduction

wjm (12/13/82)

As I mentioned in an earlier article, there are 3 types of tape noise reduction
in common use:
Dolby B (10 dB of noise reduction)
Dolby C (20 dB)
dbx (>30 dB)
Briefly they work as follows:
Both Dolby systems are "pre-emphasis/de-emphasis" systems.  They boost the
signal level above a certain frequency by a predetermined amount (which depends
upon signal level and frequency) when recording and then cut it by the same
amount on playback.  The effect is that the boost and cut cancel each other out
and the same signal comes out as went in.  However, the tape hiss was not boosted
and is reduced by the cut in the playback process.  Since the Dolby system is
level sensitive, tape decks are provided with adjustments and test tones to
calibrate it.   The difference between Dolby B and Dolby C is the amount of
boost/cut that is applied to a given input signal.  Dolby B tried to keep some
compatability where undecoded Dolby material could be listened to (albeit with
a treble boost) and this limited the amount of noise reduction it provided
while Dolby C is designed to provide more noise reduction without compatability.
dbx is a "compander" system.  In the dbx system, the dynamic range of the input
signal is compressed 2:1 when recording (the loud passages are made softer and
the soft passages louder).  On playback, the process is reversed, and the
dynamic range is expanded back to that of the original signal.  At the same
time, the noise (which is usually softer than the signal) is expanded to a
softer level (into virtual oblivion).  There are two major advantages to dbx
over the Dolby systems:  it is not level sensitive and it provides more noise
reduction.   However one cannot listen to dbx material without decoding.
I'll repeat my recommendations from the earlier item.  Since dbx is the most
effective noise reduction system, it is the one to get for home use.  Many
new decks have it built in or one can add dbx's outboard processor (a 222 for
2 head decks or a 224 to allow simultaneous monitoring from a 3 head deck).
I expect it to be available soon in car decks (if it isn't already).
For compatability for use with "Walkmen" and pre-recorded tapes the deck
should also have Dolby B.  Generally, most mass-market pre-recorded tapes
use Dolby B and the "audiophile" ones give you your choice of noise reduction
systems (I'll take dbx, thank you.).
                                                 Bill Mitchell
                                                 BTL Whippany
                                                 harpo!wjm