dbx 128 - SYSTEM PERFORMANCE; Audible Spectrum Response

28
nel 187 and the 16-channel 216, many home recording en-
thusiasts sought better quality tapes. In response to the de-
mands of consumers and small studios, dbx introduced a
variety of semi-professional units (the 150 series). These
units utilize the same signal processing as the professional
recording studio models, so tapes made with the professional
series may be decoded with the semi-professional series, and
vice-versa.
Broadcasters realized the potential for improved signal
quality if they could use dbx processing. However, broadcast
cartridges and telephone transmission lines do not offer the
excellent frequency response available in professional re-
cording studios and better hi-fi tape machines; the low and
high end of the frequency spectrum often fall off consider-
ably. Poor frequency response in a tape recorder can cause
mistracking of the RMS detection circuitry in the dbx pro-
fessional system. The rumble and low frequency errors of
warped discs, or the limited low and high frequency response
of telephone and microwave transmission lines would create
mistracking of the RMS detection circuitry. Therefore, the
dbx II system was developed, represented by dbx models
122, 124, 128 and the 142. The basic principle of operation
of dbx professional and dbx II are identical, and the amount
of noise reduction is the same, yet the two systems are not
compatible. A tape encoded with either system cannot be
decoded by the other. Similarly, a dbx encoded disc (dbx II
processed) cannot be decoded with the original dbx profes-
sional system.
The two systems were designed for different applications.
The bandpass filter in the signal path in the dbx I! system is
slightly more restrictive, rolling off 1 dB at 30 Hz. In addition,
the RMS detection circuitry in dbx II units is sensitive only
up to 10 kHz, so high frequency losses on the tape or trans-
mission lines will not create encode/decode mistracking. THE
dbx Type | dbx Type It
#49 am
tis H LH
+5
+10
+8 | Zo t|
a Cen]
Zo kd verti _
78 FT Hit
46 aN
5K 10K 20K ~10 99 50 (100 500 1K SK 10K 20K
20 «50 «(100 500 1K
Fig. 16 — Comparison of dbx Type | and Type II Processing
Type | is the curve used in the 152 and 154, as well as pro-
fessional recording studio dbx equipment. Type II is the curve used
in the 128, 122, 124 and 140 series, which is optimized for such
applications as broadcast and disc noise reduction, as well as tape
recording.
OVERALL FREQUENCY RESPONSE OF dbx I! PRO-
CESSING DOES COVER THE ENTIRE AUDIBLE SPECTRUM.
The dbx II system is also equipped with a TAPE/DISC
switch that introduces a further low end roll off (- 3 dB
@ 30 Hz) in DISC mode. This has been provided to permit
decoding of special dbx encoded phonograph records; the
roll off protects the RMS detection from mistracking due to
record warp or turntable rumble. The dbx II system is espe-
cially suited to noise reduction with cassette and cartridge
recorders which have limited frequency response compared
to a professional studio machine.
A few minor changes in the pre-emphasis and de-emphasis
curves also distinguish the dbx professional and dbx II systems.
However, both systems offer the same 30 dB of broadband
noise reduction, and a 10 dB improvement in headroom for
tape recordings. The signal processing differences between
dbx professional and dbx Il make it inadvisable to encode
with one system and decode with the other; audible tracking
errors could occur.