Ampex 351 - Page 41

HIOUCNCV IN C ’ CllJ H> SECOND
»20
<10
OB
Odb
■
— 10
10000
12
4 TRACK
MASTER,
HALF
TRACK
FREQUENCY IN CYCLES PER SECOND
0067
Typical crosstalk vs. frequency curve on adjacent
channels of an Ampex three channel Model 300. Chan 
nel 2 was recording at normal operating level and the
record head of Channel 1 was cotinected. Normal bias
and NAB equalization were used.
0
E
20000
0H9
Standard NAB post-emphasis curve for 15 ips.
lion to the track width. Technically, beyond a certain
track width it becomes difficult to maintain accurate
azimuth alignment.
If the signal-to-noise ratio is determined by the
medium itself, (the tape noise is at least 8 to 10 db
above the equipment noise) then the signal-to-noise
of the system is proportional to the square root of the
track width.
So. just how wide should the track be? As the
track width increases, closer and closer mechanical
tolerances must be held to maintain the same linear
alignment accuracy, which determines the azimuth
alignment and therefore the high frequency response
and stability. Experience has shown that, for 15 ips
recording speeds, it is practical to maintain azimuth
alignment for track widths up to 250 mils. (For lower
speeds, say at 7 ’ /2 ips. it is difficult to maintain azi 
muth alignment for tracks wider than 100 mils.)
Remembering our practical economic considera 
tions, we can put three 100 mil tracks, separated by
85 mils, on '/a-inch tape (or six tracks on 1-inch
tape). The three track, '/a-inch. equipment is widely
used in recording master tapes, and has been ac 
cepted as the best compromise between tape utiliza 
tion and track width. Different configurations of track
width and spacing, with the relative signal-to-noise
ratios of each, are shown in an accompanying illus 
tration.
Odb
15
L
I
3 TRACK
MASTER
OR DUPLICATOR
MASTER
h
H
tl
h
si
Equalization
Reproduce equalization has been standardized for
some time, with the curve in general use specified
by the NAB (standard equalization in Europe usually
follows the CCIR curve). Any pre-emphasis curve,
therefore, must be tailored to the standard reproduce
curve.
It is the feeling at A mpex that the present NAB
specifications are convenient curves, which give con 
stant overall response through the tape machine
using simple networks in both record and playback.
The design at 15 ips has been very conservative with
respect to overload capabilities, but the signal-to-noise
ratio has been inadequate. Greater attention to the
characteristics of the ear, the tape, and the music
reproduce head of the other track At high frequen 
cies, the mutual inductance and capacitance between
the two record heads causes the signal from one re 
cord head to be present in the other record head, and
therefore to get recorded on that other track. There 
fore spacing and shielding between cores is impor 
tant in both the record and reproduce heads. Obvi 
ously the closer together the tracks the more coupling
exists (assuming the same shielding). With good
shielding, an 85 mil track-to-track spacing (used for
Ampex ’ /i-inch two track, and Va-inch three track
recorders) is a good compromise — more spacing to
reduce crosstalk is unnecessary and would waste
space, but any less would result in the increased
crosstalk becoming audible above the noise.
Track Spacing
Two crosstalk effects are known to occur: At long
wavelengths magnetic coupling occurs in reproduce
between the signal recorded on one track and the
Normal record and reproduce head configurations used
by Ampex, with relative signal-to-noise ratios in re 
spect to the 100 mil track width. Dimension of six
and eight track heads on 1-inch tape are the same as
those shown for the three and four tracks on '/z-inch
tape. All dimensions are in mils.
3 TRACK 4 TRACK
(SEPARATE •DOUBLE
ERASE STEREO'
NOT FEASIBLE)
RELATIVE SIGNAL TO-NOISE RATIOS
— 1.5db + 3 7db -0.9db — I2db -3 6db
20 8 8 0
1 i 1 1
r l|
84
X
2 TRACK
STEREO
-3.6db
3J6 V/ 2
75 L&J-t — LkJ