Ampex 351 - Page 13

2-5
15 ips NAB Reproduce Equalization
The 15 ips NAB (National Association of
Broadcasters) reproduce equalization curve is
the American broadcast and recording industry
standard. The equalization curve consists of a
6 db per octave decaying slope with a 3180
microsecond low end time constant and a 50
microsecond high end time constant. This curve
is used for most of the master recordings made
by the recording studios.
3% ips Reproduce Equalization
Prior to the introduction of the 3% ips pre 
recorded tape cartridge, industry practice was
to use the 3% ips 200 microsecond reproduce
curve. This curve, which consisted of a 6 db per
octave decaying slope with a 3180 microsecond
low end time constant and a 200 microsecond
high end time constant, provided good quality
speech recordings but, because of limited signal-
to-noise ratio, it was not adequate for good qual 
ity music recordings. When the tape cartridge
was introduced, the high end time constant was
changed to 120 microseconds. This change im-
714 ips NAB Reproduce Equalization
The National Association of Broadcasters
has not set up a standard for 714 ips. However,
industry practice has been to use the 15 ips NAB
reproduce curve for 714 ips hence it is called
the 714 ips NAB curve. This curve is used for
all of the 714 ips pre-recorded tapes (both two
track and four track) made in the United States.
15 ips AME Equalization
Tape noise, or “ hiss ” is perhaps the greatest
limiting factor in the quality of present-day tape
recordings. The noise generated by the tape can 
not actually be reduced by any means outside
of improving the tape itself. However, an in 
crease in the signal-to-noise ratio can be ob 
tained by increasing the signal level. As the in 
put signal amplitude increases to a high level,
however, the amount of signal actually recorded
on the tape reaches a limit called saturation.
At this point, the signal on the tape is much less
than the input signal, or is compressed to about
one-half the amplitude or less of the input sig 
nal. Since this saturation level varies with fre 
quency, a very uneven response is obtained
when recording at too high a level. If the high-
frequency input level is increased still more
beyond the saturation point, the signal on the
tape decreases. This phenomenon is known as
self-erasure. A high-level, high-frequency signal
not only erases itself as it is being recorded, but
partially erases any other tone which is also
being recorded.
The 15 ips Ampex Master Equalization
(AME) curve is designed to obtain a somewhat
better apparent signal-to-noise ratio than is ob 
tainable with the standard NAB equalization
(see note). It was found that a greater signal
amplitude could be recorded in the 2000 to 6000
cps region than is presently allowed by NAB
equalization — without significant increase in
overall distortion. This region is the band to
which the ear is most sensitive.
proved the signal-to-noise ratio considerably
(with some sacrifice in overload characteris 
tics) and allowed music recordings of passable
quality to be made. The 120 microsecond curve
is now used for all 3% ips recordings, both
cartridge type and reel-to-reel type.
The choice between the 120 microsecond
and the 200 microsecond time constant will de 
pend on the number of recordings of each type
in the user ’ s tape library. If there are no 3% ips
recordings in the library, the 120 microsecond
time constant is preferred.
NOTE
The apparent signal-to-noise ratio is
increased by approximately 8 db al 
though the actual measured signal-to-
noise ratio remains unchanged. Note
89-0144
Issue A
for most practical purposes are adequate. How 
ever, under certain circumstances such as cri 
tical master recordings which may have many
generations of copies, a variable equalizer would
be more desirable since it can be adjusted to
follow the equalization curve exactly, overcom 
ing the effects of small variables introduced by
reproduce head differences and the like. There
is, of course, a disadvantage to variable equal 
ization in that it can also be misadjusted where 
as fixed equalizers can not.
Record equalizers are always variable since
the amount of equalization necessary to achieve
flat response will vary from machine to machine
and will vary when tapes from different sources
are used on the same machine. In all cases, there
is a corresponding record equalizer for each re 
produce equalizer.