Eddie Kramer Master Tape Plug-In User Guide

Bias
Defined: Bias is a high fr
equency signal, typically between 40 kHz and 150 kHz,
applied to the record head along wit
h the audio signal when an audio track is
placed into record. The bias current solves a critical problem when recording to
analog tape. When the amplitude of an audio signal passes through the zero
voltage crossing, the magnetic field created by the record head is not strong
enough to polarize the magnetic oxide particles on the tape. Thus, a distortion of
the original audio signal is introduced. To minimize this distortion, the bias
current is applied to break down this resistance to polarization. The audio signal
can then be recorded more accurately without the effects of low level distortion.
The amount of bias current applied is critical to the frequency response and
distortion characteristics of an analog recording.
Waves mod
eled and have provided you with two bias settings. “Nominal Bias,” the
manufacturer’s recommendation for bias adjustment
(directly from the original operator’s
manual for the modeled machine) was recommended in the early years to try and reach
the best recording levels with the minimum amount of distortion and the maximum
frequency response. This setting produced a fairly low noise level (around 60 dB lower
than the peak signal), and around 2 to 3 dB of high frequency loss with a moderate
amount of high frequency distortion.
During the ’60s, after a
number of years of working with these tapes, many professio
nal
studio engin
eers and technicians d
iscovered that by over biasing (increasing the
amplitude of bias signal) by only a small amount, they could improve the high frequency
response and at the same time lower the noise level. This was called “Over Bias” and
each tape type, studio, recording engineer and technician had their own way of
calibrating the bias to achieve their preferred sonic qualities.
For the Kramer Master Tape, we modeled the -3 dB over bias, which was agreed b
y
most engineers to be the
point where you got the best high frequency response, least
amount of distortion and best signal to noise ratio on 3M Scotch 207 tape. (Actually -.7
dB at 700 Hz for 15 ips, but set at -3 dB at 15 kHz for accuracy. You adjusted nominal
bias to accomplish the peak level of the signal and then continued beyond the peak until
the level began dropping by the desired amount, therefore the term ‘over bias’. The bias
adjustment for 7.5 was done one octave lower at 350 Hz or 7,500 Hz and at -20 dB to
avoid excessive high frequency saturation.) When you switch from “Nominal Bias” to
“Over Bias” mode, you should expect to hear a bit less noise, clearer high frequencies
(reduced distortion), and a bit more overall dynamic range (and clarity, once again the
result of less overall THD).
Flux
Defined: The
magnetic flux density recorded on a tape (level). The standard unit
in measuring the am
ount of magnetic energy recorded to tape is expressed in
NanoWebers per meter (commonly abbreviated as nWb/m). When picking an
operating level for tape (flux/level), the general rule is that the higher the
operating level, the further away you are from the noise floor but the closer you
are to the point of distortion. This dynamic is highly dependant on the tape stock
being used. For a detailed dissertation on magnetic recording flux and flux-
frequency measurements please use this URL
Waves Kramer Master Tape
User Guide
17