Fostex 250 - Interface Details; Levels and Signal Amplitude

sections INTERFACE DETAILS
This section of the manual is written for the non-engineer,
who may have only a vague idea of the meaning of levels
(whether expressed in dBV, volts or millivolts), and who prob¬
ably understands even less about impedance (expressed in
ohms or kilohms). The following subsections are written to
impart a greater understanding of level and impedance, with
specific regard to the Model 250.
LEVELS AND DECIBELS (dB)
Level is a term loosely used to describe the amplitude of a
signal or a sound. More precisely, “level” is the value of a
sound or signal relative to a given reference and usually ex¬
pressed in dB SPL (DeciBels, Sound Pressure Level) dBV
(DeciBels, Volts). The relationship between dB and volts (or
dynes, when discussing sound pressure) is not a 1:1
because the “dB" value expresses a ratio, one which is
based on logarithms.
NOTE: An understanding of the following few paragraphs
can be very helpful, but is not necessary for connecting
the Model 250 to other equipment. We have avoided
formulas and highly technical language to the extent
possible.
If the voltage level is doubled, the number of dB is not
doubled—it is increased by 6. This means that whether the
voltage goes from 30 millivolts to 60 millivolts (a 30 mV in¬
crease), or from 5 volts to 10 volts (a 5 volt increase), the level
increases by the same 6 dB. By the same rules, if the voltage
goes up by a factor of 10, the level increases 20 dB. The
same relationship applies to sound pressure level (doubling
the number of dynes is an increase of 6 dB SPL).
dBV values can be converted to specific voltages, and vice-
versa; a special slide rule or scientific calculator is usually
employed. In simple terms, however, the key to the conver¬
sion is the "reference” value for dBV. The large “V” in dBV
means that the 0 dB value is related to a specific Voltage, 1
volt in this instance. Given that 1 volt is 0 dBV, the following
table can be readily constructed:
(plus dB over 1 volt)
2 volts = +6 dBV
4 volts = + 12 dBV
8 volts = + 18 dBV
10 volts = +20 dBV
(minus dB under 1 volt)
0.5 volts = 500 mV = - 6 dBV
0.25 volts = 250 mV =-12 dBV
0.125 volts =125 mV = -18 dBV
0.1 volts = 100 mV = - 20 dBV
NOTE: Some equipment is rated using a different 0 dB
reference value. 0 dBV equals 0.775 volts, so a given
voltage, rated in dBV will be 2.2 dB higher than the dBV
operating. 0 dBm is referenced to 1 milliwatt (power, not
voltage), but in a 600 ohm circuit, the dBm number is
identical to the dBV number, so a conversion can be
approximated by assuming 600 ohms. Bear these
differences in mind when comparing specified levels in
different equipment.
NOMINAL LEVELS
Except in the case of a test signal, such as the output of a
tone oscillator or a continuous organ note, signal levels
almost always vary up and down as the sound itself changes
from very loud to very quiet. For this reason, the term
"nominal level" is used to describe the approximate operat¬
ing level of a given circuit (expressed in dB or voltage).
Remember that minimum levels (quietest passages) are
below the nominal level, and maximum levels (loud pas¬
sages) are above the nominal level.
With regard to the Model 250 and related equipment, there
are several nominal levels involved, the most significant of
which are (a) microphone level, (b) line level.
Nominal mic level = - 60 dBV = 1 millivolt
Nominal line level = - 10 dBV = 300 millivolts (0.3 volts)
MODEL 250 INPUT LEVELS
When connecting mics to the Model 250 MIC/LINE INPUTS,
remember that some microphones have higher output levels
than others; a given mic will have a higher level when it is
nearer to the sound source or when the sound is louder.
Hence, a microphone's nominal level may be higher than
- 60 dBV, sometimes as high as - 30 dBV. Instruments such
as electric guitars and pianos, may have nominal levels
higher than - 60 dBV but perhaps lower than -10 dBV. Most
consumer tape recorders, tuners stereo equipment, and so
forth utilize line levels of approximately -20 dBV to -10
dBV. This is why the TRIM (LINE-MIC) control is provided; it
allows the input sensitivity to be adjusted to suit the actual
input source.
The Model 250 RECORD IN and AUX IN jacks are meant for
connection to equipment having -10 dBV nominal output
levels. Since the RCV 1 -2 control can lower the AUX IN sensi¬
tivity, higher input levels can be accommodated.
Some sound equipment outputs are rated at +4 dBm
nominal. This equals 1.23 volts, which in turn equals about
1.8 dBV, 12 dB above the Model 250’s line input sensitivity.
The Model 250 specifications show that the unit is rated for
maximum input levels of +12 dBV (MIC/LINE and RECORD
inputs), and + 20 dBV (AUX IN). This might appear to accom¬
modate a +4 dBm nominal signal, yet such a signal may
have peaks of +18 dBm to +24 dBm (about + 16 to +22
dBV). Thus, unless a +4 dBm nominal output is reduced in
level, or attenuated with an external pad, the Model 250 input
still may be overdriven on peaks, causing audible distortion.
MODEL 250 OUTPUT LEVELS
All Model 250 outputs are rated at -10 dBV (0.3 volts)
nominal level, with a maximum output of +15 dBV (5.6 volts).
A signal that would tend to cause higher than +15 dBV out¬
put levels will merely create distortion (clipping). The VU
METERS are helpful in avoiding distortion due to excess
levels.
Since most consumer tape recorders, tuners, stereo equip¬
ment, and so forth utilize line levels of approximately - 20
dBV to - 10 dBV, the Model 250 outputs should be compati¬
ble. If an accessory has a rated input level of +4 dBm
nominal, the Model 250 output may not be adequate to drive
that accessory to full output level; this is not necessarily a
problem, although some additional noise may be introduced.
If the +4 dBm rated accessory has a gain (volume or level)
control that can be increased, as many do, then it will operate
normally when driven by the Model 250 output.
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