Ampex 351 - Page 84

Example :
-5
19-
■15
24 20
-IQ-
29
-15- •25
30
■35
Step 4:
40 -30-
.44.
-49-
•45
— 35-
54 50
-40-
Step 7:
65 ■69
-55-
Step 8:
■70
-60-
Step 9:
Step 10:
Signal-to-noise ratio computations
8-11
Step 5:
Step 6:
WHEN VTVM
READS THIS
-74 ---------------
YOUR SIGNAL-TO-NOISE
RATIO IN DB IS THIS
-5
6(1% DIST)
■10
55
56
60
Step 2:
Step 3:
89-0144
Issue A
-45 ---------
>-46 --------
-50 --------
-20 ----------------
S/N RATIO-
-25 ----------------
---- 59-
— •60-
— 64-
•34 ---------------
S/N RATIO-
-39 ----------------
DBM
4-14
-HO-
4-8-
+ 4~
0 —
VU METER CALIBRATED SO
THAT ZERO EQUALS
8 DBM OUTPUT
7 I
— 0 (3% DI ST)
---- 4 -----------------
— 6(1% DI ST)
— 9 -----------------
----10----------------
----14----------------
4 DBM OUTPUT
— 0(3% DI ST)
Place the METER AND OUTPUT
SWITCH in the RECORD LEVEL
position.
Set the oscillator to 400 cps.
Adjust the RECORD LEVEL control
to obtain a vtvm reading 6 db above
operating level ( + 14 dbm for equip 
ment with 8 dbm output).
Record the 400 cps on a section of
tape, noting where the recording be 
gins for later reference.
Disconnect the oscillator.
Set the RECORD LEVEL control to
zero. (Fully counterclockwise).
Rewind to the beginning of the 400
cps recording.
Erase the tape by recording with zero
signal.
Rewind again to the beginning of the
recording.
Read the vtvm and check the reading
against the table.
db signal-to-noise and satisfies the signal-to-
noise ratio definition.
When the VU meter is so calibrated that
zero VU corresponds to +4 dbm output add
6 db to obtain the output value to the 3% dis 
tortion level arriving at a total of 10 dbm.
Example: 10 (dbm, 4 +6)
-46 (dbm vtvm reading)
56 (db. signal-to-noise ratio)
Ampcx signal-to-noise ratio specifications
on audio instruments define in decibels the
ratio existing between the level of a steady
400 cycle tone, recorded at a level at which
distortion produced by the approach of tape
saturation equals 3% total rms, and that level
of total rms noise, in the band from 30 to
15,000 cycles, which exists in reproduction
under the same gain conditions.
Ampcx audio instruments normally arc cali 
brated so that the VU meter reads zero level
when reproducing a steady 400 cycle tone the
level of which produces 1% total rms distor 
tion due to the approach of tape saturation.
A recorded 400 cycle tone at the 3 r /r dis 
tortion level will be 6 db higher in level than
the same tone recorded at the 1% level.
Step 1.
Record Noise Measurement
To translate vtvm readings into specific sig-
nal-to-noise ratios when the vu meter is so
calibrated that zero vu corresponds to +8 dbm
output, add 6 db to obtain the output value
from the 3% distortion level, arriving at a total
of 14 dbm. Having made this computation,
bear in mind that, although the noise reading
taken on the vtvm is dbm, the measurement is
a ratio which must include the 14 dbm com 
puted to arrive at the 3% distortion level.
Therefore, the vtvm reading must be converted
to the signal-to-noise ratio.
14 (dbm, includes +8 dbm nor 
mal level and +6 dbm to
3% distortion level)
46 (dbm, vtvm reading)
60 db signal-to-noise ratio
Any reading below -46 dbm meets per 
formance characteristics specifications of 60