ã 1998 by
Nominal Level (explained) plus an SV-3700 modification
Interfacing a DAT recorder with a mixer, a piece of outboard
gear or a cassette deck isnít always quite as easy as it should be. In
the analog-only daze, it was simply a question of "plus4," "minus10" and
0VU. This month, a gain structure modification for the Panasonic SV-3700
(post D-to-A converter) will help you understand ó and manipulate ó the
"nominal" operating level. But first, a word from our sponsorÖ
IT's THE JUICE
Why are there so many different reference levels?
Quite simply, it's a matter of juice. If a device runs from a 1.5
volt battery it can only deliver a maximum level of not-quite 1.5 volts
peak-to-peak and that's with lots of distortion. But assuming 1.5vp-p,
that turns into a half a volt RMS which is about -8dBu, max. You'll
need 14 or so dB of headroom, now the nominal operating level is down around
-22 dBu, which is in the neighborhood of a passive guitar or bass.
What's dBm? Think of the "m" as the "meat" factor. A device
so specified can deliver its juice into a 600 ohm load. The "u" in
dBu implies that the load impedance is unspecified and is likely to be
high, around 10k-ohms.
When the analog meter in Figure-1 indicates 0VU,
the device to which it is connected will output its standard (nominal)
operating level. For professional and consumer recording equipment, the
standards are +4dBu and Ė10dBV, respectively. But what about the bar graph
display above? On a stock SV-3700, Panasonic chose a point 18 dB below
Full Scale (fs) as their reference. That itís not the same on every brand
and model of DAT machine can cause problems when interfacing with analog
equipment. On the DA-98, Tascam allows the user three choices: -20 dB,
-18dB and -16dB.
The "location" of the reference level determines headroom
ó the distance in dB before clipping (0dBFS) needs no translation. From
the reference level to the noise floor is another story based on the number
of bits, quality of D-to-A converter, circuit design and printed circuit
board (PCB) layout. A tone recorded at the reference level will generate
the "nominal" output.
One idiosyncrasy of the SV-3700 occurs when shorting either
pin 2 or pin 3 of the XLR to ground. This old-fashioned unbalancing method
applies to transformers but it can reduce headroom on some "active-balanced"
circuits such as those used in the SV-3700. To "safely" unbalance, use
ground and one signal pin and leave the other signal pin unconnected. The
output will be 6 dB lower but the signal will not distort.
Here come the numbersÖ
The balanced output amplifiers of the SV-3700 can deliver
a maximum signal of +24dBu (+18 dBu). Numbers in ( ) indicate the output
when operated in the "safe" unbalanced mode. The gain structure is set
to deliver +22dBu (+16dBu) when the recording level is 0dBfs. (Doing the
math: 18 dB above +4dBu is +22 dBu.) Remember that the amplifiers
feeding pin 2 and pin 3 can only deliver +18dBu "max." With only
2 dB of headroom, under normal circumstances, operation in the "unsafe"
mode will attempt to add 6 dB of level, 4dB above "max." Yee-ouch!
This means any signal recorded within 4 dB of Full Scale will distort the
output amplifiers. The Sidebar
explains how electronic numbers are born...
To fix this problem, check out Figure-2, whichshows
the complete output schematic for the right channel plus IC 905, the affected
section of left-channel circuitry. Changing the value of R929 and R930
(left and right, respectively) from 5.6kW to
2.21kW lowers the output gain by about 4 dB.
(This was the "nearest standard value" I had available.) Table
one indicates values that deliver precise dB increments. Use 1% metal-film
for low noise.)
In order to deliver the nominal +4dBu at the XLRs, a tone
formerly recorded 18 dB below full scale must now be recorded 4 dB louder.
Nothing really changed on the record side of the A-to-D converter, but
increasing the reference level by 4dB reduces the headroom by the same
amount. (You still canít record above "0.")