There is a saying "You think you understand this, but just wait until I have explained..."

This certainly applies when we talk about Volts, decibel,  impedance and so on.
Some of the Tascam service manuals have lots of reading on this subject.

As you will find very soon when you read more "in depth" about tape recorders, is that dB is mentioned very often.
dB is an expression used when comparing levels as Voltage, Power, Sound pressure, .....
Take the VU-meters for example. (VU by the way means Volume Units).
The meters do not start at 0 (zero). Instead, 0 is way up on the scale.
This is a "heritage" from the telephone companies, who wanted to measure how the phone lines performed.
By starting with 0 as a relatively high level, they could measure how much weaker the signal got when it reached
for away locations.
If it had been amplified somewhere along the line, one could also see that, as the meter had a few dB above 0, marked with +.

As I have mentioned in the section about "electrical adjustments", we can calibrate the VU-meters so that "0" corresponds
to a certain level, both magnetic, and electrical.
 

dB in electrical reference levels.

The NAB standard.
To make things even more complicated, there are different reference levels when we talk about electrical levels.
The first reference level is from the 1930´s and has to do with telephones again...
This level is named dbm, which means dB-milliwatt.
The "0" level here is called 0dBm, and is 1 milliwatt of power in a 600 Ohm impedance.
Remember that this reference level refers to "power" (watts), not voltage.

The IEC standard.
This standard is named dBV. V for Voltage.
The "0" level here is called 0dBV, and is 1,00 Volt.
This standard does not care about the impedance. This is because modern equipment have a very high impedance (50 k for example).

Is there a relation between dBm and dBV?
The problem is that we encounter several standards when we read service manuals.
However, it is possible to convert dBm to dBV and vice versa.
Usually, we want to have all levels in Volt, not Watt. If we convert 0dBm to Voltage, it corresponds to 0,775 Volt.
Since the dBm reference was intended to measure milliwatts, not Volts, we get this odd number (0,775 V).

The following table was taken from a Tascam manual, and shows the relationships:
 

dBV (IEC)	Voltage	dBm (NAB)
+6	2,00	+8,2
+1,78	1,23	+4
0	1,00	2,2
-2,2	0,775	0
-6	0,50	-3,8
-8,3	0,388	-6
-10	0,316	-7,8
-12	0,25	-9,8
-12,2	0,245	-10
-20	0,10	-17,8

When you study manuals you will find that there are 2 levels often used.
One is -10dBV, and the other is +4dBm.
dBV references are mostly used in systems with RCA connectors, and dBm in systems with XLR
connectors (balanced signals).

If you wish to know more details about measurements, you can read details in the chapter how to measure.
 
 
 

TASCAM 32

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