UNITS OF MEASUREMENT
Levels in dB
The units of signal level used in Amek literature are the international agreed quantity `dBu’. The matter of dBu, dbV, dBv and dBm can cause some confusion so the following notes may be helpful.
dB is an abbreviation for “decibel” i.e. one tenth of a Bel.
A measurement quoted in dB is describing the ratio between the quantity being measured and some reference. For some purposes a ratio is all that is necessary. For example it is quite sufficient to know the ratio by which the gain at 20 Hz differs from the gain at 1 kHz. The absolute size of the signal is not relevant.
If a dB measurement is used to describe the absolute size of something, the reference point must be known.
Early sound equipment took much of its technology from telephone engineering where 600 ohms was used as the input and output impedance. 600 ohms is the characteristic impedance of the spaced pairs of copper wires used for simple telegraph and telephone systems. With such systems, power matching was very important to optimise the signal transfer between send and receive equipment.
Signal levels were normally quoted as power ratios and a reference point of 1 mW was chosen as a convenient size. The unit was named dBm with the `m’ indicating a reference point of 1 mW and is therefore a measurement of power.
Level meters were often required to measure signals in circuits that were already loaded with 600 ohms. They were therefore designed with a high input impedance which allowed them to be connected across circuits without adding an extra load and upsetting the matching.
As the circuit impedance was known as 600 ohms, the measuring device could be a voltmeter and but scaled with the power units dBm. The voltage which produces 1 mW across a 600 ohm load is 0.775 volts so this came to be a voltage reference point.
Power matching ceased to be important in most audio systems when simple low output impedance circuit designs became available. Today it is normal for studio equipment to have output impedances somewhat less than 100 ohms and for input impedances to be greater than 10 kohms. Such systems are called “voltage matched” as the maximum voltage, not power, is transferred from the source to the load. The power transferred when 0.775 volts are applied to a 10 k ohm input impedance is only 0.06 mW so a 1 mW reference is no longer useful.
Millivoltmeters are often calibrated to measure voltage levels in dB with respect to 0.775 volts as this is the reference voltage found in 600 ohm/1 mW systems. A unit of voltage measurement which uses 0.775 is required as the term dBm is a power level and cannot properly be used to measure voltage.
dBu is the term now generally accepted as the unit of measurement using 0.775 volts as the reference. This term is used by Amek Technology and many other organisations.
The unit dBu is sometime written as dBv. It has exactly the same definition so 1 dBu equals 1 dBv.
The dBV is also an absolute unit of voltage. It expresses voltages as a ratio relative to 1 volt, not 0.775 volts. As the reference point is larger by approximately 2.2 dB, 1 dBV can be said to equal +2.2 dBu.
The term dBV is not normally used in Amek technical literature.
The units used are very important to the proper interpretation of specifications.
A noise floor of -90 dBu means the noise level is 90 dB below 0.775V. The same noise floor can be described as -92.2 dBV simply because it is further away from a higher reference point. This second figure may appear quieter at first glance but is of course the same thing!
Crosstalk may be quoted in dB or dBu. If the units are dB the figure is the amount by which the “interfering” signal will be attenuated.
If the figure is quoted in dBu (or dBv or dBV) it is important to know the level of interfering signal.
Amek/TAC specifications normally quote crosstalk in dB.
4 + 10 = 12 ?
The reference level for a lot of studio equipment is +4 dBu. Some equipment uses as reference level of -10 dBV. This is equivalent to a level of -7.8 dBu.
This creates the apparent nonsense that so called `+4′ and `-10′ systems have a level difference of 11.8 dB not 14 dB as one might expect. For most purposes the 11.8 dB figure may be regarded as almost 12 dB which is a voltage ratio of 4:1.