Volume Unit (VU) Metering
The following information is taken from a 1952 PMG publication “Radio 1 Training Manual.”
2. Volume Indicator:
2.1 The volume level of an audio signal at any particular point in a Broadcasting System is normally measured by means of a standard instrument called a Volume Indicator. The volume indicator is a direct current instrument fitted with a full wave instrument rectifier.
The Western High Speed Level Indicator was the recognised standard instrument but was not satisfactory, as the rapid movement of the pointer was fatiguing to the eye. In addition, on account of the human element, accurate readings were not always obtained. This limitation led to the development of a Volume Indicator, particularly designed for the requirements of broadcasting and the telephone plant which provides the inter-connecting service among broadcaster stations. The Volume Indicator gives satisfactory correlation of measurements under normal conditions of operation, and permits rapid accurate readings.
The Volume Indicator consists of at least two parts –
- An indicating instrument (see Fig 2) and
- An attenuator (see Fig 3)
As stated previously, it is used for the indication of volume, and has become generally known as a “VU” Meter. (VU pronounced “vee-you” and is customarily written with the lower case letters.) This word is used for the numerical expression of volume.
The volume in vu is numerically equal to the relative strength of the waves in question in decibels above or below “reference volume”.
The term vu should not be used to express results of measurements of complex waves made with devices having characteristics differing from those of the standard Volume Indicator.
2.2 Terms Used:
This term applies to the strength of speech h and music waves and is the reading given by the Volume Indicator. The indicator has specific dynamic and other characteristics, and is calibrated and read in a prescribed manner.
This is the base of the system of measurement of volume and is the scale reading for a steady 1,000 c/s wave of 1 milliwatt power into 600 ohms. (See paragraph 2.3 also)
This is the deflection to the scale point at or near which the instrument is intended normally to be read.
If a single frequency sinusoidal voltage between 35 and 10,000 c/s and of such magnitude as to give reference deflection under steady state conditions is suddenly applied, the instrument pointer will reach 99 per cent of reference deflection in 0.3 second, plus or minus 1.0 percent, and will then over-swing reference deflection by at least 1.0 per cent, and not more that 1.5 per cent. The time required for the instrument pointer to reach its position of rest on the removal of the voltage is approximately equal to the time of response.
Response Versus Frequency Characteristic: The sensitivity of the volume indicator will not depart from that at 1,000 c/s by more than 0.2db between 35 and 10,000 c/s, or more than 0.5 db between 25 and 16,000 c/s.
A correctly calibrated volume indicator with its attenuator set to zero vu will give reference deflection when connected to a source of single frequency sinusoidal voltage adjusted to develop 1 milliwatt in a resistance of 600 ohms, or with the attenuator set at n when the calibrating power is n db above 1 mW.
It should be noted here that the volume indicators available at present will not read 0 vu when 1 mW is developed in 600 ohms, and in fact the instrument will only read -4 vu. Therefore to obtain a reading of 0 vu, a power 4 db above 1 milliwatt must be developed in the resistance. These remarks only apply provided the attenuator is out of circuit.
Method of Reading:
The reading is determined by the greatest deflections occurring in a period of about a minute for programme waves, or a shorter period (for example 5 to 10 seconds) for message telephone speech waves, excluding not more than one or two occasional deflections of unusual amplitude.
The volume indicator is normally used as a bridging instrument, and when so used, its impedance must be sufficiently high so as not to influence unduly the circuit with which is used. The impedance must not be less than 7,500 ohms for use on a 600 ohm circuit.
When the volume indicator is connected to a simple resistive circuit through which a sinusoidal wave (between 25 and 8,000 c/s) is being transmitted, the root-means-square sum of the harmonics produced will not exceed 0.2 per cent.
Ability to Withstand Overload:
Because of the great variation in amplitude which this indicator may register, it has a greater ability to stand overloads than average instruments. The volume indicator is able to withstand, without injury or effect on calibration, a momentary overload of ten times the voltage corresponding to reference deflection, and a continuous overload of five times that voltage.
Two scales are provided, one calibrated in vu and the other in Percentage Modulation. The latter scale is of particular use when the instrument is installed at radio transmitters.
The connections for the volume indicator are shown in Fig 3. It will be seen that the attenuator has a characteristic impedance of 3,900 ohms.
2.4 Advantages of the Volume Indicator:
The advantages of the Volume Indicator include –
- vu indications
- Controlled dynamic range
- Increased scale length
- Improved legibility (scale)
- Less eye strain and fatigue (pointer action)
In addition, the scale card which is a special buff colour with a minimum number of markings, reduces eye strain and fatigue.
When the volume indicator is adjusted to read reference level, the programme peaks are suppressed by approximately 8 db. This means that the peak power is approximately 8 db above the volume indicator reading.
2.5 Until recently it was customary practice to measure the power level of the programme by means of a decibel meter, which is really an alternating current voltmeter calibrated to read 0 decibel when placed across a 600 ohm load carrying 0.006 watt. This corresponds to a voltage of about 1.73 volts, the normal understood reference level (when speaking of power in decibel) being 0.006 watt. Problems involving volume units are solved exactly as in any decibel problems, inasmuch as a change of any number of vu’s corresponds to a change of a similar number of decibels.
If 0 decibel is taken a 0.001 watt in 600 ohms, the terms vu and decibel may be substituted in any problem.
We note the mathematical error in point 2.5 where it states that 1.73 V in 600 ohms approximates 6 milliwatts or .006 Watts. In fact 1.9 Volts across 600 ohms delivers .006017 watts – Editor.