1.4 Desarrollo Cognitivo
1.4.1 Etapa Sensoriomotriz
Volts, amperes, and watts can be measured by inserting an appropriate meter into the circuit. If all audio signals were sinewaves, we could insert a dBm meter into the circuit and get a reading that would correlate with both electrical and acoustical variations. Unfortunately, audio signals are complex waveforms, and their rms value is not 0.707 times peak but can range from as small as 0.04 times peak to as high as 0.99 times peak, Fig. 7-12. To solve this problem, broadcasting and telephone engineers got together in 1939 and designed a special instru-ment for measuring speech and music in communi-cation circuits. They calibrated this new type of instrument in units called VU. The dBm and the VU are almost identical, the only difference being in their usage. The instrument used to measure VU is called the volume indicator (VI) instrument. (Some users ignore this and incorrectly call it a VU meter.) Both dBm meters and volume indicator instruments are specially calibrated voltmeters. Consequently, the VU and dBm scales on these meters give correct readings only when the measurement is being made across the impedance for which they are calibrated (usually 150Ω or 600Ω). Readings taken across the 2
Doubled Quadrupled Doubled Quadrupled
+6dB +6dB +6dB +6dB
Figure 7-11. Voltage, electrical power, Pw, and sound pressure, compared.
102 Chapter 7 design impedance are referred to as true levels, whereas readings taken across other impedances are called apparent levels.
Apparent levels can be useful for relative frequency response measurements, for example.
When the impedance is not 600Ω, the correction factor of 10log(600⁄new impedance) can be added to the formula containing the reference level as in the following equation:
(7-43)
The result is the true level.
7.19.1 Crest Factor
The crest factor (CF) is the ratio of the peak output to the average output. It is typically graphed in terms of the output power and is expressed in dB. For
example, the CF of a sine wave is 3dB. The CF of music may vary between 6dB and 24dB.
Crest factor is defined as ten times the logarithm of peak power out divided by average power. The actual measurements are often made using voltage.
In that case we divide the peak voltage squared by the rms voltage squared. The root mean square integral is;
(7-44)
The rms voltage squared divided by the impedance is average power.
Using voltage implies that both measurements were made across identical impedances or (open circuit). Crest factor applied to voltage waveforms is a common practice but not the defined term. Powers are chosen because when impedances vary widely over the bandwidth of interest, and consequently the bandwidth is divided into components, the powers can be added to obtain the overall crest factor. The Texas Instruments “Guidelines for Measuring Audio Power Amplifier Performance” page 25 displays actual measurements and the usefulness of power measurements. When we remember that power describes heat dissipation in power amplifiers then CF as a ratio of powers becomes evident.
7.19.2 The VU Impedance Correction
When a VI instrument is connected across 600Ω and is indicating 0VU on a sinewave signal, the true level is 4dB higher, or +4dBm, instead of 0dBm or zero level. The reason this is so is shown in Fig. 7-13. The V I instru ment use s a 50µA Figure 7-12. Sinewave voltage values. The average
voltage of a sine wave is zero.
rms Peak
Peak to peak Max
neg One
alteration
One cycle Max
pos
90°
180° 0°
270° 360°
rms = 0.707 × peak voltage rms = 0.3535 × peak to peak voltage peak = 1.414 × rms voltage
peak-to-peak = 2.828 × rms voltage
True VU Apparent VU 10 600 Z measured ---log
+
=
erms 1 T--- e2dt
0e T
⎝
∫
⎠⎜ ⎟
⎜ ⎟
⎛ ⎞0.5
=
Figure 7-13. VI instrument circuit.
6 0 0Ω
6 0 0Ω
3 9 0 0Ω
3 9 0 0Ω 3 0 0Ω 7 5 0 0Ω
3 6 0 0Ω
3 9 0 0Ω 3 9 0 0Ω 3 9 0 0Ω
L i n e L o a d A t t e n u a t o r
3 9 0 0Ω constant impedance Meter
Using the Decibel 103 D’Arsonval movement in conjunction with a
copper-oxide bridge-type rectifier. The impedance of the instrument and rectifier is 3900Ω. To mini-mize its effect when placed across a 600Ω line, it is
“built-out” an additional 3600Ω to a total value of 7500Ω. The addition of this build-out resistance causes a 4dB loss between the circuit being measured and the instrument. Therefore, when a properly installed VI instrument is fed with 0dBm across a 600Ω line, the meter would actually read
−4VU on its scale. (When the attenuator setting is added, the total reading is indeed 0VU.)
Presently, no major U.S. manufacturer offers for sale a standard volume indicator that complies with the applicable standard (C16.5). The standard requires that an attenuator be supplied with the instrument and none of the manufacturers do so.
What they are doing requires some attention. The instruments (usually high-impedance bridge-types) are calibrated so as to act as if the attenuator were present. When the meter reads 0VU (on a sinewave for calibration purposes), the true level is +4dBm.
This means a voltage of 1.23V across 600Ω will cause the instrument to read an apparent 0VU. Note that when reading sine wave levels, the label used is
“dBm.” When measuring program levels, the label used is “VU.” The VU value is always the instru-ment indication plus the attenuator value.
Two different types of scales are available for VI meters, Fig. 7-14. Scale A is a VU scale (recording studio use), and scale B is a modulation scale (broadcast use). On complex waveforms (speech and music), the readings observed and the peak levels present are about 10dB apart. This means that with a mixer amplifier having a sinewave output capability of +18dBm, you are in danger of distor-tion with any signal indicating more than +8 VU on the VI instrument (+18dBm − [+10dB] peaking factor or meter lag equals +8VU).
Fig. 7-15 shows an example of commercially available VI instrument panels used in the past that included the VI instrument and 3900Ω attenuator, which also contains the 3600Ω build-out resistor.
7.19.3 How to Read the VU Level on a VI Instru-ment
A VI instrument is used to measure the level of a signal in VU. In calibration: 0VU = 0dBm and a 1.0VU increment is identical to a 1.0dB increment.
The true level reading in VU is found by:
(7-45)
or
where,
Apparent level = Instrument indication + Attenuator or sensitivity indicator.
Thus, we can have:
1. A direct reading from the face of the instrument (zero preferred).
2. The reading from the face of the instrument plus the reading from the attenuator or other sensi-tivity adjustment—normally a minimum of +4 dB or higher. When the instrument indicates zero, the apparent level is the attenuator setting.
3. The correction factor for impedance other than the reference impedance. 600Ω is the normal impedance chosen for a reference, but any value Figure 7-14. VI instrument scales.
A. Recording and test equipment.
B. Broadcast monitoring.
Figure 7-15. Examples of commercial-type VI instru-ment panels.
True VU level Apparent level Impedance correction= +
True VU level Instrument indication
10 600
Zact ---
⎝ ⎠
⎛ ⎞
log
+
=
104 Chapter 7 can be used so long as the voltage across it results in 0.001W, Fig. 7-16.
Example
We have an indication on the instrument of −4VU.
The sensitivity control is at +4. We are across 50Ω (a 100W amplifier 70.7V output). Using Fig. 7-16, our true VU would be −4VU + (+4VU) + 10.8 correction factor = 10.8VU.
7.19.4 Calibrating a VI Instrument
The instrument should be calibrated to read a true level of zero VU when an input of a 1000Hz steady-state sinewave signal of 0dBm (0.001W) is connected to it. For example, typical calibration is when the instrument indicates −4, the attenuator value is +4, and it is connected across a 600Ω circuit. Levels read on a VI instrument when the source is the aforementioned sinewave signal should be stated as dBm levels.
7.19.5 Reading a VI Instrument on Program Material
Because of the ballistic properties of VI instruments, they exhibit what has been called “instrument lag.”
On short-duration peak levels, they will “lag” by approximately 10dB. Stated another way, if we read a true VU level of +8VU on a speech signal, then the level in dBm becomes +18dBm. This means that the associated amplification equipment, when fed a true VU level of +8VU, must have a steady-state sinewave capability of +18dBm to avoid overload.
Rule
Levels stated in VU are assumed to be program material and levels stated in dBm are assumed to be steady-state sinewave.
7.19.6 Reading Apparent VU Levels
VI instruments can be used to read apparent or rela-tive levels. If, for example, you know that overload occurs at some apparent level. You can use that reading as a satisfactory guide to the system’s oper-ation, even though you do not know the true level.
When adjusting levels using the instrument to read the relative change in level, such as turning the system down 6dB, you do not need to do so in true level readings. Instrument indication serves effec-tively in such cases.
When being given a level, be sure to ascertain whether it is:
1. An instrument indication.
2. An apparent level.
3. A true level.
4. A relative level.
5. A calibration level.
6. A program level.
7. None of the above but simply an arbitrary meter reading.
Special Note: Well-designed mixers have instru-ments that indicate the available input power level to the device connected to its output. Such levels are true levels.
7.20 Calculating the Number of Decades in a