Decibel and Meter – I

The Decibel is the logarithmic measure unit used for the sound, but depending on its application has its own scale and reference values. We leave aside for the moment the decibel acoustic (pressure, power, density and loudness) that requires arguments apart and instead concentrate on the scales and measurements in decibels used in analog and digital audio equipment.

In electrical and electronic equipment the audio signal circulates in the form of sinusoidal electrical pulses in analog field and square-wave pulses in the digital field. The unit of measurement used to define the values ​​of circulating voltage is Volt but in audio environment to have a common unit of measure and a more easy and understandable reading we use the Decibel.

The applications in which it uses more of the decibel to define a scale of voltage values ​​are the meter, which identify the signal level in input and output from the audio equipment, so as to allow a correct interfacing between the devices and the correct sending of right amount of signal, so as to obtain an optimum signal level without the system result to be under or oversized could in certain cases create values ​​of distortion and damage to the equipment.


Decibel Types

Let us now see what are the most commonly used standard for defining a decibel scale in analog domain.

Standard EBU and SMPTE

The EBU (Europe Broadcast Union) and SMPTE (Society of Motion Picture and Television Engineers) are two companies, the first European and the second American that definite the Standards (both constructive and relevant to the circulating voltage values ​​in equipment) for broadcast and radio broadcasting which also includes both the Live and Studio.

Data on references Decibel and related voltage values ​​that we will see to the professional level and are the most used and to define them is the SMPTE standard. The EBU is more focused on the European Broadcast and has slightly different values.

At one time there was only one decibel value used in any standard, the dBm.

m stay for mW (milliwatts) because the systems of work (mixer – amplifiers, processors) used as a reference the power (the product between voltage and current) instead of the single voltage (for reference it refers to the maximum average value of the signal as possible for the input and output of a device, such as mixers, amplifiers or processors as effects or even equalizers, beyond which it can generate distortion values ​​of a percentage order beyond tolerance, that for electronic equipment is 0.1 – 0,01 %).

n.b. Calculated circuits for interfacing with in reference power generated more distortion so less quality than those of today but allowed to bring for example from a mixer to the amplifier a higher power with voltage reference systems.

As today are used almost exclusively in circuits made with reference voltage as they bring more quality and less distortion, considering that to now also with a lower power is possible to obtain relevant values sound pressure.

This decibel was used at the professional level in the 60 ‘- 70’ – 80 ‘at the time of the valves, it is now obsolete but you can still find in some vintage mixer.

In practice it is to indicate the outgoing passage of 1mW on a resistance of 600 Ω. In fact, the mixer once had in their output (eg. the main out) a resistor (as seen in the explanation of the balanced lines on the subject Analog Audio Cable – III) of 600 Ω, from which did pass a power of 1mW. The voltage value of this output is 0,775 V.

n.b. The reference values ​​and standards such as also the dBm are always calculated with fader or gain at 0 dB, for which no attenuations and amplifications pre input or pre output.

Generally the standard is calculated for the output while for the inputs is referred to the reference signal level acceptable for that type of circuit (as we will see are different according to the standard), and through the meter (for both outputs and inputs) it is possible as we’ll see understand the level in dB of the voltage currently circulating.

For circuits with a reference voltage at a consumer level, there is dBV.

V stands for voltage because it has the reference value in voltage.

It is used in the consumer market but you can also find in some professional devices to interface with them consumer signals (CD players, smartphone). It refers to 1 V on a 600 Ω resistor.

It is often indicated with -10 dBV. (As a reference to 0 dB 1 mW on a resistance of 600 Ω), it has the value of 0.3 V.

Today in professional mixers or any other professional audio device in their line-out they have a resistance to 50 Ω or less, so there was the need to introduce another value: the dBu or dBv.

It is never written with v minuscule so as not to be confused with the large V letter and then it’s found almost always the dBu. It’s reported to the voltage. It can be found as a reference to 0.775V when 1mW passes over a resistance of 600 Ω and therefore identical to dBm, written only dBu because the circuit is voltage reference. Or, in the more professional systems with reference to 1,224 V because modern devices as we have said has the resistance of 50 Ω.

Very often you can find written in various devices + 4 dBu, this because placing as a reference outputs or signal inputs of a time, and then 0,775 V to 0 dB (0 dB in an input device or audio output is to indicate that the voltage level is to be operating standard), the outputs of today have a value of + 4 dBu and that is, as we have seen 1,224 V.

These are the main values of voltage used in the music from live or studio for the signal line in output or input. The line paths used in the broadcast, then in television or in the movies have higher values and that of + 8 dBu (2.24 V).


Standard at digital level

Let us now see what are the most commonly used standard for defining a decibel scale in the digital domain.

For voltage standard  in the digital domain means the signal path from  output to input devices always in the digital domain, and then after the A/D conversion and before the D / A conversion (digital signal will always and in any case be convert in analog before the sound diffusion because the physics of sound is in nature in the analog domain).

The two representative standards are always EBU and SMPTE that pose as a reference the maximum value of the square wave beyond which are generated distortion phenomena. For the EBU is + 18 dBu (always taking as a reference the 0 dBm previously seen) equal to 6.16 V, for the SMPTE is + 24 dBu equal to a voltage value of 12.28 V.

Some manufacturers mostly for marketing strategies create their own standards of use for the connection of their digital audio equipment. This is especially true in consumer because both the EBU and SMPTE standards have created a standard recital to join the consumer environment to the professional.

n.b. Working with higher voltages as digital case than the analog need is more sophisticated and delicate circuits but allows to obtain downstream of a better quality of the signal path less affected by external interference and especially also because in the digital domain, the wave is square and not sinusoidal harder to deform, in the digital domain and then as we will see in other arguments there will be other problems.


The 0 dB

To define a reference is not enough, however, to decide how they should be the input and output circuits, such as resistors and the voltages in operation, but it is necessary to allow the operator (audio technician) can understand the magnitude of these values so that you can manage it at the best.

To do this the meter (fig. 1) which, as stated allow you to view the signal level in the input and output, has been realized placing the number 0 as a reference.

Fig. 1 Meter.jpg

When the signal level reaches 0 dB means that its value is the maximum as below standard, it is therefore important to know and understand the type of standard used (thus circulating voltage), so as to interface different devices without putting in overload or downsizing the device to which the signal arrives.

Values higher ​​to 0 in the analog domain are accepted as we speak of sinusoids that have dynamic and RMS values, but generally up to a + 6 dB or + 9 dB as it is already  + 1 dB the circuit starts to become unstable (alterations of frequency and dynamic response) and begin to be generated the first distortions up to breakage of the components themselves.

n.b. In contrast to too low voltage values, so distant from the reference value, such as – 30 dB or – 20 dB, the signal response is not presented as from technical characteristics defined by the manufacturer, example, instead of having a linear frequency response has some loss in the high frequencies.

In digital meter (fig. 2) 0 dB is set as a reference for the maximum tolerable voltage before the distortion, this is because in digital they have square waves that do not present dynamic and RMS values, so even if only a voltage of 0.1 dB in + creates more distortion.

Fig. 2 level_meter_horizontal.gif

Ideally it is analog or digital in order to obtain an ideal and clean signal that fully exploits the circuit potential, the signal level of input and output should always be at 0 dB, we will see later in future explanations like this it is impossible dependent on the dynamics of input signal to the analog components or to the A / D converters which is then passed to the levels of the output signal and also depending on the recording and mixing techniques used.

Below is an example of references for analog audio signal levels in the two standards including consumer and pro not included in the two mentioned companies.

Fig. 3 levels_004.jpg

The first bar identifies the EBU standard in which the voltage value output from the audio equipment to the line level to 0 dB is + 6 dBu sometimes even + 8 dB which represents a voltage value equal to 2.24 V ( depending on whether the output resistance is 50 ohms or less).

The second bar is the SMPTE standard in which the voltage value output from the audio equipment to the line level to 0 dB is + 4 dBu which represents a voltage value equal to 1.224 V (in which we consider a resistance output 50 ohms).

The third bar is the voltage value at 0,775 V previously born to the EBU and SMPTE standards.

The bar 4 and 5 instead of the standard used in consumer level.

Below is a table summarizing the types of Decibel and their relative use in analog connections:

Fig. 4 2016-01-06_01-21-34

Knowing these values is fundamental to interfacing the various devices as a 4 dBu output line should never be connected to an input to – 10 dBV to not over-dimension the circuit and risk distortion as well as basic loss of Signal quality.

To make some examples:

A line at + 4 dBu as may be the value of the lines at the professional level (usually traveling on XLR or Jack) eg the main out of the audio mixer will be connected to a + 4 dBu input in the power amplifiers that supplies the audio speakers.

A line at – 10 dBV such as the audio output of a CD player (generally traveling on RCA or MiniJack) is connected to a – 10 dBV input line, such as the RCA inputs in some audio mixers.

Below is a summary table of Decibel typologies and their relative use in digital connections:

Fig. 5 2016-01-06_11-09-02

Below is an example of references for digital audio signal levels in the two standards including consumer and pro not covered in these two companies:

Fig. 6 1042523-Digital Headroom Levels Chart

The first bar represents the scale commonly used in digital meter with reference maximum 0 dBFS.

The second bar shows the SMPTE standard in which is known as + 24 dBu is the maximum voltage value portable.

The third bar shows the EBU standard in which is known as + 18 dBu both the maximum voltage portable.

The fourth, fifth and sixth bar shows that some manufacturers (eg. RME) do not rely on existing standards but create another one just for the connection of their equipment and to limit so the consumer purchase and loyalty.

To give some examples:

Digital equipment is born and built to work best interfaced between them, in fact when connecting digital audio equipment according to the following standard connection so that they can be synchronized and can read and transmit the audio signal (we shall see later transmission standards data used at the professional level audio to other arguments) the voltage level can be safely brought to 0 dBFS as the circuits are constructed to read and transmit these voltage values ​​(actually seen the dynamics at play it is always good to have peaks signal at most – 6 dBFS or – 5 dBFS, even more when it comes to audio files to be converted, as the conversion especially if to sampling and quantization lowest always leads to an increase in the signal level (+ 1 / + 2 dB), the same is true if you play an audio file through D / A converters or players with poor quality algorithms (such as mp3 player).

n.b. In general it is always good to stay a few points below 0 dBFS as any kind of meter also the fastest and instantaneous as the Peak Meter or Digital Meter always has a mean value and a display tolerance, and may happen coincidence that the signal is in distortion + 1 dB but the meter displays for example: – 1 dB, or 0 dB.

When connecting devices that have digital meter (as now all modern digital mixers) with analog equipment is good to be careful not to bring the input and output analog circuits to values ​​of distortion and consequent breakages. If the digital meter follows the SMPTE standard we will have to send a maximum voltage value to the analog device equal to – 24 dBFS which is the 0 dBu with voltage values ​​equal to 0.775 V or – 20 dBFS which is the + 4 dBu with voltage values equal to 1,224 V according to the reference using the input device. If follows the EBU standard we will have to send a maximum voltage value equal to – 18 dBFS which is the 0 dBu with 0,775 V voltage or – 14 dBFS which is the + 4 dBu with voltage of 1,223 V.

n.b. The 0 dBu in the digital domain represents the voltage of 0,775 V and not 1,223 V, this is because the standard has been created before the introduction of 50 Ohm value for the output resistance line level.

Fortunately, most modern digital mixer signal the exceeding of the maximum value with a red or orange color LED (fig. 7).

Fig. 7 2016-01-06_00-52-41

n.b. In some cases it may happen that even if the power amplifier sending a voltage value equal, for example at – 10 dBFS this does not go in distortion, the main reason lies in the losses that the signal in the cable crossing path and in the resistances of input of the amplifier circuit. Is possible to raise the voltage until just before feedback of distortion values ​​of the input devices, considering the dynamics in operation, so as we shall see in subsequent arguments the loudness of the music program that is going to transmit.

While in digital level whether the sending of a digital signal such as a – 3 dBFS bring in distortion the digital input of deviceconnected to it is likely that the device that receives signal follows a different standard, such as the output is on SMPTE standard while the input is on EBU standard for which a 0 dBFS SMPTE sends in a distortion EBU input. Other times especially if the sending signals at proximate to 0 dBFS can be the input or oversampling circuit that quantizes the incoming signal producing sometimes an increase in dynamic range and voltage values producing distortion values. The latter phenomenon is very present when converting audio files especially in compressed audio files.


More on Decibel and Meter:

Decibel and Meter – II ( Analog Meters )

Decibel and Meter – III ( Digital Meters and Software )

Decibel and Meter – IV ( Normalization and LUFS Meter )

Decibel and Meter – V ( Meters in Audio Equipment )

Decibel and Meter – VI ( Loudness Manager, Loudness Engineer )


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