Decibels - Standard Operating Level

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We have spoken about the audio chain and the different stages a sound signal goes through to be transformed step by step into what we need. Every stage receives an electric signal at its input, it manipulates it and supply the result of the manipulation that has taken place at the output. The output signal that ensues from a stage will feed the the next stage's input which itself shall apply a further transformation, and so on. To get a better picture let's imagine that the last stage is a power amplifier [Amplification ] connected to a loudspeaker system.

What happens if the output from one stage has a much higher voltage than the amount the next stage is capable of handling? The answer is not at all obvious and there are many factors involved; generally speaking we can say that the second stage shall have to handle a signal which is too high and for which it hasn't been calibrated, thus resulting in distortion [Distortion by saturation ] . The distortion's intensity shall increase the more the signal goes beyond what the second stage is expecting to receive. But what exactly is the second stage expecting to receive? The answer to this question is the SOL, acronym for Standard Operating Level.

Let's give a practical example: say that the SOL of a certain module is 1 Volt. This will mean that the audio signal passing through this stage shall have maximum values of around 1 Volt or slightly more (to be more precise SOL is measured in dB, but for now we'll stick to normal values).

In the following table we have the SOL values and their corresponding voltages in various operating contexts:

Table 3.2. Reference values for the Standard Operating Level 

Operating contextdBVolt
Professional+4 dBu1.2 V
Semi-professional-10 dBV0.32 V
Broadcasting6-8 dBu1.55 V - 1.95 V
Standard Hi-Fi user-10 dBu0.25 V

We can elicit some interesting considerations from this table. Firstly we see how every context has its own dB reference value: dBu in the professional field, semi-pro and broadcasting, dBV in the semi-professional field. But what is particularly interesting is the voltage column on the right. We can see how in the professional field the voltage level relating to the SOL, is 1.2 V, whilst in the Hi-Fi field it is 0.25 V. The first signal has an amplitude which is about 5 times greater than the second and thus allows a far more faithful sound reproduction to take place (to convince you of this fact, try and imagine that you are working with a SOL level of 0.000001V. Would you still be able to spot the difference between two voltages such as: 0.0000015 V and 0.0000016V?). So, the higher the SOL (namely, the higher the voltage level at which a circuit operates) the more accurate the reproduction of the waveform. Obviously quality demands cash; circuits that work with higher voltage levels are more expensive, and this truth is applicable in general and shall be reiterated many times in this course.