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About noise and distortion measurements

Probably, it will not be a great exaggeration to say that the main parameters that we pay attention to when choosing equipment are noise and distortion levels. Why?

Perhaps – because almost any others – dynamic, frequency, etc., if you want and have some qualifications, can be easily changed in any direction you need, and these are almost unchanged. That is, it is possible to change something, but this will require a complete alteration of the entire product, and it is unlikely that it will ever be realized in reality.

Thus, these two parameters are “the objective reality given to us in sensations,” and to live and work with them. How to measure them, and – more importantly – to correctly interpret the result?

As previously written, for a correct measurement of noise, a quadratic voltmeter is necessary first of all. Conventional voltmeters – testers, etc., including digital ones – are unsuitable for these purposes. Why?

Yes, because they all measure other values ​​- average straightened, peak, etc., etc. In this case, often on the scale can even be written “RMS”, but this is not true, because this scale is only graduated in these values, but what is written above is actually measured. With such instruments, only a sinusoidal signal can be accurately measured: for the “sine” between its various values ​​(average, peak, effective) there are strictly defined relations, and corrections for them have already been made in the design of these instruments. Due to this, when measuring a sinusoidal signal, the results are reliable, but when measuring noise – alas! ..

In order to help you understand a little about this variety of different values ​​of the same signal, below are three formulas by which they are calculated:

About noise and distortion measurements (Let’s talk about how you can measure noise and distortion)
The average value of the signal, it is also a constant component (DC)

About noise and distortion measurements (Let’s talk about how you can measure noise and distortion)
Mean Rectified Signal

About noise and distortion measurements (Let’s talk about how you can measure noise and distortion)
RMS value of the signal, it is also effective

So, if you want to get reliable results when measuring noise, then first of all make sure that the voltmeter that you use for these purposes is truly quadratic, and not its “surrogate brother”.

So, you took a suitable device, connected it to the output of the device you are examining, and – can you already measure? It is possible, but better – it is not necessary. What is still missing? A few unobvious things – an oscilloscope. It would seem – why? It seems like the noise gathered to measure, and not look? Yes it is. But…

Noises in real studio equipment are very small, and make up (in the worst cases!) Fractions of a millivolt. Due to their smallness, even the most insignificant presence of other signals can strongly affect the measurement results, distorting them “beyond recognition”. Therefore, in order to know for sure that we are measuring the level of noise, and not something else unknown, it is desirable (but rather necessary) to additionally carry out visual control of the signal under study. (By the way, this is always useful – in order to know what exactly is being measured, otherwise you can “intend” this!)

“Theoretically” there can always be “a lot of things” in a signal, for example, background, ultrasonic interference from digital circuits, etc. etc. And in order not to make a mistake, it’s better to “see” this signal.

The oscilloscope must not be connected to the output of the instrument under study, but to the special output of the voltmeter. Practically in any “normal” voltmeter there is a special socket – “EXIT”. The signal already amplified inside the voltmeter is fed to it, and by applying the signal to the oscilloscope from here, you “kill two birds with one stone.”

Since the level of the initial noise voltage is very small, then applying it directly to the oscilloscope, you can – most likely – see nothing at all, because the sensitivity of most oscilloscopes is insufficient to analyze weak signals. In addition, if you connect the oscilloscope to the input of the voltmeter, then the oscilloscope itself will very likely be able to introduce noise to the input circuits of the voltmeter, and then – goodbye, objectivity of measurements!

Well – now that we have connected a voltmeter to the output of the device under study, and an oscilloscope to its output, are we ready to take measurements? Almost, but not quite. (Do not hurry!)

The fact is that modern “sound signal processing devices” (in the broad sense) are assembled, as a rule, on very high-speed, high-frequency elements – transistors and microcircuits. The spectrum of their noise can extend very far beyond the sound range, and since the voltmeter measures “everything”, its readings can therefore significantly differ from the values ​​perceived “by ear” in the sound range. How to be

Yes, it’s very simple – to include in the circuit a measuring filter that limits the frequency band supplied to the voltmeter by audio signals from 20 Hz to 20 kHz.

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