Decibels (db's)

[pwdrive]

Some times pictures help me, I have watched a few of this guys presentions in the past, my initial response after the opening gambit of the first one I watched was nah, I can't listen to this, however, I actually found that he got the message across to me.

This one is a primer on the decibel:

https://www.youtube.com/watch?v=mLMfUi2yVu8

Lawrence.

[Joe]

I'll have a look later when suitibly 'Braced.'

- Joe

[pwdrive]

Hi Joe, no need to be braced it's a relaxed presentation.

Lawrence.

[Skywave]

I was not impressed at all!

For a start, I found his style of delivery annoying and distracting. The section where he had 3 amplifiers in cascade and was talking about the 'gain' of each of those stages was very poor: he made no reference to whether the dBs he was referring to were voltage dBs or power dBs. He equated 'magnitudes' to a voltage or a current, but, by implication, not power. Technically then, he is incorrect: a 'magnitude' refers to a size of 'something': that 'something' could be a voltage, current, power, resistance - anything. When he was discussing frequency response of an amplifier, he kept referring to the frequency axis in units of dBs! He also made no reference to source and load resistances and that they often need to be taken into consideration when making dB calculations. There was also various other errors, but these come into the category of hair-splitting, so I have made no reference to them (not wishing to start an argument here! )

But overall, his biggest mistake was his failure to point out, right at the beginning of his 'lecture', that dBs are fundamentally a measure of the ratio of two powers and that using dBs to compare two voltages (or currents) can indeed be done, and indeed is, but when that is done, great care is required.

To me, I feel that anyone who is confused about 'dBs' and then watches that is going to be even more confused by the time they've reached the end of it (and then taken a deep breath! ). Overall I feel that I could have done a much better job.

Al. / Aug. 20, 2013 //

[Skywave]

I'll have a look later when suitably 'braced.'
Joe

Hi Joe, no need to be braced: it's a relaxed presentation.
Lawrence.

Well all I can say to that, Lawrence, is that your definition of a 'relaxed' presentation certainly isn't mine! I distinctly gained the impression that he was 'high' on some drug or whatever (not that I'm speaking from personal experience, I hasten to add! )

Al. / Aug. 20. 2013 //

[Joe]

He seemed unusually calm, Al.

I suppose it comes down to what you're used to. To me "half", "0.5" or "50%" is more natural that "-6dB" in the same way that 70°F is more immediately comprehensible than 21C.

- Joe

[Skywave]

O.K., Joe, perhaps it's just me then - but I felt that his enthusiasm (if that's what it was) was of such a magnitude that it seemed to cloud his thinking and made for a 'jumpy' and poorly-constructed presentation. Perhaps I should mention that during my working life I once held the post of the company's technical instructor, delivering courses to staff and customer-engineers alike: the preparation and rehearsal of lectures were essential to ensure a smooth, error-free performance. It was a job I really enjoyed, but it was hard work, and yes, it had its compensations, such as plenty of world-wide travel. I suppose that I am essentially using that experience as a yard-stick here. But then again, I always wanted to be a teacher.

As for dBs versus arithmetical and percentage ratios, don't overlook the essential and useful feature of working with decibels: that of being able to add (or subtract) to determine the end result, as opposed to a similar chain of calculations but which involve multiplication (or division).

Al. / Aug. 20, 2013 //

[pwdrive]

I was not impressed at all!

For a start, I found his style of delivery annoying and distracting. The section where he had 3 amplifiers in cascade and was talking about the 'gain' of each of those stages was very poor: he made no reference to whether the dBs he was referring to were voltage dBs or power dBs. He equated 'magnitudes' to a voltage or a current, but, by implication, not power. Technically then, he is incorrect: a 'magnitude' refers to a size of 'something': that 'something' could be a voltage, current, power, resistance - anything. When he was discussing frequency response of an amplifier, he kept referring to the frequency axis in units of dBs! He also made no reference to source and load resistances and that they often need to be taken into consideration when making dB calculations. There was also various other errors, but these come into the category of hair-splitting, so I have made no reference to them (not wishing to start an argument here! )

But overall, his biggest mistake was his failure to point out, right at the beginning of his 'lecture', that dBs are fundamentally a measure of the ratio of two powers and that using dBs to compare two voltages (or currents) can indeed be done, and indeed is, but when that is done, great care is required.

To me, I feel that anyone who is confused about 'dBs' and then watches that is going to be even more confused by the time they've reached the end of it (and then taken a deep breath! ). Overall I feel that I could have done a much better job.

Al. / Aug. 20, 2013 //

He did actually refer to voltage at the beginning of his explanation of those three amplifier stages.

He did show the difference in the formulas between V (or I) and power when converting to db's

He emphasized earlier on the that db's was just an expression of a ratio, to me that's all db's is.

He explained why using a logarithmic scale has it's advantages in magnitude terms for the Y axis and the X axis as per the spectrum display.

I personally think he initially used the term db's in the X axis purely to illustrate the advantages of displaying time as an order of magnitude which reinforced his early explanation the db's is a ratio, he then clearly went on then to show that order of magnitude in real numbers relating to time/frequency.

That for me was the essence of his presentation, it's just a ratio.

Yes suffixes have to be assigned and I am sure he did mention that somewhere.
It was a simplified explanation and he did not try to hide that fact, like he said it's just a ratio, it could be as a sound pressure level that we can hear or voltage/current/power that we cannot see.

As to his style, I can see he's not going to be everyones cup o' tea, that was my initial response when I first clocked one of his videos as I said previously, however I found his explanation ok and on a level I could relate to....despite his squeaky voice.

Lawrence.

[pwdrive]

A sort of related subject:

X1/X10 'scope probes and why the frequency response is poor on many of them when switched to the X1 function ie: don't condemn the 'scopes rated bandwidth....

Yes it's that bloke who did the db's video as mentioned, an explanation from others are available.

Lawrence.

EDIT 1: 'scope probe:

https://www.youtube.com/watch?v=OiAmER1OJh4

EDIT 2: Cockroft Walton Multiplier (same presenter)

https://www.youtube.com/watch?v=ep3D_LC2UzU

EDIT 3: DB's again, a more comprehensive slower pace explanation (That's yer lot for my links)

http://www.fourier-series.com/rf-concept...ibels.html

[AlanBeckett]

Are you going to add a Link in there young Lawrence?

I agree with Al in that I prefer a more 'sober' approach. I'm afraid that the modern 'frantic' Presenter isn't my scene either

Alan


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