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Interface, dynamics, headroom, and ideal input level

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Re: Interface, dynamics, headroom, and ideal input level

Postby Hugh Robjohns » Fri Dec 18, 2020 7:57 pm

merlyn wrote:There's a well accepted definition of 'unit'. The decibel like the litre is a 'Non-SI unit accepted for use with SI units'. :)

In the same way that the litre is built upon the metre, the Bel is built on the Neper... And it's a relative rather than absolute quantity.

If the OP's friend had been aware of that he may have realised you can't add and subtract them as you please. :)

Usually, you can -- that's kind of the whole point... But the context definitely matters, of course!

If I have a signal at +4dBu and make it 6dB louder, it will come out at +10dBu.

If a converter is aligned such that +4dBu = -20dBFS then if I raise the signal level by 6dB in the digital domain it will be at -14dBFS, and the D-A converter will give +10dBu...

But whatever... I'm not convinced the OP's friend was struggling with the maths. It seems to me more a misunderstanding of fundamental concepts relatesignally amid range, signal levels and noise floors!
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Re: Interface, dynamics, headroom, and ideal input level

Postby Wonks » Fri Dec 18, 2020 10:11 pm

merlyn wrote:There's a well accepted definition of 'unit'. The decibel like the litre is a 'Non-SI unit accepted for use with SI units'. :)

You really need to go back and read and understand what you've written.

The litre is definitely an SI unit!

You are however trying to compare dimensionless 'units' (which are really names for ratios or a calculated value) with true dimensioned units like the litre, metre, amp, volt etc.
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Re: Interface, dynamics, headroom, and ideal input level

Postby merlyn » Fri Dec 18, 2020 11:35 pm

Maybe you could edit the Wikipedia page and put them right :

https://en.wikipedia.org/wiki/Internati ... se_with_SI
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Re: Interface, dynamics, headroom, and ideal input level

Postby Hugh Robjohns » Sat Dec 19, 2020 12:09 am

Wonks wrote:The litre is definitely an SI unit!

Hmmm... We'll, kind of... But not really! ;)

As I said earlier, it comes down to what meaning you associate with the term 'unit'. Most of us could list a bunch of 'units' we readily use within the SI structure, but it's actually a bit more complicated than that.

Merlin is quite correct in the pedantic wormhole about the litre -- I had to go back and refresh my memory about this, too.

There are only seven 'base SI units'. They are the kilogram, metre, second, Kelvin, ampere, Mole, and candela.

...and if you doubt Wikipedia (always wise...) the National Physics Laboratory is quite clear about it: https://www.npl.co.uk/si-units

These base units are supplemented by 12 'derived SI units' for things like area (m^2), speed (m/s), magnetic field strength (A/m)... etc -- all based on variations or combinations of base SI units.

Then there are 22 'derived SI units with special names', like Hertz, Pascal, Newton, Joule, Volt, Watt, and many more. For example, Hertz is 1/s, Newton is metre kg s^2, Pascal is Newton metre^2... and so on.

In this strict context, the litre is genuinely defined as a 'non-SI' unit (along with the tonne, and the Neper and Bel, amongst others). This is because it is used as a popular convenience, in place of the correct SI derived term -- volume or capacity is defined in the SI units as a cubic metre (m^3). The litre, of course is a volume of 1 decimetre cubed -- a much more practical measurement for daily tasks, but not strictly the SI standard of volume!
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