[sdiy] DACs on-board 12 vs external 16 vs 24 bit

Neil Johnson neil.johnson71 at gmail.com
Wed Nov 6 17:38:41 CET 2013


> I've had a feeling you'd bring up the dBm calculations. AFAIK dBm is
> normally specified into 600 Ohm, unless specifically noted otherwise.
> However, even with 600 Ohm meters, the impedance dissipating the power
> is denoted. The manual always has that info.

I beg to differ.  "dBm" as written makes to reference to any impedance
whatsoever, the only reference is 1mW (the "m" suffix).  The impedance
is only useful when you wish to convert that into volts, and as you
say in audio that is traditionally 600 Ohms, hence 0dBu is the voltage
generated by 1mW into 600 Ohms = 0.775V.  But in reality 600 Ohm
inputs and outputs are rarely used in the audio world, hence the need
for dBu and dBV.

> 600 Ohm is the typical value for dBm as used in audio engineering, and
> I have a feeling that this page is focused specifically for that. No
> RF concepts are mentioned.

Yes I suspect you're right.  But that is no excuse for unwritten
assumptions on a page which is otherwise an excellent tool.

> I too think it would be a good idea for them to mention where the dBu
> and dBV references come from.

We agree :)

Just don't go above 0dBFS!!!!!

This video (I think I linked to it before) has some audio examples of
the effects of bit depth and dither, including shaped dither:

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

Neil
--
http://www.njohnson.co.uk



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