Reply To: This is impossible!

March 4, 2023 at 1:44 am #5082
James Johnston

    Indeed, and 32 fixed point, let’s consider that, please. (this really is acoustics but we’ll do it here).

    From noise floor to peak level is 6.02*32= 192.64 dB dynamic range.

    The noise floor of the atmosphere is somewhere in the 6dB SPL to 8.5dB SPL range at your ear drum. You won’t be rid of that until you have no atmosphere on both sides of the ear drum, which presents a few issues.


    So we’ll take the 6dB limit.  That means your peak level is 198.64 dB SPL.

    194dB SPL is a waveform that goes between 0 pressure and 2 atmospheres, yes, from perfect vacuum to 2 atmospheres.  So, right away, there’s not going to be any kind of linearity involved.  But let’s assume that it’s only positive peaks for a minute, forget that trivial little detail.

    198.64-194=4.64 dB above 1 atmosphere.  Divide by 20, exponentiate, that’s 1.7 atmospheres ABOVE 1 atmosphere, or about 25 PSI.  Now, 25PSI overpressure is rather a lot, it tends to be used only in “military” applications, to say the least.   It’s not lethal, but “serious hearing damage” is quite possible, also buildings may collapse, and the like.  In reality, it will take out windows, flatten weaker buildings, etc.


    So, yeah, 32 bit fixed point is a touch of overkill for capture.

    But, how do we get 198.64 dB of dynamic range in an electrical circuit given the charge on an electron?  Well, shot noise alone is 1/sqrt(number_of_electrons ) in the circuit. So you need about 2^64 electrons per second, which is thereabouts of 1.8 E19, or roughly speaking a peak current of a bit over +- an amp into the mike preamp, for a dynamic mike, and similar preposterous things for other kinds of mike.

    So, 32 bits for capture is ridiculous.

    For computation 32 bit fixed OR float may not be enough. Doing those 10Hz 3rd order butterworth highpass filters at 96kHz is “interesting” indeed, as RBJ and I have both pointed out a few times.