On 17/06/2018 12:18, Claude Frantz wrote:
If I understand well, an ADC itself can only operate in fixed point
mode. If a floating point output signal is expected, I think that there
should be a switchable attenuator at the input of the ADC, in order to
avoid a loss of resolution. I ignore if real sound cards are built in
this manner. Finally, it's important that the level of the input signal
is at the optimum level so that the ADC can operate at the best possible
resolution. My question is: Have the different level sliders any action
(attenuation or amplification) on the level of the signal entering the
ADC ?
A further question: Is there any tool (perhaps in ALSA) which offer the
ability to verify and adjust this level at the input of the ADC, to the
optimum value ?
Hi Claude,
all consumer ADCs, to my knowledge, are integer output. Where floating
point sample streams are used it is normally only for applying
algorithms purely in the digital domain for DSP. Converting a digital
sample stream from integer to floating point does not "improve" the
resolution, it just allows multiple calculations to be applied to it
without introducing large cumulative errors.
Sound cards usually have some crude gain control before the ADC,
normally as one or two switched fixed gain stages, these are intended to
allow connection of low output microphones as an alternative to using a
line level input. The operating system normally exposes this gain
setting as a "Microphone boost" control or similar. The gain steps are
usually 10dB and 20dB so you might find a slider with 0dB, 10dB, 20dB,
and 30dB levels from the combinations of a 10dB and a 20dB switchable
gain stage in series. Higher end sound cards might have analogue gain
potentiometers with a physical knob to adjust levels into the ADC.
Other than the above all other controls will be in the digital domain
and should be set to 100% i.e. 0dB.
73
Bill
G4WJS.
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