Determine and report maximum ADC level - for clipping/oversteering indication

[hayguen]

HDSDR shows the RF level in dBFS - assuming, that the I/Q samples it receives are really the sampled values.
strong signals, might get filtered out with the DDC step before HDSDR gets the I/Q data and computes the RF dBFS value shown in the upper right corner of the RF spectrum .. which now might be too low not indicating the correct value ..

[howard0su]

How can I report +20db always on LNA that RX888 has? or HDSDR doesn't care about dBm at all?

Current code has a gain manually calibrate (Oscar did that).

Shall I use a fixed gain like 1.0f/32767/1000? I tried that and HDSDR reports -50dbFS, which is far above LTC2208 datasheet which is about -110dbFS.

[hayguen]

don't think, that HDSDR cares for any "dBm" - which is more than unknown if you have different cable length or have switched in an attenuator or external LNA ..

manual calibration is available with Options (F7) / "Calibration Settings" in the "S-Meter Calibration" tab.
clicking on the power value below the S-meter switches "S" values to plain "dB" values.

seeing the text field below "Last Calibration in that dialog:

Time:    20210107_101612Z
Input:   ExtIO_SDDC.dll
LO:      103.000.000Hz
RF-Gain: +29,7dB
IF-Gain: +11,2dB

containing LO frequency and the RF/IF Gain values .. reminds me on how many premises the dBm power depends on - especially with the R820T2-tuner on VHF!

HF with direct sampling might be easier ..
we could suggest a new ExtIO function delivering an initial "Correction Value [dBm]" for the S-meter calibration.

my issue is independet from power .. to allow exact determination of the ADCs clipping/oversteering in dBFS - similar to #98

[hayguen]

additional note on

1.0f/32767/1000

the ExtIO does deliver 32-bit float values. these values should be nearly +/-1.0 peak-to-peak for 0 dBFS,
that when recording RF files in PCM16/PCM24 there is no dramatic loss of dynamic range.
float values with abs() >= 1.0 would surely produce a mess! many audio editors also don't like such files.

too small values would produce/loose sensivity, cause of truncation of lower bits. also not good.
suppose, Oscar's calibration solution might have this problem!?

[hayguen]

Mario (HDSDR) just wrote me to press Ctrl + Shift + J to get dBm values below the S-meter.
couldn't find that in the menus/options in the GUI

from http://www.hdsdr.de/hdsdr_keyboard_shortcuts.htm:

Switch S-Meter and RF Spectrum compensation:
 off: without '= ' prepended to strength value (=default)
 on: with '= ' prepended to strength value,
   compensates strength value for RF/IF gain and attenuation - when known

there are some other interesting functions in the Lab section:
http://www.hdsdr.de/hdsdr_keyboard_shortcuts.htm#lab
have a look there!