And http://superkuh.com/rtlsdr.html says:On keenerds driver for rtl_power its [-D direct_sampling_mode, 0 (default/off), 1 (I), 2 (Q), 3 (no-mod)]. I assume rtl_power_fftw would have something similar, if not you'd need to edit the code to add it in
So if you work through the context, direct_samp=3 seems to only be relevant if you are using the V2 or older style rtl-sdr dongles, and are using the 'keenerd' code base, and are trying to use that dongle to do direct sampling without any hardware modifications. Otherwise the values 0, 1, and 2 are as I explained in my previous post.No hardware change, software mod direct sampling
It has recently become possible to use direct sampling with no hardware modifications at all. It is still very experimental and performance is bad. In Oct 2012 Anonofish on the r/rtlsdr subreddit had discovered the PLL would lock for a small ~ 3686.6 MHz - 3730 MHz range far outside the normal tuning range and there seemed to be signals there. In January 2014 ##rtlsdr IRC channel user tejeez figured out this bypassed the tuner (mixer leakage) and implemented a set of register settings (R820T IF frequency, IF filter bandwidths, r82xx_write_reg_mask(priv, 0x12, val, 0x08) replaced with r82xx_write_reg_mask(priv, 0x12, val|0x10, 0x18)) that would exploit this to enable HF reception. Shortly thereafter keenerd assembled everything into a relatively easy to use patch-set.
If you want to give HF listening a try with no risk keenerd has added these changes rtl_fm and rtl_power in his experimental rtlsdr repository. To use the no mod mode with rtl_ tools append the argument, "-E no-mod". To use the no-mod direct sampling in something that uses gr-osmosdr, like gqrx or GRC flowgraphs, add the following to the the "device string" parameters: ie "direct_samp=3". Plug your HF antenna into the normal connector, no hardware mods needed.
Or, to put it another way, if you are using V3 rtl-sdr dongle and mainstream code base, direct_samp=3 doesn't do anything, which is what I observed.