There are several approaches taken by manufacturers to enhance legibility of speech within noisy signals.
Since the human voice is centered at ~2,500 Hz, with speech extending roughly from 500 to 4,000 Hz, this is the range that needs to be emphasized.
Heil prefers the parametric EQ approach, where he uses 2 or 3 channels and enables changing the center frequency, bandwidth and amplitude in each. Most new radios have something similar built in, often minus the ability to change the center point or the audio bandwidth. His favorite boost frequency is 2,500 Hz. The frequencies below and above 2,500 are cut.
This is very much like the standard graphic equilizer, but with the focus on frequencies used in ham radio audio. There is little to no use for frequencies below 500 or frequencies above 4,000.
Several other manufacturers just cut frequencies below 1,000 Hz and above 3,000 Hz which has a similar effect and with a little gain added to the entire frequency band.
Then there are the Digital Signal Processors (DSPs). This is where it gets fuzzy for me. These systems sample the signals, sample the noise, and separate out what it believes to be voice and voila, a more legible voice minus most noise.
For those who have used EITHER or BOTH techniques via an external component to enhance voice clarity, how have they worked for you? Which one does the better job?(And yes, yes, I understand that eliminating noise BEFORE the audio stage is preferred. This post RF audio topic is directed toward those of us who have done all we care to do on the RF end and want to accomplish what we can on the output/audio side.)The external, stand alone products I am considering are these:
XIEGU GBR-1
https://qrznow.com/xiegu-gnr-1-noise-reduction-unit-full-review/Timewave DSP-599 or DSP-9 (Both discontinued???)
West Mountain Radio CLRdsp
http://www.westmountainradio.com/product_info.php?products_id=clr_dspBHI (various models), mainly parametrics
https://www.bhi-ltd.com/Are there others you have had good experience with?
Please describe the type of noise your solution has best suppressed.
My noise is unidentified neighborhood or atmospheric broadband. I already shut down everything electrical in my house, run my radio off of a lead acid battery. Going out 100 feet around my house with an HT I get solid S9 on 2 meters on up.
So, at this point I am only interested in potential radio audio output processors that enhance voice clarity buried in noise, whether parametric EQ, DSP, or a combination.
Oh, and a few of the demonstrations I have seen on a couple of these products demonstrate achieving better voice clarity than can be achieved with the NR in even newer radios. But I take those with a grain of salt until I get more user feedback. I have an Icom IC-7100. And yes, I've played with and set the internal tone and noise settings.