RTCM: high pass filtering in hardware vs. software

AllStarLink
1

Greetings:

I am at the very early stages of helping a club set up a multi-receiver, single transmitter RTCM repeater controlled by ASL3, and an Arcom RC-210. If it were up to me, the RC_210 would be left out of the chain, but they still want it in-line for some specific reasons.

Anyway, I’m wondering what the best way to go about filtering RX audio would be. The RTCM has it’s own filter, but apparently, so does the channel driver.
I thought about disabling both, and just putting a notch filter at the fundamental PL frequency in rpt.conf, but thought that might screw up DTMF decoding as it does with simpleusb when the high pass filter is disabled.

Basically, I’m looking for the fullest sound with the least amount of compromise. The PL of the repeater is unfortunately pretty audible at 162.2 Hz. The current system, even with filters in place, you can still hear harmonics of that frequency in the chain. I’m hoping to avoid that problem with the RTCM, fed by a pair of flat Motorola CM200’s, but as it’s not currently in a state for testing, I don’t know which of the options is less intrusive.

Thanks and 73
N2DYI

2

From what I could tell in the RTCM manual it mentions only the voter.conf plfilter cfg, which should work the same as plfilter in simpleusb or rxhpf=0 in usbradio. Those should be very sharp (48dB/oct) filters that pretty much knock out everything below 300Hz, and with a narrow notch or two at higher harmonics (324 Hz, 487 Hz, etc) that ought to work very well with zero chance of interfering with DTMF tones (which are closer to 1KHz). Doing that sharp of filtering in analog would be pretty involved and not likely practical. An 18 or 24 dB/octave Butterworth HPF might improve ADC dynamic range a dB or 2 but probably wouldn’t make any difference with properly set levels.

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