Pre-emphasis or Not for VHF digital modem tones


Greetings --

I posted this a while back in  but didn't get much of a response, so I'm reposting it in this group.

I'm somewhat new to using fldigi at VHF.  In doing testing and observing on the air signals, I see that some VHF stations are using pre-emphasis in their transmissions and others are not, often with some of those VHF stations not aware that they either are or are-not transmitting with pre-emphasis.

The difference appears to come about because some operators inject their fldigi TX audio into the microphone jack of their VHF transceiver, because that's the only option available to them (so their signals are transmitted with pre-emphasis).  And other operators inject their fldigi TX audio into the 6 pin (or 8 pin) mini-DIN jack on the back of their VHF radio when their radio has such a jack, because it's convenient way to connect to the transceiver, and those transmissions in most cases appear to be without pre-emphasis.

For high-speed wide-bandwidth operation at VHF is there any consensus among experienced VHF fldigi operators as to whether it's better to make those transmissions with or without pre-emphasis?  I would presume it's best for everyone to do the same thing (pre-emphasis or no-emphasis), but is it better for everyone to do it one way or the other?  If no pre-emphasis is the best option, should those who are forced to inject fldigi audio into the VHF transceiver's mic jack add some sort of external de-emphasis circuit before the mic jack, so that their transmitted audio spectrum has a flat frequency response?


Paul, ad7i

David Ranch

Hello Paul,

I think the main Fldigi list would be the best place for this qurstion since it really has nothing to do with the NBEMS type usage of Fldigi. 

Anyway, the issue of mismatched emphasis is mostly a matter of the software decoding the signals.  In a packet setup using old TNC2 hardware, the mismatch or "twist" really hurt the the decode performance.  More modern systems like KPC3s and software TNCs like Direwolf, UZ7HO Soundmodem, etc. all deal with the mismatch pretty well.  Generally speaking, all HAMs should use the right configuration but some modern radios do the WRONG thing on the "1200bps" RX pin and TX pin on the rear 6pin "DATA" jack and NOT apply the expected de-emphasis and emphasis!  Beware!

So the question of how Fldigi deals with it will generally depend on the modes you're using.  The potential risk I see is a remote station transmitting via the 6pin discriminator input at maximum levels and then a local station applying de-emphasis on the signal and lowering the high frequency levels by ~6dB. 
The wider the bandwidth, the more potential impact there might be.  I bet that the Fldigi modem will deal with the different levels just fine.