Danny Douglas wrote: > > I have a hard time visualizing the need for a narrow filter, for such > narrow modes. You can sit, in PSK for instance, slap up against > another PSK signal and still copy much weaker signals. Thats the > whole purpose of the narrow band digital modes to start with. I use > WinWarbler (now) to do my digital transmission in both PSK and RTTY, > and when I want to do something like Olivia or MFSK etc. I go to > MixW. I particularly like WinWarbler because it has the wide band > copy ability in PSK. I.E it will automatically copy (and show all the > channels) in a 2 or 3 KC bandwidth - at the same time. Using a narrow > filter in there would completely negate that fantistic capability. > > I have never had to use my narrow filters in order to copy a digital > signal, and yes I have played with them to see what difference it > made. Todays rigs, with their dsp filtering just seem to bypass any > need at all for additional filters for digital operation, though I do > see the need for CW filters, and have 250 and 270 cy fliters in my > two rigs because I use my ears, and not the computer to detect and > read that mode.
Not meaning to be argumentative, but I believe most of the above to be objectively untrue from a technical standpoint. A strong adjacent PSK31 signal inside the passband will desensitize any rig's receiver by activating the AGC such that if you try to receive a weaker adjacent signal, you will be unsuccessful. If you disable the AGC often the stronger signal will simply overload the receiver. IF filtering is essential for preventing this, and countless times filtering has made the difference between good copy and no copy. DSP filters are only helpful if they are in the IF, preventing AGC action by signals outside the passband. Software solutions, such as those in MixW or PSK Deluxe, cannot affect the rig's AGC action and are no substitute for good IF filtering. de Roger, W6VZV
