Back
to
|
|
Email |
Your 756Pro's filters are very different than normal filters in conventional radios. They are digital filters capable of until now unheard of performance and versatility. In order to use them properly it helps to understand the differences they exhibit as opposed to normal filters. Not in a technical sense but in how they perform in the real world.
Normal filters have a long slow curve and the "real" performance is not easy to quantify. Meaning the the numbers are confusing as to how the filter actually performs. The filter numbers usually indicate the width of the filter from where it drops by 6db. Where the filter starts to drop is usually a lot narrower. Where the filter peters off to nothing is a lot wider. For this reason in terms of "open" bandwidth the filter is actually not as wide as the specs. In terms of rejection the filter is actually a lot wider.
Take the Icom specs for the stock 756 2.4khz filter. Here we have a 2.4khz filter. Or do we? Well the specs are -6db at 2.4 and -60 at 3.8. Does sound a bit ambiguous, doesn't it? So here we have a filter that starts to trail off long before 2.4, because at 2.4 it is already down by 6db. On the other hand this filter is not in total reject all the way out to 3.8 where it is 60db down. So here we have a filter that has serious attenuation well below 2.4, hence not passing full bandwidth. Yet even though it is too tight for full audio, it still doesn't have serious rejection for a long ways out. This is why for a real tight filter you need about a 1.8 and the received stations sound terrible. And for full audio to 2.7 you need a 3.3 filter and you hope nobody crowds in next to you because this filter is going to trail off way out past 3.3 well into the next "channel" so to speak.
Theoretically with DSP technology if your DSP processor and it's support circuitry is fast enough and powerful enough and the rest of the radio is up to par, you could have any filter specs you desire. This is what Icom seems to have done with the 756Pro's DSP technology (well it's close at least). So with the Pro you can have filters that are broad and flat across the desired bandwidth that also cut off right away instead of trailing off slowly at a much greater width than the filter's usable bandwidth.
Here are Icom's 756Pro filter specs with a 2.4khz filter selected. More than 2.4 at -6db, less than 2.8 at -60db. My translation of this combined with on the air testing indicates the filter is seemingly totally flat out to the spec or just a few hz beyond. And at about 300hz past spec (or 150hz on either side or slightly less) it is 60db down.
Obviously this is having your cake and eating it too. So with these kind of filter specs you can have a "full audio" filter that doesn't hear people 3.5khz out. And you can have a very narrow anti-QRM//Contesting filter that doesn't give nearly as bad audio as a normal filter.
You'll find you can run a lot narrower filter than you would expect and still get full audio from the station you are listening to. This is because the filter is flat right out to it's specs and not sloping off before the edges. So for this reason you don't need as wide a number to get good sound. I've found I can get very good receive audio with lots of bass and good highs down to about 2.2kz. My favorite filter is about 2.5 or so for easy listening and it's also pretty good under crowded band conditions, although 2.2 to 2.4 is still pretty good sounding and pretty much bullet proof against QRM except very strong nearby stations with excessively wide transmit audio. You really don't need to run any wide filters like 3.0 like you needed to run in the past to get full audio.
Conversely you'll find that you can run wide numbers and not hear the stations on neighboring frequencies. Same reason, tight filter skirts. So if you find you prefer around a 2.4 or 2.5 filter for maximum audio quality, this filter will also be quite tight as far as not hearing QRM from nearby frequencies. So when you use a 2.4 filter it is wide and flat out to 2.4 and it cuts off completely just a few hundred hz beyond 2.4, rather than a long slow taper out to 3.5 or further.
So in other words, except in extreme circumstances you'll be running your favorite filter most of the time, instead of switching all the time between a wide and narrow filter. Because your favorite filter, likely around 2.2 to 2.6 will be as tight as a conventional 1.8 and still give you the full audio of a 3.0. Having said that I still find myself switching to my narrow filter a lot of the time simply because I can use a 1.8 to 2.0 for extreme side QRM rejection AND continue to hear fairly good audio quality. As I keep saying, with these filters you can have your cake and eat it too.
The main display "BW" number is the current bandwidth. It is the selected filter setting, minus whatever high or low cuts you have made with the PBT controls. So if you are on a 2.5 filter and have made .5hz of cuts the BW indicator will show 2.0.
The main display "SFT" indicator is how far you have shifted your center line,in other words the center of the current filter including whatever cuts or shift you have made with the PBT controls. So if you cut the top by 400 hz the SFT will show -200 and the bandwidth will close in by 400 hz.
When you turn both PBTs at once you'll only change the shift (SFT), but the bandwidth (BW) will remain the same. At first it's a bit confusing but it all makes sense in the long run.
You simply press and hold the filter button to get the filter menu up. Here you can visually see the actual pass band, shift and high and low cuts relative to the mode you are operating. The filter menu and the main display SFT and BW figures as well as the actual filter pass band automatically flips when you switch modes (LSB to USB). So in other words, the same filter you make up for LSB will transpose and work exactly the same in USB mode without having to worry about everything being backwards as far as your passband shifting from being negative relative to the carrier frequency to being positive.
All filter settings, whether your shift, cuts or bandwidth are memorized on the fly. Meaning you don't have to do anything to set or lock your settings, the radio just does it. And of course the filter settings are also specific to the mode. Meaning your SSB filters are completely independent from your CW or RTTY filters. So when ever you leave a filter, that filter will be exactly the same when you return to it.
On SSB the filter centerline is 1500hz. This means whatever your bandwidth is if you don't shift the filter it will always center on 1500hz. Also you can not shift the filter beyond the 1500hz centerline. So for instance as you narrow a filter the 1500 line on the menu must be within the filter. And if you narrowed the filter down you'll eventually end up with a 0hz filter at 1500hz. The filter centerline is different from mode to mode.
On the filter menu you see the PBT1 and PBT2 indicators at the bottom as well as two overlapping silhouettes on the main graphic. These silhouettes indicate the twin PBT settings, they are NOT two overlapping filters, this is only a convenient, and sensible way to show your settings in an understandable way. So as you move either PBT up or down where ever the silhouettes overlap is your actual filter band pass.
The numbers at one end and the other end on the graphical display are your top and bottom skirts. These are the points at which your filters begin to cut off or reject incoming signals. These are really the most important things to think about in filter setting. The left number is the filter's bottom edge and the right number is the filter's top edge. On SSB these numbers virtually translate into the actual audio range you will pass from whatever station is on your frequency. So providing the received station has "full audio" a 0 to 2350 filter setting is 2350hz wide filter that starts at 0hz and is open all the way to 2350hz. Move the bottom edge up to 150 and Bingo you loose whatever audio content that station has below 150hz. Move the top edge down by 200hz and same thing happens, you loose the top 200hz of the station's audio content.
I'm going to try and make as many points here as I can instead of giving you a step by step hoping you now have a feel for what the filters do and how to translate the numbers in your display.
Secondly, you are missing out on a big part of the "756Pro experience" if you are not using a large, high quality stereo speaker on your 756Pro. I use 200 watt rock and roll speaker with a 10 inch woofer and I have switches disconnecting the tweeter and midrange elements so it works perfect for full range SSB audio from about 0 to 3000hz, AND it cuts whatever hi frequency hiss content there is. So if you're using any kind of communications speaker you will not necessarily make the same observations on audio response in regard to filter settings that I mention here.
Any filter narrower than 3.0 being based on a 1500hz centerline is going to need a negative shift or you will loose your bottom end audio. For instance, a 2.4 filter needs a -300 shift in order to hear all the way from 0hz on up.
I find anything higher than about 2600hz on my top skirt actually sounds "too wide" and too much crackly//hissy content is heard. This sounds contrary to popular belief but don't forget. Everything you know about SSB fidelity and filter is based on long sloping filter curves that are no where near as wide and flat or as sharp cut off as your 756Pro's filters.
There is no point in ever running a filter that is open below 0hz. All this will do is open your receiver passband further toward any neighboring station's passband. In other words running a 3.0 filter and shifting it down to remove hiss is not a good idea. You'll remove the hiss and it'll sound good BUT you'll be opening up yourself to QRM with no gain in audio response. The correct way to do it is to set up a filter that you can shift down WITHOUT bringing the baseline down below zero.
On your tighter filter settings don't forget to begin to cut the bottom as well as the top of the passband. The first 150 or 200hz only contains the very deepest bass audio so you can sacrifice that as well as the top end in making up a tight filter.
I find it easier to make up three standard filter settings rather than always be messing with the PBTs trying to get everyone to sound right and fight QRM at the same time.
I have my wide filter set from 0hz to about 2.5 or so as this is all the high content I want and I also hear all the bass. My medium filter I cut down more on the top end and only bring the bottom end up a bit so on this filter I still hear most of the bass and I am only sacrificing highs. On my narrow filter I have serious cuts on both top and bottom. I try to get this one where it still sounds acceptable yet has serious QRM rejection both above and below.
I'm going to keep posting bits and pieces to add here from time to time as things come to mind.
