I disagree. Roofing filers serve the same purpose, are located in the same
place and are even the same physical part they have been for the past 30
years. Roofing is simply a new term invented to describe a filter needed to
overcome the shortcomings of DSP. With adequate "roofing filters" there is
little to be gained from DSP in the cw realm. They have nothing to do with
the "front end". They are in the IF chain as they have always been. The
Orion is a superhetrodyne receiver which was invented 70+ years ago. It has
a first converter, IF and detector. The roofing filter is located in IF
stage just like it has always been. The IF is downconverted to a very low
frequency (still considered an IF) and fed into the "DSP" system we so
highly revere. The DSP provides, amoung other things, a detector and agc
system. If the roofing filter were narrow enough, the DSP would not have
much to do on cw. After all, once the filters have removed the QRM from both
sides of a signal, all we need is a product detector to convert it to audio
so we can hear it! Shucks, cw is just a tone! What is the point of
converting it to digital pulses then trying to reconstruct it into a tone?
Now with SSB were we must wide wide wide filtering like 2.4khz just to
understand the stuff it's a different story. Now we can use the DSP to act
as an automatic notch filter and eliminate cw tones which BTW mostly exist
on 40 meters. DSP is also used in the transmitter's circuit to good
advantage to intregate voice procession etc.
There has been some talk about DSP filtering "not ringing". This is very
true in the JA imports but TenTec's filters seem imune from ringing, even
the narrow 250hz filters sound very natural. If you want to hear some
ringing, fire up a PROII with a 50hz filter and tune across 80 meters at
night. A little time spent with that in your headphones should send you
running for a quiet spot.
"I noticed the same thing as I started getting to know my new Orion. I
think that part of the confusion is that the roofing filters are not the
signal bandwidth filters that we are used to seeing advertised in other
rigs. The roofing filter sets the overall operational bandwidth that the
receiver front end sees. The signal bandwidth is set using the DSP
bandwidth control."
Steve N4LQ
----- Original Message -----
From: "Bernard(wtrone)" <wtrone@comcast.net>
To: <tentec@contesting.com>
Cc: <ditsnbits@tentec.com>
Sent: Thursday, August 05, 2004 5:53 PM
Subject: Re: [TenTec] Orion Roofing Filter Cut In
> I noticed the same thing as I started getting to know my new Orion. I
> think that part of the confusion is that the roofing filters are not the
> signal bandwidth filters that we are used to seeing advertised in other
> rigs. The roofing filter sets the overall operational bandwidth that the
> receiver front end sees. The signal bandwidth is set using the DSP
> bandwidth control.
>
> Having said that, I think that TenTec confused the situation by using
> 1.8 and 2.4 kHz values for their "SSB" roofing filters. Plain and simple,
I
> think that these filters are a mistake for those of us who enjoy mostly
> using SSB! Why? They are too narrow.
>
> I think that it is reasonable and prudent to have a roofing filter
that
> is somewhat wider than the desired signal bandwidth. How much wider?
20%?
> 50%? 100%? I don't know, but I suppose that there are performance and
cost
> trades that establish the optimal range.
>
> I don't know about the rest of you, but I have great difficulty
copying
> SSB with a bandwidth less than 2.0 kHz. I have had rigs with 1.8 and 2.0
> kHz signal bandwidth filters and I just didn't use them. They did me no
> good. I normally set my Orion signal bandwidth between 2.1 and 2.8 kHz
> depending on the band conditions. On automatic filter selection, I just
> don't use the 2.4 kHz roofing filter. It is too narrow. I could, of
> course, select it and lock it.
>
> So, back to the TenTec mistake, IMO. I think that they should have
> selected a "SSB" roofing filter of about 3.5 kHz and forgotten about the
1.8
> and 2.4 kHz filters. A 3.5 kHz roofing filter probably would have allowed
> DSP bandwidth settings of 2.8 kHz or less. I think that would have been
> "ideal" for us SSB operators. The 6.0 kHz filter, IMO, is a little too
wide
> for DX who are using a 5.0 kHz split. We need something narrower for the
> heavy QRM.
>
> I do have a question for any of you who have both the 1.8 and 2.4 kHz
> filters. Have you tried setting the DSP bandwidth at 1.75 kHz or so, and
> then tried both the 1.8 and 2.4 filters on various SSB signals? Did you
> hear any difference?
>
> Don't get me wrong, I really like my Orion. I just think that TenTec
> selected the "wrong" SSB filters. I suppose that they were more focused
on
> CW operation.
>
> Just one man's opinion.
>
> Comments??
>
> 73s
>
> Bernard, WA4OEJ
> ----- Original Message -----
> From: <K4IA@aol.com>
> To: <tentec@contesting.com>
> Cc: <ditsnbits@tentec.com>
> Sent: Thursday, August 05, 2004 8:42 AM
> Subject: [TenTec] Orion Roofing Filter Cut In
>
>
> > I am confused by the scheme used to automatically cut in the Orion
roofing
> > filters.
> >
> > Why does the filter not cut in until the DSP bandwidth is substantially
> less
> > than the filter?
> >
> > For example, the 2.4 filter doesn't switch in until the DSP is at 1.990.
> >
> > The 1.8 doesn't engage until the DSP is at 1.400 This is way too narrow
> for
> > SSB - thus there is no effective roofing filter for narrow SSB and one
> could
> > argue this roofing filter is useless for SSB. Yes, I know you can set
the
> > passband for +150 and still be able to decipher SSB at that narrow a DSP
> setting,
> > but speech is very difficult to understand.
> >
> > The 1.0 comes in at .740 Is this one of the reasons some have switched
> out
> > the 1.0 in favor of the INRAD 600 in this position?
> >
> > The 500 is at 350. I don't have the 250 so I can't comment on that one.
> >
> > Wouldn't it make more sense to have the roofing filter match the DSP?
> >
> > Would it make sense to have the roofing filter kick in "early" as the
> INRAD
> > 600 does when placed in the 1.0 position?
> >
> > Radio k4ia
> > "Buck"
> > Fredericksburg, Virginia USA
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
>
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|