Elecraft

Hearing the effect of narrower roofing filter

15 Messages

K7TV

Hearing the effect of narrower roofing filter

Reply Threaded

All,

My K3 (with one extra roofing filter for CW) is not here yet, so I am asking this question without direct experience.

It will be interesting to do an A/B comparison to see how the receiver sounds with/without the narrow crystal filter for a given bandwidth.
I suspect this kind of test is of interest to many K3 owners. However, to be meaningful, it must be carried out properly. I want
to ask the group: 1. How best to carry out such a test, and 2. What say those that have already tested.

As to how to carry out the test, I think I read somewhere that the narrow filter can be disabled from the radio front panel. (Someone please
correct me if I am wrong.) Now doing so, without changing the DSP bandwidth, is going to change the overall bandwidth unless the
narrow crystal filter being disabled is already considerably wider than the DSP bandwidth.

The case where the crystal filter is narrower than the DSP width setting is obviously going to result in less noise when the crystal filter is in, so that is not the case I am interested in.

If the crystal filter is nominally the same width as the DSP filter, that is an interesting case. The composite width would then be narrower
than the DSP alone (actually with say an SSB crystal filter). To make the comparison properly, when one takes out the narrow crystal
filter (replaces it with an SSB crystal filter), one would probably need to change the DSP filter width to a narrower setting in order to get
the same overall filter width. This wouldn't work out unless the required change in DSP width happens to match one of the available
DSP width settings.

If the crystal filter is moderately wider than the DSP setting, say using (vs not using) a 1 kHz crystal filter with a DSP setting of 500 Hz, that is an interesting situation that should be common in the K3. In this case maybe one can leave the DSP at 500 Hz, and just listen for any change.

What kind of change am I talking about? If there is a signal within the crystal filter bandwidth that is strong enough to pump the hardware AGC,
then I would expect to hear increased qsb to the down side. The point of the experiment would then be to assess how bothersome this
effect is in a real situation. Is the narrow crystal filter worth it? While doing this, I would really like to see an indicator that shows the hardware AGC kicking in. Is there any way to see this on the K3, short of hanging a scope on the AGC line?

On another line of thinking, reality tends to be complex, and, especially after the hardware AGC trigger level was raised (I think that is what was done??), maybe the composite of random noise and multiple strong signals in the crystal filter passband can produce some very short-lived signal peaks that either overload the ADC or by non-linear effects can generate additional audible noise in the case where the narrow crystal filter is not used? Whatever the cause, I am very interested in any noise increase that is actually audible in a real-life case with the K3, as a result of not having the ideal crystal filter for a given effective receiver bandwidth, especially for the case where NO ONE strong signal within any of the available crystal filters is strong enough to activate the hardware AGC.

What has been observed by the group?

73, Erik K7TV
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Darwin, Keith

RE: Hearing the effect of narrower roofing filter

Reply Threaded

-----Original Message-----
From: elecraft-bounces@...
[mailto:elecraft-bounces@...] On Behalf Of Erik N Basilier

My K3 ... interesting to do an A/B comparison to see how the receiver
sounds with/without the narrow crystal filter for a given bandwidth.

...

What has been observed by the group?
---------------------------

I've not attempted such a complex test as you described (which I snipped
out). I have the 500 Hz filter in my rig. As I narrow the DSP from 2
KHz to 550 Hz, I can hear a predictable narrowing of the passband with
each step. When I step to 500 Hz and the narrow roofing filter kicks
in, I can hear a distinct change, more than just the DSP narrowing I'd
heard while stepping from 1000 Hz to 550 Hz. The effect is very
noticeable. I don't think you'd have to construct some sort of
(artificial) double-blind test to see what it does. Besides, what you
really want to know is how does it play on the air. The best way to
learn that is to play on the air :-)

I have been able to get AGC pumping if I try. In all cases, going from
550 Hz to 500 Hz bandwidth took care of the pumping (roofing filter
works)

- Keith N1AS -
- K3 711 -
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Jan Erik Holm

Re: Hearing the effect of narrower roofing filter

Reply Threaded

Darwin, Keith wrote:
> each step. When I step to 500 Hz and the narrow roofing filter kicks
> in, I can hear a distinct change, more than just the DSP narrowing I'd
> heard while stepping from 1000 Hz to 550 Hz. The effect is very
> noticeable.
>
> - Keith N1AS -
> - K3 711 -
Keith,

Can you describe this "change" closer.

73 Jim SM2EKM

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Darwin, Keith

RE: Hearing the effect of narrower roofing filter

Reply Threaded

Keith Darwin wrote:
>> each step. When I step to 500 Hz and the narrow roofing filter kicks

>> in, I can hear a distinct change, more than just the DSP narrowing
I'd
>> heard while stepping from 1000 Hz to 550 Hz. The effect is very
>> noticeable.

Jim replied:
> Keith, Can you describe this "change" closer.

We'll, let's see. When the 500 Hz filter kicks in, the resulting audio
becomes more focused. There are noticeably less highs and esp. lows.
Any low rumbly "grunge" from off freq. signals drops to zero and high
freq chatter becomes much weaker. I like the effect and find the rig to
be more pleasing to use at 500 Hz than at 550 Hz due to the steeper
compound filter skirts. I'm considering a 1000 Hz roofing filter to
make the rig sound better between 550 and 1000 Hz bandwidth.

- Keith N1AS -
- K3 711 -
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Julian, G4ILO

RE: Hearing the effect of narrower roofing filter

Reply Threaded

Darwin, Keith wrote:

I've not attempted such a complex test as you described (which I snipped
out). I have the 500 Hz filter in my rig. As I narrow the DSP from 2
KHz to 550 Hz, I can hear a predictable narrowing of the passband with
each step. When I step to 500 Hz and the narrow roofing filter kicks
in, I can hear a distinct change, more than just the DSP narrowing I'd
heard while stepping from 1000 Hz to 550 Hz. The effect is very
noticeable.

That's just what I find, too. Particulaly when operating PSK, when I reduce the filter with towards 500Hz I hear the tones of out of band stations gradually being reduced by each step. When the 500Hz filter kicks in the tones suddenly disappear altogether.

Julian, G4ILO. K2 #392 K3 #222.
G4ILO's Shack Ham Directory KComm for Elecraft K2 and K3

hank k8dd

Re: Hearing the effect of narrower roofing filter

Reply Threaded

----- Original Message -----
From: "Jan Erik Holm" <sm2ekm@...>
Cc: <elecraft@...>
Sent: Tuesday, July 08, 2008 8:31 AM
Subject: Re: [Elecraft] Hearing the effect of narrower roofing filter

> Darwin, Keith wrote:
>> each step. When I step to 500 Hz and the narrow roofing filter kicks
>> in, I can hear a distinct change, more than just the DSP narrowing I'd
>> heard while stepping from 1000 Hz to 550 Hz. The effect is very
>> noticeable.
>>
>> - Keith N1AS -
>> - K3 711 -
> Keith,
>
> Can you describe this "change" closer.
>
> 73 Jim SM2EKM

Get two pieces of plastic pipe about a foot long.
One six inches in diameter.
One three inche in diameter.
Close one eye and look at the mountains in the horizon and you will see a
wide view. 6000 Hz
Put the six inch pipe up to the other eye. You will see a portion of the
horizon - like 1000 Hz.
Put the three inch pipe up to your eye. 500 Hz.
The effect, as Keith says, is very noticeable!

73 Hank K8DD

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David Cutter

Re: Re: Hearing the effect of narrower roofing filter

Reply Threaded

Fine observations when you change the filter setting knob, but what makes you think you are hearing the effect of the roofing filter and not the dsp filter?

David
G3UNA

>
> From: "hank k8dd" <k8dd@...>
> Date: 2008/07/08 Tue PM 02:29:08 BST
> To: "Jan Erik Holm" <sm2ekm@...>
> CC: elecraft@...
> Subject: Re: [Elecraft] Hearing the effect of narrower roofing filter
>
> ----- Original Message -----
> From: "Jan Erik Holm" <sm2ekm@...>
> Cc: <elecraft@...>
> Sent: Tuesday, July 08, 2008 8:31 AM
> Subject: Re: [Elecraft] Hearing the effect of narrower roofing filter
>
>
> > Darwin, Keith wrote:
> >> each step. When I step to 500 Hz and the narrow roofing filter kicks
> >> in, I can hear a distinct change, more than just the DSP narrowing I'd
> >> heard while stepping from 1000 Hz to 550 Hz. The effect is very
> >> noticeable.
> >>
> >> - Keith N1AS -
> >> - K3 711 -
> > Keith,
> >
> > Can you describe this "change" closer.
> >
> > 73 Jim SM2EKM
>
> Get two pieces of plastic pipe about a foot long.
> One six inches in diameter.
> One three inche in diameter.
> Close one eye and look at the mountains in the horizon and you will see a
> wide view. 6000 Hz
> Put the six inch pipe up to the other eye. You will see a portion of the
> horizon - like 1000 Hz.
> Put the three inch pipe up to your eye. 500 Hz.
> The effect, as Keith says, is very noticeable!
>
> 73 Hank K8DD
>
> ____________________________________________________________
> GET FREE SMILEYS FOR YOUR IM & EMAIL - Learn more at http://www.inbox.com/smileys
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Darwin, Keith

RE: Re: Hearing the effect of narrower roofing filter

Reply Threaded

Not sure, David, if your Q was to me or to Hank. I can only answer for
me.

When I adjust the filter width knob, I'm most definitely hearing the
combined effect of the DSP bandwidth reduction and the change in roofing
filters. But, since I can change DSP without changing the roofing
filter (by stepping from 1500 Hz down to 550 Hz) I can get a good feel
for how the DSP filter sounds. Then, stepping to 500 Hz, I can see the
combined effect of one more DSP step plus the insertion of the 500 Hz
roofing filter. The observed big change from 550 Hz to 500 Hz is
*mostly* due to the roofing filter.

- Keith N1AS -
- K3 711 -

----Original Message-----
From: elecraft-bounces@...
[mailto:elecraft-bounces@...] On Behalf Of
d.cutter@...

Fine observations when you change the filter setting knob, but what
makes you think you are hearing the effect of the roofing filter and not
the dsp filter?

David
G3UNA

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Dick Dievendorff

RE: Re: Hearing the effect of narrower roofing filter

Reply Threaded

A couple of related points, which may not be new news to anyone:

You can set the K3's filter bandwidth configuration to cause the roofing
filter to switch at points other than the bandwidth printed on the filter.
Maybe you would like your (nominally) 500 Hz filter to switch in at 450 or
550 Hz. My 8-pole 250 Hz filter is a bit broader than 250 Hz, and I've set
it to switch at 300 Hz.

You can also turn off a filter (set its bandwidth to zero temporarily) to
evaluate whether a change you're hearing is due to the roofing filter or the
change in bandwidth provided by the DSP.

And you should attempt to adjust the filter gain compensation so that there
is no change in the volume of the note you're tuned to. Rene, who guides
the assembly of K3s built by Elecraft, told me that he uses a voltmeter on
the speaker terminals when adjusting the gain compensation and can get
usually get within a half decibel.

I notice a definite qualitative change as the narrower filters switch in. It
sounds quieter, and it seems like the volume is being turned down. I think
I'm responding to the reduced low and high frequency components that are
supposed to be cut off. The DSP-related changes seem more gradual.

Dick, K6KR

-----Original Message-----
From: elecraft-bounces@...
[mailto:elecraft-bounces@...] On Behalf Of Darwin, Keith
Sent: Tuesday, July 08, 2008 7:43 AM
To: elecraft@...
Subject: RE: Re: [Elecraft] Hearing the effect of narrower roofing filter

Not sure, David, if your Q was to me or to Hank. I can only answer for
me.

When I adjust the filter width knob, I'm most definitely hearing the
combined effect of the DSP bandwidth reduction and the change in roofing
filters. But, since I can change DSP without changing the roofing
filter (by stepping from 1500 Hz down to 550 Hz) I can get a good feel
for how the DSP filter sounds. Then, stepping to 500 Hz, I can see the
combined effect of one more DSP step plus the insertion of the 500 Hz
roofing filter. The observed big change from 550 Hz to 500 Hz is
*mostly* due to the roofing filter.

- Keith N1AS -
- K3 711 -

----Original Message-----
From: elecraft-bounces@...
[mailto:elecraft-bounces@...] On Behalf Of
d.cutter@...

Fine observations when you change the filter setting knob, but what
makes you think you are hearing the effect of the roofing filter and not
the dsp filter?

David
G3UNA

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6:33 AM

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Bill W4ZV

RE: Re: Hearing the effect of narrower roofing filter

Reply Threaded

K6KR wrote:

>I notice a definite qualitative change as the narrower filters switch in. It
sounds quieter, and it seems like the volume is being turned down. I think
I'm responding to the reduced low and high frequency components that are
supposed to be cut off. The DSP-related changes seem more gradual.

Dick made some very good points about using the K3Utility to enable different filters and about the need to set gain correctly for each filter. Here's a simple test that may surprise you:

1. Disable all filters but your 2.7 or 2.8 for CW (using Dick's K3Utility).
2. SLOWLY rotate WIDTH from 1.5 to 0.05.
3. Note how the character of the noise changes.

I find abrupt changes in noise character between some DSP settings...and this is *not* due to the roofing filter since that is held constant at 2.7k. The abrupt changes I hear using a 350 Hz CW pitch setting are:

1.1 - 1.0
0.90-0.85
0.70-0.65
0.50-0.45
0.30-0.25
0.20-0.15

Do you see a pattern in the above? I hear something which repeats almost every 0.20 increment...(and I bet Lyle can explain why). I recall seeing this on Spectrogram when I was closely looking at DSP BWs shortly after I got my unit. We did discover some serious offset problems at the low end (which Lyle has since fixed), but I recall the DSP BW changes were never exactly linear as you reduced BW. This is not a particular concern to me but curious nevertheless.

73, Bill

Brett Howard

RE: Re: Hearing the effect of narrower roofing filter

Reply Threaded

I agree with this assessment and when I get my rig back plan on upping
my 250 filter to kick in at 350. But I had 4dB of gain on the 250 and
it still seemed like things got a lot quieter when switching to the 250.
I'd have to measure and see if I had the actual same level there (and I
can do that now that I have the XG2) :).... But I did have 4dB thrown
in for good measure! :) I found myself using the 250 all the time for
rag chewing... But I use 250 on my K1 for its lowest filter position
too..

I tend to generally like things pretty quiet. I use 850 for scanning
the band (as that is as wide as the K1 can go). I use 500 to 550 for
calling CQ if they're further away than that then I guess I don't get to
hear them... And then I crank it down to 200 to 250 when I'm talking
with a guy. It not only ensures that I'm pretty close to zero beat with
him but also makes things quieter and gets rid of a lot of the
electrical hash I have in my area. The K1's noise blanker doesn't hold
a candle to the noise in my area but the K3 was doing a pretty fantastic
job!

On Tue, 2008-07-08 at 08:02 -0700, Dick Dievendorff wrote:

> A couple of related points, which may not be new news to anyone:
>
> You can set the K3's filter bandwidth configuration to cause the roofing
> filter to switch at points other than the bandwidth printed on the filter.
> Maybe you would like your (nominally) 500 Hz filter to switch in at 450 or
> 550 Hz. My 8-pole 250 Hz filter is a bit broader than 250 Hz, and I've set
> it to switch at 300 Hz.
>
> You can also turn off a filter (set its bandwidth to zero temporarily) to
> evaluate whether a change you're hearing is due to the roofing filter or the
> change in bandwidth provided by the DSP.
>
> And you should attempt to adjust the filter gain compensation so that there
> is no change in the volume of the note you're tuned to. Rene, who guides
> the assembly of K3s built by Elecraft, told me that he uses a voltmeter on
> the speaker terminals when adjusting the gain compensation and can get
> usually get within a half decibel.
>
> I notice a definite qualitative change as the narrower filters switch in. It
> sounds quieter, and it seems like the volume is being turned down. I think
> I'm responding to the reduced low and high frequency components that are
> supposed to be cut off. The DSP-related changes seem more gradual.
>
> Dick, K6KR
>
>
> -----Original Message-----
> From: elecraft-bounces@...
> [mailto:elecraft-bounces@...] On Behalf Of Darwin, Keith
> Sent: Tuesday, July 08, 2008 7:43 AM
> To: elecraft@...
> Subject: RE: Re: [Elecraft] Hearing the effect of narrower roofing filter
>
> Not sure, David, if your Q was to me or to Hank. I can only answer for
> me.
>
> When I adjust the filter width knob, I'm most definitely hearing the
> combined effect of the DSP bandwidth reduction and the change in roofing
> filters. But, since I can change DSP without changing the roofing
> filter (by stepping from 1500 Hz down to 550 Hz) I can get a good feel
> for how the DSP filter sounds. Then, stepping to 500 Hz, I can see the
> combined effect of one more DSP step plus the insertion of the 500 Hz
> roofing filter. The observed big change from 550 Hz to 500 Hz is
> *mostly* due to the roofing filter.
>
> - Keith N1AS -
> - K3 711 -
>
> ----Original Message-----
> From: elecraft-bounces@...
> [mailto:elecraft-bounces@...] On Behalf Of
> d.cutter@...
>
> Fine observations when you change the filter setting knob, but what
> makes you think you are hearing the effect of the roofing filter and not
> the dsp filter?
>
> David
> G3UNA
>
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David Cutter

Re: Re: Hearing the effect of narrower roofing filter

Reply Threaded

The reason I'm particularly interested is because I fancy doing diversity
reception and de-select my roofing filters in the main rx so there is no
phase shifting relative to the second rx without filters.

Have you de-selected the roofing filters to do a comparison with and
without?

David
G3UNA

When I adjust the filter width knob, I'm most definitely hearing the
combined effect of the DSP bandwidth reduction and the change in roofing
filters. But, since I can change DSP without changing the roofing
filter (by stepping from 1500 Hz down to 550 Hz) I can get a good feel
for how the DSP filter sounds. Then, stepping to 500 Hz, I can see the
combined effect of one more DSP step plus the insertion of the 500 Hz
roofing filter. The observed big change from 550 Hz to 500 Hz is
*mostly* due to the roofing filter.

- Keith N1AS -
- K3 711 -

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David Cutter

Re: Re: Hearing the effect of narrower roofing filter

Reply Threaded

But if you leave the setting at its nominal, can you hear the difference as
that bandwidth is selected? I'm not sure I understand the advantage you are
gaining by bringing the crystal filter in later; I'll do if there's
something to be gained, it sounds an interesting approach.

David
G3UNA

Darwin, Keith

RE: Re: Hearing the effect of narrower roofing filter

Reply Threaded

Nope, I have not. My operating (near zero right now due to "life"
getting in the way) is 99.9% CW, old school, with manual keys. I'm not
pushing the envelop so I'm not trying new & novel things. These days, I
pretty much just use my rig rather than playing & testing with it.

- Keith N1AS -
- K3 711 -

-----Original Message-----

Have you de-selected the roofing filters to do a comparison with and
without?
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Dick Dievendorff

RE: Re: Hearing the effect of narrower roofing filter

Reply Threaded

I'm not completely sure what to do with this capability, other than perhaps
have the switchover occur at a bandwidth that more closely matches the
filter you actually have. What I've done is tell the K3 that my 250 Hz
filter is really 300 Hz (it's really probably more like 370 Hz).

I also have a 400 Hz filter. I didn't make a particularly wise choice of
filter bandwidths. To those who have pointed out the error of my ways, my
defense is that I chose the filters on first order day before the curves
were published on the web site. I don't view this as a severe issue. Maybe
I should have ordered the 200 Hz 5-pole instead, but I got it into my head
that maybe the 8-pole filters were "better" in some way. I didn't repeat
this mistake when I ordered filters for the subreceiver.

Dick, K6KR

-----Original Message-----
From: elecraft-bounces@...
[mailto:elecraft-bounces@...] On Behalf Of David Cutter
Sent: Tuesday, July 08, 2008 10:18 AM
To: elecraft@...
Subject: Re: Re: [Elecraft] Hearing the effect of narrower roofing filter

But if you leave the setting at its nominal, can you hear the difference as
that bandwidth is selected? I'm not sure I understand the advantage you are

gaining by bringing the crystal filter in later; I'll do if there's
something to be gained, it sounds an interesting approach.

David
G3UNA

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No virus found in this incoming message.
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Version: 8.0.138 / Virus Database: 270.4.6/1540 - Release Date: 7/8/2008
6:33 AM

No virus found in this incoming message.
Checked by AVG - http://www.avg.com
Version: 8.0.138 / Virus Database: 270.4.6/1540 - Release Date: 7/8/2008
6:33 AM

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