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Coax filter

C

cameo

Jan 1, 1970
0
Years ago, when Comcast added Internet service to the pre-existing cable
TV service in my neck of woods, the technician, after running a short
pigtail coax cable from the wall outlet to a two-way splitter to serve
both the TV and the cable modem, he also installed a small hex barrel
shaped filter into the cable leg that runs from the splitter to the TV.
The filter is about half inch thick and 2 inch long and looks made of
stainless steel.

Not being an electronics expert, I never really understood the purpose
of that filter and I did not notice any difference either im my
broadband modem or TV performance after I removed it. Can anybody
enlighten me what that filter is supposed to accomplish? If anything, I
would have expected something like that in the cable leg that runs from
the splitter from the modem, not in the TV leg, but what do I know?
BTW, the following code is stamped on the filter: SNB6-5/136 kV 182, if
that means anything after all these years.
 
C

cameo

Jan 1, 1970
0
If it goes to the TV, it is a high pass filter to keep the uplink RF
from overloading the TV. Some tuners have no internal filtering, and
broadband causes problems.
I see ... What frequencies are used for the TV and the broadband?
 
R

Robert Baer

Jan 1, 1970
0
cameo said:
Years ago, when Comcast added Internet service to the pre-existing cable
TV service in my neck of woods, the technician, after running a short
pigtail coax cable from the wall outlet to a two-way splitter to serve
both the TV and the cable modem, he also installed a small hex barrel
shaped filter into the cable leg that runs from the splitter to the TV.
The filter is about half inch thick and 2 inch long and looks made of
stainless steel.

Not being an electronics expert, I never really understood the purpose
of that filter and I did not notice any difference either im my
broadband modem or TV performance after I removed it. Can anybody
enlighten me what that filter is supposed to accomplish? If anything, I
would have expected something like that in the cable leg that runs from
the splitter from the modem, not in the TV leg, but what do I know?
BTW, the following code is stamped on the filter: SNB6-5/136 kV 182, if
that means anything after all these years.
Methinks it is to reduce interference to the TV that is generated by
the cable modem.
That cable modem generates a lot of hash, and interferes with cell
phones up to at least 5 feet.
 
C

cameo

Jan 1, 1970
0
It means quite a bit. Descriptions are nice, but (accurate) numbers
are more important.

<http://www.ppc-online.com/broadband/group_trap.php?id=69>
An SNBR-5/136 is a band reject filter that drops everything from 5 to
136Mhz, which makes little sense. The model should be something like
SNBR-45/550. Please check your numbers. Also, what color is the
stripe? Such a filter is what is normally used for cable data only
connections where the uplink channels are above about 550Mhz and the
return channel is below 30MHz. It's purpose it to prevent you from
getting free analog cable TV service if you only subscribe to cable
modem or telephony service.

You misread my post, Jeff. It's SNB6-5/136. not SNBR-5/136. But it might
be too old a code to find any documentation on it now. I think the
Comcast technician installed it about 15 years ago. There is no color
code on it, BTW. It's all mice stainless steel shiny. It is also not a
cylindrical shape as your picture shows, but hexagonal. Otherwise it's
similar dimensions.
 
C

cameo

Jan 1, 1970
0
The use of the filter depends on what type of service you're getting.
What combination of cable data, cable TV, MoCA, and/or cable telephony
are you getting from Comcast?

I only have a ClearQAM HD TV (doing double duty as a PC monitor) and the
Motorola cable modem operating from that splitter. The TV is connected
through an STB that only provided SD picture but I can bypass it to get
the local HD signals of the local stations. They all are powered from
the same power circuit via power strip.

I also have an old analog set in another room that gets the TV signal
through a Comcast DTA since Comcast dropped all analog signals in my
area and encrypts everything except the local HD broadcasts. I do get
good reception on both TV sets and good data speeds as well from the
modem, despite the real old RG-59 cabling. Even with the coax filter
removed. I get no telephony or other Xfinity services besides the
broadband and Digital Economy TV package.
 
T

TheGlimmerMan

Jan 1, 1970
0
Methinks it is to reduce interference to the TV that is generated by
the cable modem.
That cable modem generates a lot of hash, and interferes with cell
phones up to at least 5 feet.


What shit brand of cable modem are you referring to?

The modern Motorola units don't do that. and they fire off at over
30Mb/s.

And the TV (band) is inside coax and won't get anything injected into
it, unless you (or your installer) are an idiot at cutting a proper
fitting as well.

The digital (read Internet service side) segment is yet another set of
cable channels and the digital broadband 'service' is contained within
standard 6MHz channel slots.

So, the whole thing is packaged within the standard cable TV channel
slot on the digital side.

I thought the filter was for the cable modem, not 'for the TV side' as
it were. On our system, it is a specific channel notch filter, which is
what the numbering is about. It is no different than the "channel traps"
they used to use to deny a customer a certain channel. If they bought
that channel's subscription, they would come remove the "trap" from your
taps out on the pole. That is a channel TRAP filter, which passes all
channels except for the trapped channel.

This is a channel PASS filter that filters all channels except for the
digital one, and that branch feeds our cable modem segment.

In your case, since it is on the cable TV feeder side, I suspect that
it too is a channel TRAP filter (a narrow notch filter), which traps the
channel the digital side is on and passes the rest.

I know even the TV stuff is digital now. I am using the term to refer
to Internet service in this post.
 
T

TheGlimmerMan

Jan 1, 1970
0
Hex shape suggests an attenuator, not a filter.

It is a specifically set device to 'attenuate' a specific channel or
band of channels. IOW, a notch filter.

I doubt the feed is so hot they need to cap the leg feeding the TV
band. But it does appear to be only a 6dB pinch. But that is a guess...
as well.
 
C

cameo

Jan 1, 1970
0
What shit brand of cable modem are you referring to?

The modern Motorola units don't do that. and they fire off at over
30Mb/s.

Mi modem is Motorola's SB6121 that I bought myself.
And the TV (band) is inside coax and won't get anything injected into
it, unless you (or your installer) are an idiot at cutting a proper
fitting as well.

The digital (read Internet service side) segment is yet another set of
cable channels and the digital broadband 'service' is contained within
standard 6MHz channel slots.

So, the whole thing is packaged within the standard cable TV channel
slot on the digital side.

I thought the filter was for the cable modem, not 'for the TV side' as
it were.

That's what I expected, too.
On our system, it is a specific channel notch filter, which is
what the numbering is about. It is no different than the "channel traps"
they used to use to deny a customer a certain channel. If they bought
that channel's subscription, they would come remove the "trap" from your
taps out on the pole. That is a channel TRAP filter, which passes all
channels except for the trapped channel.

That's how it was here, too, except it was the cable box on the
street as we don't have poles in my street.
This is a channel PASS filter that filters all channels except for the
digital one, and that branch feeds our cable modem segment.

In your case, since it is on the cable TV feeder side, I suspect that
it too is a channel TRAP filter (a narrow notch filter), which traps the
channel the digital side is on and passes the rest.

I know even the TV stuff is digital now. I am using the term to refer
to Internet service in this post.

Well, Comcast here now enforces channel subscription remotely by
sending signals to their STBs and DTAs. Only the local stations can
still be received with digital TV sets till they encrypt those, too.
 
C

cameo

Jan 1, 1970
0
Yep. That's why I suggested you verify the part number. An
SNB6-5/136 is very close to a PPC part number, but is not on the PPC
web pile. Also a 5-136MHz band pass or reject filter makes no sense.

Also, could I trouble you to answer my question and disclose which
services you're getting from Comcast? I can then guess(tm) which
filter might be appropriate.

You skipped that part in my last post, I'm afraid. I described the
services in detail.
I have some old MHZ CATV catalogs from that era that might have
something. I'll dig tomorrow. However, as I vaguely recall, all the
customer premesis filters were cylinderical to make them difficult to
remove without a special tool. The hard line connectors for the amps,
couplers, and splitters were all hexagonal, to make them easy to
install. I don't recall any filters inside hex packages. See for
yourself:
<https://www.google.com/search?tbm=isch&q=cable+tv+filter>

OK, a picture is worth a thousand words, I guess. So here are two pics
of my filter in question:

http://i50.tinypic.com/158b475.jpg

http://i49.tinypic.com/1z3t46t.jpg
 
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