mtdean at thirdcontact
Feb 25, 2005, 7:52 PM
Post #1 of 8
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Amplifiers, filters, cable, and PVR-x50's (was Re: Gotta buy a vowel: compiling patched tuner.c - RESOLVED)
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David Cain wrote:
> ... I've now accomplished what I set out to do and have UHF channels
> (albeit not channel 69, but since that's UPN here it's no great
> loss... ;)
> ...
> OK, I'm going to go to bed and dream of amplifiers for my poor OTA TV
> signal...
Just thought I'd chime in here with some info I've been meaning to
post. A lot of people--especially those using the LG TAPE (type 47) and
TCL 2002N (type 50) tuners for which I provided the tuner
definitions--have complained of poor picture quality when using the
tuner. After much discussion (i.e. see
http://www.gossamer-threads.com/lists/ivtv/devel/16163 for one of many
threads), "remote" testing, and a *lot* of repetition of my assurance
that the tuner definitions are correct according to the tuner
datasheets, I (and, to the best of my knowledge, the others involved in
the discussion) came to the conclusion that the picture quality was a
systemic problem, most likely due to signal quality.
Even so, if your signal quality is poor, there are a lot of things you
can do to enhance it. I set up a Myth box for a friend who has 4
PVR-250's (all with the LG TAPE tuner) and his picture quality improved
from unusable (even with 1 tuner and no splitter) to excellent (better
than he could get with a direct run to a TV)--even on an 8 1/2 foot TV
screen (using a projector--which magnifies the imperfections in the signal).
Shane did a lot of research to figure out how to improve his signal, and
his efforts paid off for him, so I wanted to ensure that others who may
be in a similar situation know there's still hope. Since Shane hasn't
gotten into the IvyTV mailing list, I'm posting the information for
him. I hope others find this information useful, and if anyone has more
to add or corrections/clarifications, please post.
The points listed below should help to improve your signal quality, and,
therefore, your picture quality. Each point will bring varying amounts
of improvement--ranging from not noticeable to "wow" improvements--if
used individually. However, you'll probably get the best improvement
using a combination of suggestions. If you can, I highly recommend
taking all the steps. I had been trying to convince Shane to switch
from his analog cable connection to digital cable or satellite cable
because his picture quality was so much lower than the quality I got
from my satellite receiver with S-Video to the PVR-x50; however, since
making the last of the changes described, I can barely tell a difference
between his picture and mine.
- Ensure you are using good connectors on all cable (including patch
cables). Screw-on connectors are very poor quality and leak a lot of
signal. Crimp-on connectors provide a much better signal with much less
leakage--but only if properly installed (if not properly installed, they
can actually provide more leakage than screw-on connectors). You will
need a good quality (large and heavy) crimper to get a good connection
(check with friends/work/etc. to see if you can borrow one to save you
the $40-$100 for the crimper). Cheap/small/lightweight crimpers
typically cause you to create lower-quality connections than screw-on
connectors would provide. Shane and I replaced all the connectors in
his system (on all cables and patch cables).
- Use RG-6 cable--not RG-59--especially for long runs (greater than
about 20feet). Even better--as long as you're going to the trouble of
replacing the cable--spend a couple extra bucks to get some RG-6
quad-shield cable. Remember, also, that cable quality can also have an
impact (Canare and Belden make good cable, but you might get lucky with
no-name/house brand spools). Since you'll likely buy a spool of cable,
you might as well use the leftovers to make your patch cables from
RG-6. Although it probably won't affect attenuation much for such short
runs, it definitely won't hurt to use RG-6 for these, too.
- Don't over-split the signal. 2-way splitters will attenuate the
signal less than 4-way splitters (about 3.5dB and 7.0-7.5dB
respectively), so choose appropriately. If not using a cable/output,
terminate it. You should be able to make a 5-900MHz splitter work, but
might get better quality from a 5-1000MHz splitter.
- Use an amplifier--especially when using a splitter. The amp
should cover the range from 50MHz to 900MHz at minimum. Anything more
is unlikely to provide any benefit (NTSC uses frequencies up to
806MHz). The amplifier should be connected before anything that can
attenuate the signal--amplify the signal, not the noise. Ideally, it
should be connected directly to the line from the cable company. Since,
in reality, this is seldom possible (lack of power outside, need for an
HPF (see below), etc.), get it as close to the cable company's line as
possible (or, as close to the antenna for those not using cable).
- If you have a cable modem, you may get interference on low
channels when uploading data to the 'net (typically, uploads are at <
54MHz while download is striped in with the channels). If so, you can
get a high-pass filter (HPF) and place a 2-way splitter at the cable
company's line. One side goes to your cable modem and the other to the
HPF. The line from the HPF goes to the amp, then to your PVR's.
Shane's equipment. Note that this is by no means the best equipment
available, but it's working well for him. Some of the equipment he
purchased before doing the research. For other equipment, he chose
"good enough" rather than the best.
- Cable: cheap RG-6 cable (~$30/1000ft spool)
- Connectors: crimp-on connectors connected using the Ideal
Crimpmaster #30-503 (about $30 for the die alone (if you already use the
Crimpmaster for making 10-base T connections) or about $60 for the
die/tool combo). It's not the best on the market, but was about the top
of my price range, and I trusted the quality because I was using the
same tool for making ethernet cables. If you're lucky, you might be
able to find a the tool/die combo for about $30 (I got mine for that
when Home Depot quit carrying them several years ago). Note that it
takes a lot of pressure to crimp an F-Connector and it will hurt your
hands... :) More expensive ones hurt less...
- Amp: cheap Arista 50-900MHz with a variable 30dB gain control.
- Splitter: cheap Arista 5-900MHz 4-way splitter (
http://www.aristagroup.com/Arista_prod/video_cat/video_acc3.htm#a2 ).
- HPF:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67277&item=5753889145&rd=1&ssPageName=WDVW
All signal levels given in the diagram below are approximate. All are
calculated/guesstimated using approximate expected signal attenuation
for various devices except the level at the entry to the house, which
was obtained by hooking Shane's cable modem directly to the cable
company's line and calling tech support and asking them to query the
modem for signal strength. The cable/connectors are assumed to cause no
attenuation, but in reality the cable alone could cause as much as
7-10dB/100ft. Losses at connectors vary depending on the quality of the
installation of the connector.
Shane's hookup:
cable from (+3.5dB in)
cable co ------> 2-way splitter ------> cable modem
(+3.5dB | (0 dB)
measured) |
v
HPF (0dB)
|
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v
(-1.25dB in) amplifier (set at ~ +11.75dB)
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v (+7dB in)
(+10.5dB in) 2-way splitter ---------> 4-way splitter
| (0dB out)
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v (+7dB in)
------ 4-way splitter --------
| | | |
v v v v
PVR-250 PVR-250 PVR-250 PVR-250
(all feeds to PVR-250's at approx 0dB)
The 4-way splitter on the right is to allow Shane to pipe the TV signal
to other rooms in his house. That way, he has the option to hook up
TV's (without having to build more Myth frontends) in other rooms. If
you take it out, you'll get less attenuation, but he has a very clean
signal, even with it there.
Good luck, and HTH.
Mike
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