On Sat, 2009-05-09 at 01:19 -0400, Nick Nobody wrote:
> On Friday 08 May 2009 22:47:15 Andy Walls wrote:
> > On Fri, 2009-05-08 at 21:58 -0400, Nick Nobody wrote:
> > > On May 8, 2009 08:04:11 pm Andy Walls wrote:
> > > > On Fri, 2009-05-08 at 16:56 -0400, Nick Nobody wrote:
> > > >
>
> Everything is grounded properly and I'm using the same coax everywhere (75
> Ohm, RG-6, 95% shielded, rated for 2200MHz).
OK.
> I'm using a splice at the moment
> to reduce any potential loss that I might incur from using a splitter (only
> one device is connected at a time).
That's bad, if by splice you mean two cables wired to the incoming feed
at the same time. In such a case, without a splitter, you'll get signal
(voltage wave) reflections due to impedance mismatches (which will look
like noise) and the power is splitting anyway. Also at the end of an
unterminated cable you'll get reflections from impedance mismatch that
will travel back up and split down to the other end - more apparent
noise.
Reflections essentially show up as increased noise floor in the tuner,
degraing your signal to noise ratio.
With digital modulations that have lots of Forward Error Correction
(FEC), the Bit Error Rate (BER) vs Signal to Noise Ratio (SNR) curves
are close to a "brick-wall". There is a very narrow band (~ 2 dB) where
the signal will go from a very low BER (good picture) to a very high BER
(no picture) as your S/N ratio drops.
This is the joy of digital TV reception that the FCC did not convey top
the public. :( The FCC didn't fully assess the impacts to OTA
broadcast to outlying areas either, I suspect. What does
http://www.antennaweb.org say you can expect for digital reception of WFFF at your location? What
is the bearing that antennaweb.org say you should point you antenna vs
the bearing at which you have it? (antennaweb uses magnetic north)
Antennaweb should be running a Longley-Rice propagation model to give
you some decent expectation of availability.
> This will eventually be replaced with a
> splitter once I get everything sorted.
Bring an unspliced, unsplit line straight to the HVR-1600's digital TV
input. If that doesn't work well, you'll need a higher directivity
antenna and/or a low noise preamp.
> Right now the HVR-1600 is working properly, I can get FOX but I suspect that
> it's because it's night time. I'll report back in the morning once the sun
> comes back out.
Weather fronts and temperature varaition betwenn you and the
transmitting station also make a difference at times.
> One thing that I don't understand is how come the television reports that it's
> getting such a high signal strength (about 75%, and I still get a picture all
> the way down to about 30% while turning the antenna)
What does you TV report as SNR? At what SNR does your TV start getting
errors on a station? How close is WFFF's ditial station to that
threshold SNR on your TV?
My Sony TV will start getting errors at an SNR of 13 dB for ATSC OTA
8VSB modulation. The channel will have no picture if the SNR drops to
11 dB. IIRC...
> while the capture card
> can't get a lock
Managing SNR by obtaining a good receive system noise figure makes a lot
of difference. Remeber that your TV has lots of power and space for
components to build a much more complex receiver with better, but
larger, components and good shielding than a card in your PC's PCI slot.
For example, I doubt your TV uses a tiny silicon tuner chip like the
MXL5005s. It probably uses larger components with better noise figures
and less loss.
Also the tuner AGC take over point (TOP) between the MXL5005s and
CX24777 (aka S5H1409) could be set suboptimally in the linux drivers,
resulting in a suboptimal receiver system nosie figure. I have not
tried to experiment with this as I could only adjust the TOP on the
S5H1409 properly.
> (I can't verify what the signal level is on the capture card
> because femon -H always reports "signal 0% | snr 0%"). Are all ATSC
> capture cards this picky?
The 0% signal is due to a s5h1409 driver deficiency, ignore it. The SNR
should be around 2.3 or 2.4 dB as reported by the s5h1409 driver for
ATSC OTA reception to work.
Receiver implementation makes the difference in the face of low incoming
S/N ratio. Receivers that have more space and power for components will
typically have better performance. The amount of HVR-1600 real-estate
dedicated to the MXl5005s (in the small tuner can) and the CX24777,
compared to the real-estate afforded to the digital tuner components in
your TV, is probably quite different.
In the face of low incoming S/N, you best bet is to reduce the noise to
as low a level as possible (eliminate EMI and reflections due to
impedance mismatches) and manage the total *system* noise figure.
The noise figures (NF) of components before the first gain stage
(cables, connectors, etc.) will dominate the noise figure of your
receive system:
http://en.wikipedia.org/wiki/Friis_formulas_for_noise The NF in dB for a cable, connector, or other passive component is the
same as it's loss in dB. So looking at Friis' equation, to get the best
possible receive system noise figure, an LNA (low noise amplifier) as
close to the antenna as possible is what one wants to manage your
receive system NF.
Regards,
Andy
> I swapped out the cables and nothing changed, I wish it was that simple :(
> Thanks for your continued help,
>
> nick
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