[mythtv-users] Poor recording of races only

[Stephen Worthington]

On Tue, 14 Oct 2014 00:49:23 +0000 (UTC), you wrote:

>I've finally gotten my Ceton InfiniTV 4 PCie working on Mythbuntu.  <YAY!>  Anyway, I've set up a bunch of shows to record, all of them HD.  Many of those shows record just fine.  However, I'm a racing fan, and every race I've recorded so far has been of really poor quality except one.
>
>All of those recordings show up with a yellow border.  I think that means the recording is damaged from what I've read.  When I watch the recording, every time a scene changes, the screen gets very blocky (macroblocking?) and the audio cuts out, then it will return to a beautiful recording for a moment until the camera changes, then it repeats.  It gets worse and worse as the show goes.  I haven't let it run, but I have noticed that the file size is very small.  For example, a normal TV show recorded for 1 hour at 5.xx Gb.  A formula 1 race (3 hours) recorded at 0.5 Gb.
>I've also had some problems with BBC America.  Some recordings are great.  Some show up as yellow like the races.  Of those, some just hang the frontend.  Others come back with a message saying the file is missing.
>
>The one race recording I recorded without problem was a NASCAR race that was not scheduled to record, but I turned it on in Live TV.  It looked great, so I went back to the program guide and tried recording it.  It recorded fine.  As I said, these shows are all HD.
> Initially, I was unable to record anything when I set up the box.  I was getting buffer errors.  I found that my network was running at 10Mb/s.  I fixed that so it's running at 1000MB/s, but I still have some Cat5 (not 5e) cable in the walls.
>I looked at the channels that I've recorded on and here's what I got for signal strength and S/N ratio:TNTHD - always good:  Signal Strength - 3.8 dBmV, Signal to Noise Ratio - 32.6 dBWMAQHD - always good:  Signal Strength - 4.1 dBmV, Signal to Noise Ratio - 33.4 dBWBBMHD - always good:  Signal Strength - 5.9 dBmV, Signal to Noise Ratio - 34.6 dB
>BBCAHD - sometimes good recordings, other times not:  Signal Strength:  2.3 dBmV, Signal to Noise Ratio:  34.1 dBNBCS1HD - always bad:  Signal Strength - 3.9 dBmV, Signal to Noise Ratio - 34.8 dBFS1HD - always bad:  Signal Strength - 3.5 dBmV, Signal to Noise Ratio - 32.1 dB
>Since it seems like a channel thing (higher channels have problems, lower channels don't), I figured I'd look at improving my signal strength.  I've had a Motorola 1-2 amplified splitter (http://www.amazon.com/Motorola-Signal-Booster-BDA-S2-Amplifier/dp/B0017I1PVC/ref=pd_sim_e_5?ie=UTF8&refRID=0REJCWJTGHQ0WF945342) on the line coming into my house for a while now.  It helped a lot as before I couldn't tune.  I've got one of those lines going to my old STB, and the other going to a 1-2 splitter.  From the splitter, one line goes to the cable modem, and the other line goes to my backend's cable tuner card.  I've always read that splitters degrade signal, so I tried disconnecting the STB and connecting one long line directly from the Motorola amplifier to my cable card.  On all channels, SS goes up by 2 - 3 and S/N goes up a bit too.  I've read that a perfect signal strength would be 0 dBmV, but I'm wondering if that's wrong.
>Can anybody offer any suggestions on what I should look at to be able to record these shows?
>Mike

Basic physics applies to splitters, as in all things.  If you split a
signal into two, then each split signal will be less than half the
signal strength of the incoming signal.  The "less than" bit is due to
whatever losses occur in your splitter - some are much more lossy than
others.  The only reason that splitters work in aerial systems is that
tuners usually have a very wide range of signal strength that they can
work with, so halving the strength in a two-way splitter still leaves
a signal that is easily enough for a tuner to work with.

When your signal strength as received at the aerial is too low but of
OK quality, or you are reducing the strength by using splitters,
amplifying it helps reception.  But if the signal strength at the
aerial is so low that the quality of the received signal is not good
enough for the tuners to use, amplifying it does not help, and in fact
degrades the signal quality further as amplifiers are not perfect.

Aerials are a compromise - they normally have one frequency they work
best at and their performance drops off the further away from that
frequency a channel is being transmitted on.  So higher and higher
frequency channels (in your case) will likely be getting lower and
lower quality as received by the aerial, even if they are all
transmitted with the same strength and quality.  Which they are not -
transmitters vary in quality and transmission power, even when all
your channels are transmitted from the same site.  Aerials are also
more or less directional - if your channels are transmitted from
different sites, your aerial will likely be pointed in a compromise
direction somewhere between the transmitters.

So, as you can see, aerial systems can become very complicated.  I
would suggest that the first thing to find out is if the quality of
your signal for the bad channels is good enough to work with at all.
That means plugging your aerial signal as directly as possible onto
one tuner without any splitters (or amplifiers initially).  Can you
record from it then?  You may need to try adding amplification, but no
splitting.  If that works, then you can probably organise amplifiers
and splitters that will work.  But if it does not work, then the
problem is the aerial itself.  You may need a different aerial to pull
in the problem channels, for example, one aerial pointed in the
direction of each transmitter site and selected for the frequencies
involved.

If the quality is OK with just one tuner on the aerial, then you may
need to get a better signal path from the aerial to your tuners.  The
ideal setup with amplifiers is to have one amplifier/splitter at the
aerial or as close to it as possible (eg where the aerial cable enters
the house).  That amplifier/splitter would have individual aerial
cables going from it to every tuner (including your TVs).  Also,
amplifiers and splitters come in widely varying quality - choosing
good ones is important when dealing with lower quality signals.  It
really helps to have a professional quality signal meter when setting
up aerial systems like that, and some experience with aerials - so if
you have a good aerial company you can hire to do the job, that may be
the best option.  There seem to be plenty of bad aerial companies
around though.