[mythtv-users] Good signal strength? was: Re: Alternative to Silicondust HDHR3-6CC-2X3?

Joseph Fry joe at thefrys.com
Thu Sep 6 20:00:34 UTC 2012

> Be careful here, I bought a power splitter and it actually caused the loss
>>> of some channels.
>>> One of which my wife likes to watch.
>>> I had to pay to have the CCo come out and raise the power enough to
>>> overcome the passive splitter losses.
>>> Needless to say, this did not make me a bigger fan of the CCo.
>>> There seems to me some wonderful and creative engineering going on here.
>>> Some of the main channels are very fussy about the plumbing downstream
>>> from the cable while
>>> the junk channels would probably be OK using old speaker wire.
>>>   My TV provider is Comcast, if that makes a difference.
>>> _______________________________________________
>>> mythtv-users mailing listmythtv-users at mythtv.orghttp://www.mythtv.org/mailman/listinfo/mythtv-users
>>>   I bought an amplified splitter off eBay. I have a 2-way passive
>> splitter at the ingress, with one cable going to the modem and the other
>> the amplified splitter. From there all my runs go direct to their
>> respective devices, including my HDHR prime. It works quite well for my
>> home setup. When I get home I can post the manufacturer and model of the
>> splitter, if anybody cares.
>> In a perfect world the only cables going to my TVs would be Ethernet, but
>> the tech isn't quite there yet, and with my penchant for watching live TV
>> combined with the copy-protection flag crap that so many channels use, it
>> may never happen.
> Putting the amp at "the ingress" is the safest place to put one... using
> it further down the line, after having already reduced the signal with
> splitters and long cable runs, will often result in reduced signal QUALITY,
> which is far more important than signal strength.   Obviously if your
> signal coming in is very strong to begin with, you may get away with it,
> but typically the cable companies only put out a signal strong enough to go
> through 7-10 dB worth of loss, which will occur with a 4 way split and a
> long cable run.  Amplifying the signal after a 7dB drop will likely result
> in a high strength but poor quality signal... at least in my experience.
> Looking at my existing InfiniTV PCIe, the signal strength for all four
> tuners varies from about -4 dBmV to -5 dBmV while idle, and around -2 to -3
> dBmV while playing a channel with right around 35 dB signal to noise
> level.  Is that good?  Bad?  Indifferent?  Am I likely to run into any
> problems if I split it two, possibly three ways?
Those numbers look pretty good.  On my current channel I am seeing -3.8
dBmV and a 34.3 SNR and have no dropouts that I've noticed.

Now if you split it two ways, your signal strength will drop another 3.5
dB... so you will probably be at -5 or -6 dBmV.  The standard says a tuner
is supposed to work with a signal as high as 16dBmV and as weak as -12dBmV;
0dBmV is nominal.  Of course all tuners are different.

If you split the line 3 ways, one leg will have a 3.5dB loss and the other
two will have a 7dB loss... pushing you pretty close to the bottom end.
 The best I can say is to try it... keeping in mind that the SNR is more
important than the strength.  A SNR of 35dB means that the signal is about
1000x greater than the noise level (on average)... adding splitters
shouldn't raise the noise level on the line much if at all.... as the noise
is being reduced by the same level as the signal.  However you will likely
see SNR increase because you have more devices connected, more cable, and
more connections... all of which can act as an antenna or otherwise inject
noise into the signal; additionally, all tuners have built
in amplifier that will add noise when trying to boost weaker signals to
usable levels... this amplification stage is largely what dictates the
quality of a tuner.

Note not all channels will have the same SNR.  For example my 4G LTE
Verizon phone will completely kill the SNR of some of the channels on my
HDHR prime if it's transmitting too close to it.  I've had to keep my phone
as far away from it as I can because of the amount of noise it generates in
those frequencies.
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