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Old 5-Nov-2014, 1:17 AM   #42
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,747
And now to put some of your numbers into an estimate, I will use the guidelines from ATSC A/74:

ATSC Recommended Practice:
Receiver Performance Guidelines

Document A/74:2010, 7 April 2010
http://www.atsc.org/cms/standards/a_74-2010.pdf

5 RECEIVER PERFORMANCE GUIDELINES
5.1 Sensitivity

Quote:
A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 (i.e., the FCC Advisory Committee on Advanced Television Service, ACATS, Threshold of Visibility, TOV) for input RF signal levels directly to the tuner from –83 dBm to –5 dBm for both the VHF and UHF bands.
and from ADTech:
http://forum.tvfool.com/showpost.php...3&postcount=31
Quote:
I think he's fine as long as he doesn't amplify the C2V's output. If we accept the simulator's estimate of about -25 dBm at the antenna, add in 8 dB for antenna gain, subtract 1-2 dB for the insertion loss of both the AC7 and integrated U/V combiner, subtract 3 dB of coax loss (about 50'), and subtract the Roamio's tuner noise figure (??), we're still well under -20 dBm signal power at the inputs to the individual tuners which would be expected to tolerate maximum signals up to nearly 0 dBm.
As you can see, ATSC says that a tuner should be able to handle a signal as strong as -5 dBm and provide satisfactory reception. ADTech has calculated that the signal into your Roamio is under -20 dBm, which is 15 dB weaker.

for more than one signal ATSC says:

5.2 Multi-Signal Overload
Quote:
The DTV receiver should accommodate more than one undesired, high-level, NSTC or DTV signal at its input, received from transmission facilities that are in close proximity to one another. For purposes of this guideline, it should be assumed that multiple signals, each approaching –8 dBm, will exist at the input of the receiver.
Your signal is also weaker than that.

In my previous post about THREE TYPES OF OVERLOAD, a -5 dBm signal would fall between type 1 and type 2 overload, but closer to type 2.

When I was experimenting with my Apex DT502 digital to analog converter box just after it came out, I connected two preamps in series between the antenna and the Apex. When I looked at the monitor, there was no signal on the screen. At that time I didn't know as much about overload as I do now, so I didn't understand what was wrong. It was type 2 overload causing complete loss of reception.

The signal from the antenna was +20 dBmV, which is the way my SLM (signal level meter) measures signals, or equal to -28.8 dBm. The gain of the first preamp was 25 dB, which brings us up to -3.8 dBm. The second preamp gain was 15 dB, which is +11.2 dBm into the tuner. The tuner was not damaged. I haven't tried type 3 overload because I don't want a toasted tuner.

My newest TV is a SONY KDL32R400A. Using a splitter to send a signal to the TV and my SLM, I measured the signal level at 100 on the Diagnostics Screen signal strength scale and it was +40 dBmV, which is equal to -8.8 dBm. The engineer who designed the circuit must have been reading ASTC A/74.

In this country engineers use the symbol dBm for signal power; antenna installers and cable guys use dBmV. Other countries use dBμV (note the little tail on the u which means "micro"), which is dB microvolts. It's also signal power but its reference level is defined in microvolts. The conversion factor between dBmV and dBμV is 60, so 40 dBmV is equal to 100 dBμV, which is what the Sony engineer was thinking. 100 on the signal strength scale equals 100 dBμV.....nice!

I then inserted a variable attenuator (using the method described in the link in my signature) and brought the signal level down to dropout, which was about -85 dBm and noticed that the units on the signal strength scale were equal to about 1 dB. So, I was getting a TV and a signal level meter for the price of a TV.

My older SONY KDL22L5000 signal strength reading never gets up to 100 no matter how strong the signal. The reading is derived from the AGC (Automatic Gain Control that controls the sensitivity of the tuner) voltage. The newer SONY must have a different type of circuit, perhaps an RSSI chip like in cell phones which gives you strength in bars.

I had a chance to measure another R400A, and it gave me the same reading at 100 on the signal strength scale.
Attached Images
File Type: jpg SONYSSVSdBmVchart.jpg (59.6 KB, 1217 views)
File Type: jpg SONYgoodSIG.jpg (72.9 KB, 1097 views)
File Type: jpg SONYbadSIG.jpg (105.4 KB, 1152 views)
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Last edited by rabbit73; 6-Nov-2014 at 5:07 AM.
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