[opendtv] It's Not Just The Antenna

  • From: dan.grimes@xxxxxxxx
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Wed, 21 Jan 2009 09:52:28 -0800

As I mentioned earlier, I am having problems with ATSC reception on 
certain channels using my Philips 3575H, even though I have near perfect 
reception using an outdoor antenna.

First, I attempted to look at the RF to see if the issue lies there.  I 
used a spectrum analyzer and varied the RF level up and down with pads.  I 
don't think it is an RF issue because adjusting the level did not change 
the effect on the channels.  Most channels are solid but KLVX (PBS) is 
getting some minor dropout and is the weakest signal. 

The channel that locks up the ATSC receiver is KVWB 21.1 (DTV22, 
NyNetworkTV).  Like I said earlier, it breaks up, then goes to black and 
white, then completely disappears and locks up the receiver.  This is true 
with the RF ranging from -5 dBmV to -20 dBmV  (the major networks are 
stronger).  I must then switch to NTSC or DVD and then back to reset the 
receiver.  Sometimes it takes a while before it locks back into the 
channel and I cannot change the channel until it does so.

I have also experienced similar problems with KVCW 33.1 (DTV29), but not 
to the extent of 21.1.

According to the analyzer, the C/N is between -9 and -11 for the minor 
ATSC broadcasts.  But the analyzer is old (circa 1980) and abused so I am 
not confident that the self noise isn't quite high.

Is there anything else in the RF path that I can check?

I do not have an 8VSB, ATSC, or PSIP analyzer to go further with testing.

I called the station and left a message but have not yet received a 
response.  I'll let you know if I find anything further.


From the past:

Response #2:

"Dan installed an external antenna and experienced macroblocking and some
lockups at channel changes.

"Given his conditions, he might try to insert a 3- or 6-dB l- or t-pad 
his antenna line and see if too much power is the problem.  It usually is.

"Lockups at channel changes could be cause by too much power, or could be
caused by bad PSI/PSIP mismatch (some of the middleware stacks in mid and
low cost receivers haven't implemented this properly or in a way that 
with real-world situations.) It can also be caused by overshooting the bit
budget, in inartfully designed receivers.

"A spectrum analyzer might help diagnosing power issues, but a PSI/PSIP
analyzer will help (someone with knowledge) diagnosing any mismatches."

John Willkie

Response #1:

"These are probably not RF problems.  They are problems upstream of the 
transmitter.  There is no separate color subcarrier in DTV to drop out 
luma continues to be received.  Nor is there a separate sound subcarrier. 
the station is compression coding NTSC source material, it's probably 
encoding equipment that is the problem.  However, there are problems on 
satellite digital distribution system to the stations.  the macroblocking 
up on occasion even on NTSC reception."

Al Limberg


The antenna is not the miracle fix for ATSC reception.  I installed a high 
quality outdoor antenna (Winegard, Belden cable, and all high quality 
components in between) and I'm in a part of the Las Vegas valley that is 
quite clean of tall buildings (with the exception of the airport behind 
me) and in direct view to the transmitting antenna.  My analog reception 
is near perfect and the meter on the Digital Stream box shows a solid 
green, 92 to 96 percent, for almost all DTV stations.  However, my Philips 
3575H (no meter that I can find) locks up once in a while when changing 
channels, I often get macroblocking, audio drops occasionally and 
sometimes the color drops out.  And one of the sub-channels (NBC 3.2, 
Weather) doesn't display at all.

I have not put a spectrum analyzer or other equipment on it yet to see 
what RF issues I might be experiencing.

So a perfect antenna and RF path doesn't solve all the problems.


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