[opendtv] Re: It's Not Just The Antenna
- From: "John Willkie" <johnwillkie@xxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 21 Jan 2009 10:04:36 -0800
Dan;
Aside from not having the gear, if it ain't RF, then the most likely
culprits (in order of significance) are
1) transport stream bit rate (should be constant; doesn't mean that
the encoder is actually doing that)
2) bad elements in MPEG-2 PSI and/or PSIP (such as tables changing
dynamically with or without changes in version_number)
3) repetition rates in PSI and PSIP tables
4) data mismatch between PSI and PSIP (most importantly, the
transport_stream_id in the PAT header must match the transport_stream_id
field in the header of the TVCT
5) video/audio buffer under/overflow
Hooling up the free version of TS_Reader to a PCI-card receiver can help
diagnose all of the above save for 5).
John Willkie
_____
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de dan.grimes@xxxxxxxx
Enviado el: Wednesday, January 21, 2009 9:52 AM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] It's Not Just The Antenna
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.
Dan
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 into
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 deals
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 8VSB
DTV
transmitter. There is no separate color subcarrier in DTV to drop out while
luma continues to be received. Nor is there a separate sound subcarrier.
If
the station is compression coding NTSC source material, it's probably their
encoding equipment that is the problem. However, there are problems on the
satellite digital distribution system to the stations. the macroblocking
shows
up on occasion even on NTSC reception."
Al Limberg
Original:
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.
Dan
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