Not sure this is the right place to post this but perhaps someone here
will have an answer. I have two TVs on both cable and satellite. I
have a local Public TV Channel that occasionally has no sound on one
TV. Picture is OK but no sound. The sound on the other TV is just
fine. I can use the VCR as a tuner and the sound is OK. Another
problem I've noticed is when watching the NBC nightly news on a local
NBC station via cable, the sound is not sycronized with the speakers
mouth. It's OK on the other TV or if I use the VCR as a tuner. This
only occurs when on cable and not the satellite. Both TVs are on the
same cable feed. Anyone got any ideas what's going on?
Don in Tracy, Calif.
The symptom of a cold solder joint. The sound circuitry is good. A
good old slap on the TV cabinet should bring the sound back
temporarily. The TV heats up and cools everytime you switch it on or
off. That causes expansions and contractions and weak solder joints
do lift from the PCB, or generate hairline cracks.
If you are handy with a solder iron and willing to poke around the
innards open up the back cover, with the power disconnected of course.
With the power on again, you should have video and perhaps audio. Use
a wood dowel or some non conductive stick to wiggle the wire cabling.
If you get lucky the audio cable (or component eg. volume control,
speaker connection) cold solder should show up. Take a close look at
the solder joints, especially where the plug pins are soldered onto
Don't be too nervous about the high voltage. Normal precautions
should suffice. If you do get zapped it is unpleasant but you don't
see any TV techs with frazzled nerves or burnt hair walking around.
In an earlier life servicing computer monitors and equipment I had
been zapped only once.
This this how I fixed my TV's problem with intermittent video
problems. The top few video scan lines would spread out every now and
then particularly when the furnace fired up in winter. I opened up
that hernia box quite a few times over the years to locate the fault
without success. It was frustrating as I could not reproduce the
problem at will, thus bringing that monster box to the repair shop
would be pointless. Furthermore all the electronics was on one PCB
and the wiring-packaging was too compact to allow meter probes to
reach the test points. On the last try, I happened to brush against
the cable and the video problem appeared. Jiggling that cable harness
reproduced the problem consistently. The rest was easy.
Anything more than locating and resoldering the cold solder joint is
probably a shop job. There are hardly any home repair replaceable
electronics components nowadays. Ask for a repair estimate first. If
your TV is more than five years old it has probably more than paid for
itself and not worth repairing. Its cheaper and better to by a new
If the video and sound both go to the satellite and back, they should
be delayed the same amount. I never noticed this (even with
satellites) in the days before digitizing video - only after
digitization, so I think it is because of that.
The television network has fragmented, and there are now many more ways
of getting TV to the end user than there once were.
Encoding or recoding the signal to do statistical multiplexing (if you can
look ahead several seconds, you can fit n channels into a given
bitrate at better quality than if you just encode them all with minimal
This introduces a several second delay.
Sometimes this is counteracted by adding in delays to other signals, to
make them happen more or less in time.
A hop to a GEO satellite and back takes only .2 seconds or so.
Even 2 hops only adds .5 seconds.
Sound and vision is always in sync on every broadcast channel
The problem is that how do you sync different channels.
If someone's listening to an FM broadcast of a sports event, and
happens to turn on a satellite TV, there may be a delay of up to
around 10 seconds.
Do you delay the FM signal by 10 seconds?
And what if there is a digital cable station getting its feed digitally
from that satellite, and reencoding.
This may add another 5 seconds, so do you up the delay to 15 seconds?
Here is the device to get them back in sync
that was mentioned in
but one of the devices has to be used before they get out of sync and
another at the end to get them back in sync. So would CNN (say) have
to use one before the audio and video get out of sync, and then the
local cable company have to use one to get it back in sync?
It turns out that is the reason:
"...However, the nature of the sync problems TV is having is just the
opposite: The video lags the audio. That's because of digital
"There's a long and a short reason for this; you'll be happy to know
that I'm only able to recite the short one: The process of digitally
encoding and decoding video is more complicated than for audio, and so
it takes longer. Running through a number of encoding and decoding
processes where it always takes video longer to make the trip than
audio, and the problem gets worse.
Lip sync on the TV station was correct; on the cable channel it was
badly out of sync.
...the problem won't get solved unless you insist that it is. Declare
war on lip sync problems and get them fixed. "
The technical discussion is getting confusing.
I had taken courses on TV servicing for interest (though never used
that professionally as there is no money in consumer electronics
servicing.) If you refer to any textbook, the front chapters with
the basic concepts will do, there will be a block diagram on TV
circuitry and on the TV broadcast signal.
On the analog TV sets. The audio and the video are embedded in the
same signal - the composite video signal. The signal gets into the TV
set via the antenna or the cable to the "Tuner" block where you select
your channel. Then it enters the "IF Strip" block followed by the
"Video Detector" block. From here the signal is split and output to
the "Audio" block and the various "Video" blocks. The signals may be
distorted, or missing audio or video only. But there is no way one
signal will be out of synch with the other.
I haven't read up on digital TV circuitry but a quick search on the
Internet turned up < http://www.sencore.com/custsup/pdf/TT171.pdf
and there is a useful block diagram in page 2. It is still a single
composite video signal which does not get separated into its audio and
video components until the second PIF stage (cable ->Tuner -> PIF ->
etc.) and therefore both the audio and video arrive in synch and
cannot be out of synch whatever happens inside the TV set.
A few select descriptions from this URL will be useful:
From my understanding of the above Sencore explanation digital TV is
not true HDTV as HDTV will be wholly digitized at the broadcast end.
In a similar vein...
We use Comcast cable for TV, Telephone and Internet service.
I have noticed that about once a week, particularly when watching TV
"Premium Channels" or "On Demand" programs there is a high pitched
"chittering" background noise on the audio, characteristicly like an
"automatic level control" system (It increases in level when the program
audio is quiet and noticably decreases when, for example, an actor
starts speaking.) It's reminiscent of "alternator whine" on an AM car
radio twenty years ago.
The noise sometimes persists for as long as 15 to 30 minutes, then it
goes away. If I switch the cable box around to to a few other channels
when the noise is present, there isn't that background noise on any of them.
Unless I'm really engaged in the program (Like sometimes it's SWMBO
who'se watching it, but I'm reading the newspaper in the same room.)
that damn noise starts boring through my brain and is hard to "tune out".
Last night we were watching a pay "On Demand" movie and that annoying
audio noise was present for about half the movie. I found myself getting
irritated at having to put up with it, particularly as we were "paying
extra" for the program.
I've called Comcast's customer service about it, but they act like they
don't understand me. (Big suprise.)
Anyone else experienced this particular problem, and got it fixed?
Jeff Wisnia (W1BSV + Brass Rat '57 EE)
"If you can smile when things are going wrong, you've thought of someone
I have that on most channels, some are worse than others. My guess is
that it is caused by the video signal being compressed for
transmission and de-compressed. It takes time to do the
compression/de-compression, and they don't bother to keep the sound
and video in sync.
I don't know. For me it seems to vary by channel and may be better or
worse at different times. I had the problem with my old house in a
different city, and different cable provider too. And I've seen the
same problem in motels I've stayed in.
Swap the TV's and see if the problem stays with the TV or
stays at that location. 99.9999999% odds are that it stays
with the TV. Are you SURE it's not set to SAP? From the way
you answer, it sounds like you might not even know what it is.
Hmmmmm, I see.
You may want to seek professional help.
TP / Network Man __________________________________
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