[TechAssist] Re: RCA Proscan PS27115, Chassis CTC197BC. Intermittent loss of audio, just a pop sound when channels change?

  • From: Jon Garver <jongar@xxxxxxxxxxxxx>
  • To: techassist@xxxxxxxxxxxxx
  • Date: Mon, 31 Oct 2005 12:29:32 -0500

Got a few calls on the same problem Friday.
Haven't tried this myself but here is an explanation and possible fix.
---------------------------------------------
RCA CTC195, CTC197, CTC203 Audio-Dropout "Glitch"

THOMSON AUDIO DROP-OUT

There has been a situation when in some RCA/GE televisions a low-level, 
scratchy-sounding noise comes out of the speakers instead of normal 
audio, or when video is muted. These problems can occur when invalid XDS 
data is broadcast by television stations and received by certain Thomson 
television receivers. XDS data is digital information placed within the 
video signal's vertical interval of an analog (NTSC) television 
transmission. This data is used by the microprocessor in the television 
receiver. It may be transmitted from any analog television channel.
_________

Explanation of XDS data: In each frame of video, line 21 contains a 
single stream of data, containing different types of data packets. Field 
1 of line 21 contains two captioning channels (CC1 and CC2) and two 
"text" channels (TEXT1 and TEXT2). All four of these data channels share 
that 600 cps data stream, and the information is sorted out using packet 
headers. Field 2 contains a matching set of data channels (CC3, CC4, 
TEXT3, and TEXT4), and can also contain extended data services (XDS) 
packets.

The XDS (extended data services) provide information about the current 
program, TV station, and network. Unlike the caption and text data, they 
are packets rather than continuous streams of data.

XDS packets include:

Name, length, and start time of current show Type of show, based on a 
set of category codes Program content advisory (see "V-chip data" below) 
Network name Station name and number Time of day National weather 
service warning codes
________

The "Thomson audio drop-out problem" occurs when a broadcaster sends 
invalid XDS data - that is, data that does not contain truncation 
packets. There are certain XDS encoders on the market that broadcasters 
may use that will do this. Thomson television receivers using 16-bit 
microprocessors (i.e. CTC197/PTK195 chassis) are programmed to decode 
XDS data by counting data packets. When the microprocessor encounters 
corrupt data with no truncation packets, the data overflows into memory 
registers where it does not belong, the first of which relate to the 
audio processing. When the microprocessor encounters this corrupt data 
in the audio (or even video) registers, it interprets it as the start of 
a shut-down, and mutes the audio (and/or video).

When the unit is unplugged and replugged, it empties these registers of 
the corrupt data.

It is good to know that this problem was resolved in newer chassis 
designs that use 32-bit microprocessors (i.e. MM101/102 and DTV306/307).

But the vulnerable Thomson televisions will do this as they are 
receiving invalid data from a station to which the MAIN TUNER is tuned. 
This is why it is so important to find out what channel, and more 
specifically, what program the viewer was watching at the time of the 
anomaly.

The real solution: Broadcasters can upgrade their equipment so that the 
data they send is valid
(includes the necessary truncation packets), so that their signal is always
100% valid, and does not mess up these Thomson receivers. Some retailers 
have someone assigned to contact suspected broadcasters. Once the 
customer identifies the program (or commercial) was being watched 
(including the time of day) when the incident occurred, they should 
report it to the home service specialist.

A final note: XDS data may be inserted live, or played back on video 
tape by a broadcaster. It can also show up on a VHS tape that the 
customer recorded from a previously-aired television program.
-----------------------------------------------------------------------
A fix that I have been recommending to our techs is to place a 1mfd 
capacitor to ground on U13101(15). This swamps the closed caption data 
to the micro and stops the reception of the defective data packet. In 
the event that later on the resolve the defective data problem the 
capacitor can be removed. Obviously the closed caption feature will be 
crippled with this capacitor installed.



Jon Garver
Heritage Television Service
204 Heritage Rd.
Monticello, IN. 47960-1510
Voice & FAX: 574-583-9681
Alternate Email: HeritageTV@xxxxxxxxxxx

Robinson Electronics wrote:
> Can any of you Techs. when with some info on this problem? Are there any cure 
> from Thomson on this?
> The customer said he was watching TV and the audio just quit, but would make 
> a pop sound when the channels were changed.
> I checked the set out, plugged it in and the sound is great, no problem found.
> I changed out some caps in the audion circuit in case of low esr, checked ok 
> though:
> C11601, 11604, 11611, 11623, 11628, 11907.
> I checked the audio output TDA7499, not getting hot, checks ok.
> Played for about a hour, called the customer to pick up.
> The customer said it played good for about 2.5 hrs. then quit after a 
> commercail came on, now just a pop sound when channels change. looks like 
> after unplugged for a while, it resets and the sound is good.
> I saw some notes indicating Q11900 "mute xstr" as a common audio problem!
> 
> Has anyone seen this, found a cause or cure for it?
> Can anyone help us with this? Any info appreciated.
> Thanks.         David.
> 
> Robinson Electronics
> 1075 Sweeten Creek Road #51
> Asheville, NC 28803-1757
> Ph./Fax. 828  274-3787
> Toll Free. 1-888 ROBELECT
> www.robinsonelectronics.com
> www.robelect.com
> Mailto: RobinsonElectronics@xxxxxxxxxxx
> 
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