RCA CTC195, CTC197, CTC203 Audio-Dropout "Glitch"
THOMSON AUDIO DROP-OUT
There has been a situation when in some RCA/GE televisions
low-level,
scratchy-sounding noise comes out of the speakers instead of norma
audio, or when video is muted. These problems can occur when invali
XDS data is broadcast by television stations and received by certai
Thomson television receivers. XDS data is digital information place
within the video signal's vertical interval of an analog (NTSC
television transmission. This data is used by the microprocessor i
the television receiver. It may be transmitted from any analo
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 2
contains two captioning channels (CC1 and CC2) and tw
"text" channels (TEXT1 and TEXT2). All four of these dat
channels share that 600 cps data stream, and the information i
sorted out using packet headers. Field 2 contains a matching set o
data channels (CC3, CC4, TEXT3, and TEXT4), and can also contai
extended data services (XDS) packets.
The XDS (extended data services) provide information about the curren
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 weathe
service warning codes.
The "Thomson audio drop-out problem" occurs when
broadcaster sends invalid XDS data - that is, data that does no
contain truncation packets. There are certain XDS encoders on th
market that broadcasters may use that will do this. Thomso
television receivers using 16-bit microprocessors (i.e.
CTC197/PTK195 chassis) are programmed to decode XDS data by countin
data packets. When the microprocessor encounters corrupt data with n
truncation packets, the data overflows into memory registers where i
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/o
video).
When the unit is unplugged and replugged, it empties these register
of the corrupt data.
It is good to know that this problem was resolved in newer chassi
designs that use 32-bit microprocessors (i.e. MM101/102 an
DTV306/307).
But the vulnerable Thomson televisions will do this as they ar
receiving
invalid data from a station to which the MAIN TUNER is tuned. This i
why it is so important to find out what channel, and mor
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 i
valid (includes the necessary truncation packets), so that thei
signal is always 100% valid, and does not mess up these Thomso
receivers. Some retailers have someone assigned to contact suspecte
broadcasters. Once the customer identifies the program (o
commercial) was being watched (including the time of day) when th
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 custome
recorded from a previously-aired television program.
So the fix is to place a 1mfd capacitor to ground on U13101(15). Thi
swamps the closed caption data to the micro and stops the reception o
the defective data packet. In the event that later on the resolve th
defective data problem the capacitor can be removed. the close
caption feature will not work with this cap. installed