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-   -   TBC-1000 incorrectly processing PAL60 (https://www.vintage-radio.net/forum/showthread.php?t=187279)

unitelex 3rd Jan 2022 11:46 am

TBC-1000 incorrectly processing PAL60
 
I have been asked to fix an issue with a Datavideo TBC-1000 time base corrector.

The owner reported herringbone stripes on the video image.

This unit has three boards, a small psu regulator board, a digital processing board and a 4 way distribution amplifier board. The PSU board is a buck SMPS so the electrolytics were replaced by new low ESR types and confirmed OK generating clean 5V output.

The distribution amplifier was tested as OK by feeding the non processed video signal direct to it, achieving good outputs on all 4. So the problem seems to be related to the digital board. A number of electrolytics were changed for good quality types, and both Crystals were checked by substitution with new and confirmed with the scope.
Interestingly digital board was also offered as a PCI sized plug in card (TBC-100) without any PCI bus so it could be used in a PC connected to the PC's separate AV card. This is evident from its shape and unused connectors.

The digital board has a 8051 MCU (Winbond clone) which seems to do the initial setup via SPI and flashes an LED. The incoming composite or Y/C input is processed via two SAA7111A devices (have not figured out why 2 are used) The digitised frame is stored in 4V3422 3Mbit RAMs (4 off). The output signal is generated by a single Conexant BT864A providing Y/C S-video and composite.

I am using a Panasonic TV TX-L37G10B. The TV manual states it can decode PAL, PAL 525/60 M.NTSC NTSC

If I use a European content DVD, I can see the DVD player is outputting composite PAL50 4.43MHz subcarrier colour burst. This gives a clear image on the TV both directly connected or via the TBC-1000. No problem there.

Using a PAL60 4.43 (eg from a DVD player and US content DVD) direct into the TV (Panasonic TX-L37G10B) I get a clear colour image also.
Sending the same composite signal via the TBC-1000 the TV briefly shows a good colour image (less than one second) then it degrades to a monochrome image with slanted vertical stripes. The scope shows that the TBC-1000 video output is 60Hz but with a 3.57MHz colour burst. The Panasonic TV is not able to decode the chroma.

So it seems the TBC-1000 is applying the wrong subcarrier colour burst when faced with a PAL60 input or the TV just cant handle this combination.

Co-incidentally the Winbond MCU uses a 3.5794 crystal for its clock (cheap xtal), this is not the source of the 3.57 colour burst however, I confirmed this by changing this crystal to 4.43MHz and it made no difference.

I checked the data sheet for the SAA7111 and it *seems* to imply that it uses the frame rate to automatically detect PAL vs NTSC format.
But the colour does decode correctly for a brief moment suggesting that the device is at least capable of doing the correct decoding, but is changing mode. The two SAA chips and the Conexant and MCU are connected via SPI for configuration. On a hunch I powered the unit normally with a PAL50 signal then held the MCU in reset and started the PAL60 DVD. Interestingly this made no difference. The colour image appeared momentarily and reverted to the mono image with stripes. So the MCU is not changing the mode. It looks like the SAA7111 is detecting the 60Hz frame and assuming it needs to process the chroma as NTSC, or possibly more likely the Conexant BT864A is changing the mode.

I suspect the TBC-1000 is operating as intended. Its design is more than 10 years old. If it was intended to improve time base on VHS PAL50 or NTSC then it seems suitable for this. But seems incapable of at least one combination of PAL60 or more modern disc recordings.

Does anyone have any insight into this, I would be interested to know if the chipset is configurable to achieve the PAL60 with 4.43 subcarrier.

Thanks
Chris

jjl 3rd Jan 2022 12:48 pm

Re: TBC-1000 incorrectly processing PAL60
 
It sounds like the TBC is outputting PAL-M which was the broadcast standard used in Brazil with the NTSC field rate of 59.94 Hz and a 3.575611 MHz colour subcarrier when presented with a PAL60 input. Note that PAL60 is not an officially recognised PAL standard.

John

unitelex 5th Jan 2022 11:16 pm

Re: TBC-1000 incorrectly processing PAL60
 
OK thanks John, I agree.

Concluded it is working as intended.
Picture quality is very good with a standard PAL signal.

Chris


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