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FleaUNO NTSC DAC
08-07-2019, 07:51 AM
Post: #1
FleaUNO NTSC DAC
Hope you don't mind me posting a question about a product that's long past it's expiry date, but Uno is still one of my favourite toys and I'm still developing with it.

So one quick question: is there any trick to getting the NTSC DAC working? If I generate a signal using my own resistor ladder then it works fine, but if I do it through the AV cable I get spiking that looks like there's capacitor in parallel to the load. Sure enough, when I look at the schematic I see C47, but it's marked DNF and I can't locate it on the board either:

[Image: D1hMzdv.png]

My constraint file uses the locations from the sample app:

LOCATE COMP "NTSC_DAC[3]" SITE "27" ;
LOCATE COMP "NTSC_DAC[2]" SITE "28" ;
LOCATE COMP "NTSC_DAC[1]" SITE "143" ;
LOCATE COMP "NTSC_DAC[0]" SITE "31" ;

I haven't ruled out the possibility that I've somehow borked my DAC pins, but I can't help but feel there's something I'm missing?
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08-12-2019, 12:54 PM
Post: #2
RE: FleaUNO NTSC DAC
Hello and thanks for your question!

I have re-checked the schematic and what you posted looks correct. Are you sure that you are referencing to ground in the connector, and not one of the audio channels by mistake? I have also replied to your recent email with an NTSC demo attachment. Kindly give that a try that first and let me know if that works for you...

For your information, the pads for C47 are located right in front of the left anchor pin of "DV OUT" connector (looking at the board from the top, with all of the Uno's rear connectors facing away from you).

I left out C47, because I thought, for some strange reason "Hey, wouldn't it be cool if I can generate waveforms over and above NTSC video frequencies?" Rolleyes

Hindsight being perfect as always, I probably would've further tweaked both the audio and video DAC circuits in future FleaFPGA Uno runs. Sad that it didn't happen.

For example: Instead of installing a capacitor in C47, I could also have gone with say, a 190 Ohm resistor in it's place (would also have tweaked other DAC resistors to suit). Further, adding diode clamps or a discrete bipolar transistor buffer stage could help protect the FPGA from the potential transients found in analog TV video links. Same goes for the audio portion (i.e. adding diode clamps).

Cheers,
Valentin
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08-12-2019, 04:36 PM
Post: #3
RE: FleaUNO NTSC DAC
Have responded via email, but updating here as well for anyone else reading.

The issue was my cable. I found it in one of the many boxes of cables in my garage and must have been from my old camcorder or something because the pin-out is completely wrong. And yes, I was sending the composite signal down one of the cable's audio channels....which would certainly help explain why I thought there was a large capacitor lurking around somewhere! Big Grin

Thanks again for the response and help.
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