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Group Project: Building the Hedghog 405 standards converter

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Honorable Member Registered

The hedghog is taking a bit more shape now, I had trouble with the phono sockets so gave up on the original case and used another of the type I used for the PICGEN 405 generator. This gave me more space so I used front and rear BNC's for video in and out and DIN for audio, I also used a 4 pin XLR to suit my workshop DC distribution. 

There is a strange patterning fault on the video however, it looks like PSU noise but I tried it with a linear supply and it was still the same, it looks like an analog fault, the patterning can be minimized by fine tuning, but both patterns are fine.

IMG 0122
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IMG 0117


Posted : 22/08/2019 9:40 pm
Posts: 4563
Illustrious Member Registered

Have you tried a different TV, its possible that if one of the traps in the IF is not correct it may be letting through a spurious signal from the generator.

The pattern looks about the same frequency as the 0.8Mhz grating on the Philips test card, you can judge what is the frequency by comprising with the frequency gratings on the test card. Then look for beats between the various frequencies being generated, vision carrier, sound and Chroma.

If the modulator is double sideband it may be a beat from the other sideband.

Sound/Chroma beats can be a problem but with 6Mhz sound it may be a high 1.6Mhz, if you are using 5.5Mhz, that would be just over 1 Mhz.

Just some thoughts.


Posted : 22/08/2019 10:25 pm
Posts: 252
Reputable Member Registered

Hi mfd70
It has come along well. The larger case will keep it nice and cool.

If the test patterns are clean one possibility for the patterning could be pick up from the audio/video leads to the sockets. The FPGA operates at high speed and so has the ability to generate unwanted digital noise.
I used PCB mounted sockets to help prevent pick up.

Is the pattering still there when not connected to a audio/ video source? If so disconnect the audio/video leads from the Hedghog PCB to check for pick up.




Posted : 22/08/2019 10:55 pm
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