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Forum 141

Converting 405 lines to 625 lines using a Hedghog II

Standards Converters & System Modulators

Last Post by FRANK.C 4 years ago

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FRANK.C

(@frank-c)

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I like to make it clear from the outset that this thread relates only to the Hedghog II. The original Hedghog has not got the hardware to do 405 to 625.

In August 2019 Jeffrey Borinsky successfully built a 625 to 405 standards converter. He detailed how he did it in this thread on Golborne Vintage Radio Forum. The big thing was that he found a way to use a video decoder that was designed for 625/525 lines only, to successfully acquire a 405 signal. This was a big development and has allowed me to write new firmware for Hedghog II which will allow it to convert from 405 to 625 as well as doing 625 to 405. No change to hardware is required just a firmware upgrade

Jeffery was able to make the SAA7118 video decoder in his converter lock to 405 lines by under clocking it. I tried under clocking the TVP5150 in a Hedghog II and it worked. But the data clock from the TVP5150 had too much jitter in it to give good results. However, I found that over clocking it worked much better and produced a data clock clean enough for Hedghog II to use.

When in 405 to 625 mode its data clock is close to 35 MHz which is quite a bit above its normal 27 MHz. To date it has been tested in a number of Hedghog II and each one worked well with the TVP5150 showing no signs of distress or any sort of ill effect. But as the TVP5150 is working so far outside its normal operating area no guarantee can be given firstly that every TVP5150 will work and secondly that if it does work that it won't be damaged. In my opinion I think that the TVP5150 will be OK but as I have said no guarantees.

I have no suitable VCR to record/playback 405 lines, so I don't know how well it will preform with a VCR as source. If someone does try it with a VCR I would be interested to know how it worked. So maybe you could post here with the results.

I have uploaded new firmware to the Hedghog web page which allows a Hedghog II to work as a 405 to 625 converter as well as converting from 625 to 405.

Stephen forum member Freya is supplying fully constructed Hedghogs

Frank

www.electronics.frankcuffe.ovh

Posted : 04/04/2020 4:42 pm

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