Hedghog 625-405 standards converter

[peter scott]

The main use for a WC-01 is to make 240 line Baird and other standards that need field rate conversion.

Don't forget that there are other ways to generate odball signals and they don't need to restrict the bandwidth in the way standards converters do.

http://www.nostalgiatech.co.uk/Baird%20240%20lines.htm

Peter

[`FRANK.C]

Many thanks to Trevor and Jeffrey for putting Hedghog through its paces for me.
Yes please, do more tests.
As Jeffrey has correctly said the aperture switch just selects between  3, 2, and 6 line interpolation, The 3 line has been adjusted to give a softer interpolation.

I will release the full details for anyone to to construct it, SM assembly isn't too bad. The board isn't exactly high density and most of the components are relatively big (0805) so easy enough to hand assemble, although better magnification than a bench magnifier is needed for some of the IC's.
The FPGA board in the link that Jeffrey has given is the correct one.

The only source that I can find for the modulator chips is Aliexpress.
Hedghog can be built without the modulator section and used with another modulator.
A thick white line on the PCB encloses most of the components that can be omitted if the modulators are not being built.

Frank

[ppppenguin]

Peter has done much good work on using PC graphics cards to geenrate different standards. Getting correct field sync waveforms is still a problem though Peter has made some progress in this area.

[Murphyv310]

Hi.
Well I've not progressed with the PC route as yet. What with Anne having an OP and unfortunately myself having some other problems, the thought of pulling the PC out and working on it was too much. I've been doing small things since January.
I've decided that I will have a go some time in the next couple of months but would need to change to Windows on the PC unless I can load an old version of Linux and go down that route, I though am not so good at computer programming etc and am likely mess up and go on a huff!

[`FRANK.C]

Hi
The filtering on the video output is just a 9 MHz low pass filter which is part of the video amplifier. Before the DAC it is just over sampled at four times pixel rate.
By its nature the 9MHz filter produces over and under shoot, I have the coefficients of the over sampler adjusted to give minimum over and under shoot. This may not be the correct way to have them set.

The test card is a bit modern looking for a vintage set and is harsher than one generated from an image but after all my antics with PIC test cards I just couldn't resist .
I should point out that the frequency gratings are 1, 1.5, 2, 2.5 and 3 MHz. They are correct on 4:3 output but on 5:4 because the same card is cropped and stretched the frequency's will be lower.  I don't have access to the VHDL files right now so cant say exactly how much lower they are.

Frank

[ppppenguin]

I didn't look to see if you had oversampled the output. Well done for making a 4x oversampler. The simple output filter shouldn't produce any significant overshoot. The point about oversampling is that the output filter can be very simple. An example...

Sampling frequency 9MHz, wanted BW 3.5MHz. The filter has to go from a flat passband at 3.5MHz to a good stopband at 4.5MHz and stay at least 40dB down above that. Doing this with good phase response is hard. Very hard. Something like a 5th order elliptical with phase compensation sections. You used to pay the likes of Matthey and BAL a lot of money to do this.

Now let's oversample 4 times to 36MHz. The passband is still flat to 3.5MHz. The stopband is now from 18MHz. you've got over 2 octaves to get down to 40dB so a 3 pole filter should be OK. Just 1 coil and 2 caps. I suspect you'd be aiming at a fairly soft Bessel response, possibly even Gaussian. Very easy to design and very uncritical.

It won't matter here, but there's also a need for sin(x)/x correction. This is because a DAC produces steps rather than unit impulses. For the non-oversampled example it's more than 1dB increase in response at 3.5MHz.Must be linear phase. Again Matthey etc would sell you this for lots of dosh. For 4x oversampling it's about 0.15dB. In this design you can forget about it. In a high precision system that I designed I used a 3 pole FIR with small -ve outer coeffts to do it. Not 100% accurate but I reckon I was within 0.02dB overall, probably better. When doing this kind of filter which has greater than unity gain at some frequencies you need to watch that you don't hit the endstops and roll round. From 255 to 0 or 0 to 255 in an 8 bit system. If you're a bit close, the fix is to wind down the low freuency gain a bit by tweaking the coeffts.

To test the oversampler and output filter you simply program in suitable test waveforms. Pulse and bar, sine wave multiburst are both good. P&B shows phase response as well as freequency. The ultimate test is a strange looking sin(x)/x waveform and to be honest I've not done this even on my own designs. You can also program impossible waveforms such a big step changes directly to the DAC or via the oversampler. This allows you to investigate the impusle repsonse of both analogue filter and oversampler.

[`FRANK.C]

Thanks Jeffrey for that detailed explanation, I may revisit the oversampler coefficients.

I just checked the VHDL files to see how much down the test card frequency gratings are in 5:4 mode. The 4:3 pixel clock is 8.743 MHz while the 5:4 pixel clock is 7.938 MHz so the frequency gratings in 5:4 will be approximately 90.74% of what they are in 4:3. Which will be approximately  0.91, 1.36, 1.81, 2.27 and 2.72 MHz.

Frank

[Murphyv310]

Hi.
Didn't get much time to do anything with the converter yesterday as it was shopping day then when I did get some time I was trying the composite video output on the Raspberry Pi.
Maybe today or tomorrow.

[`FRANK.C]

Hi Trevor
No rush at all with the testing.
I was thinking today about the effect you were seeing on the frequency gratings. As Jeffrey correctly pointed out the aperture switch only effects vertical interpolation and should have no effect on frequency response, however if the bars of the gratings are not perfectly vertical they will get interpolated and I think it is plausible that this is the effect you are seeing.

Frank

[Murphyv310]

Hi Frank.
Im sure you are right. I have fed the converter with test card C from my PC Via an HDMI to composite converter and the effect is not seen. So it could be some raggedness from the Imogen. The best source of stills is certainly from the RT Russell TCD which I passed on last year. I watched a couple of programs this afternoon on the Ekco T377 via your converter and have been delighted with the result, works brilliant. Next thing is to re- run a coax from the shed to the house. So I can sneak a set in when Anne is out!!! Either that or keep it indoors with a free view box for those times.