8-Step Greyscale.

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8-Step Greyscale.

Postby Steve Anderson » Sat May 26, 2007 5:26 pm

Here's the second .wav file I have created. It's also at 48kHz, 8-bit mono, 60 seconds of a NBTV geyscale pattern from black to peak white. This should result in a 400Hz line frequency with eight luminance steps. There is line sync, but no frame sync, being line-repetitive it shouldn't be needed. I will get around to the missing pulse in due course.

There are 120 samples per line, eight of which are used for the sync pulse (167uS) and each greyscale value is of 14 samples (292uS). Being 8-bit video a few of the greyscale values aren't exact, but within a small fraction of a percent.

The other thing of note is that on this PC it came out 'the right way up', sync was negative and white was most positive.

Again, I would very interested in any feedback.

Steve A.

I won't post any more until I know I'm heading in the right direction.
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Updated file.

Postby Steve Anderson » Thu May 31, 2007 2:58 pm

I have found out my error in creating these wave files in respect of them being 2 bytes too long. The updated the one above (8Grey60.wav) is now correct in all respects as far as I can see.

But one thing I have noticed is some interesting artifacts generated by the soundcard. If you look at the .jpg below you'll notice 'steps' and overshoots in the waveform, these are stable and don't vary. Each one is about 10uS in duration which implies that somewhere twice the sample rate is being used, 96kHz.

I have examined the file at hex level using Hexedit and the values are spot-on for 8-bit audio. It's as if some form of digital filtering is being used. It's probably of no consequence to NBTV, I just wonder why.

Keep in mind that this is not really a 'proper' soundcard, but the on-board sound from the motherboard. When you consider the motherboard cost less than US$50 (without processor) I guess it's no suprise.

Anyone seen something similar?

Steve A.
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Postby Klaas Robers » Thu May 31, 2007 8:07 pm

Steve, place a low pass filter after the sound output and everything is removed. A suitable capacitor alone will do I think.

Another strange thing are the "half way" samples, or have you programmed it in that way?
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Postby Steve Anderson » Thu May 31, 2007 8:51 pm

Klaas Robers wrote:Another strange thing are the "half way" samples, or have you programmed it in that way?


No, the "half way" samples are not part of the file, the values are not contained within it at all. Being 10uS long it implies 96kHz being used, perhaps as a digital filter.

If you play back the file and normalise it to 1Vp/p the values in the file should reveal the following:-

00h = 0.000V (Sync tip)
4Ch = 0.298V (Black, nominally 0.300V)
66H = 0.400V (1st grey step)
7Fh = 0.498V (2nd grey step, nominally 0.500V)
99h = 0.600V (3rd grey step)
B3h = 0.702V (4th grey step, nominally 0.700V)
CCh = 0.800V (5th grey step)
E6h = 0.902V (6th grey step, nominally 0.900V)
FFh = 1.000V (Peak white)

If you look at the 'data chunk' (yes, that is the correct term), from byte 45d onwards, it only contains those values. The preceeding 44d bytes are the header information.

I will try the same exercise at a much lower sample rate and see if the same thing happens. If it were, at a sample rate of 8kHz (far too low for NBTV) there might be these "half way" steps with a duration of around 60uS.

I'll keep you updated.

Yes, a simple low-pass filter would remove them, but even without one I doubt they would be visible, I am just curious as to the reason they are there.

Steve A.

Postscript.

In fact it seems that some processing is being done in the digital domain to simulate an analogue filter. Of what sort I'm not sure, compare the two photos and you can see the similarity, although there are differences.

The analogue version (Clean.jpg is a Sallen and Key filter at 15kHz at -12db/octave), whereas the other seems to be a digital version of a Chebyshev filter, possibly around the Nyquist cut-off of 24kHz, hence the ringing.

Forget the fact that one is ascending, the other decending, it shouldn't make any difference.
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Postby Klaas Robers » Thu May 31, 2007 10:21 pm

Yes, it might be a digital filter working at 2x oversampling. Then a FIR-filter with 6 coefficients (-¼, ¼, 1, 1, ¼, -¼)/2 or something like that will show this behavior.

On the Nipkow disc you won't see it of course, because there is integration of light in the moving holes.
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Postby Steve Anderson » Fri Jun 01, 2007 1:11 pm

Klaas Robers wrote:Yes, it might be a digital filter working at 2x oversampling.


I repeated this with a sample rate of 8kHz, which only required four bytes to be changed in the header. Using the same data the waveform frequency was 66.7Hz as expected.

The 'half-way steps' where still there and of the same duration, but a lot more of them. So I guess it is the result of oversampling and/or digital filtering.

Anyway, I was just curious, I don't think it's a problem for NBTV.

Steve A.

I might try it at 96kHz, just for the fun of it!
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96kHz sampling.

Postby Steve Anderson » Fri Jun 01, 2007 1:56 pm

Well, I did try it at a sample rate of 96kHz which with the same data produced a 800Hz line rate. I was quite suprised that my cheap motherboard supported 96kHz.

The 'half-way steps' were still there saying that something now was running at 192kHz.

...I'm not going any further, for the moment anyway.

But isn't that bandwidth tempting? *Drumming fingers on table* What do we do with it?

We could increase the frame rate to 50Hz eradicating the flicker. Keeping the 12.5Hz frame rate we could increase the number of lines to 64 with the equivalent of 96 pixels per line. Or a bit of both.

We could add in a colour sub-carrier from (say) 15kHz to 40kHz. *Fingers still drumming*...Hmmm...

We could use a FM based system like SSTV and get rid of all the low frequency problems that NBTV has....

Steve A.
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8-bit greyscale (2nd version)

Postby Steve Anderson » Fri Jun 01, 2007 5:52 pm

Here's my attempt at another greyscale, yes I know it's boring, but I feel I'm making progress.

Firstly it has the 'missing sync' for frame sync. Secondly, to be able to visually see you have frame sync, the right-hand lines (1-16) should get brighter from bottom to top of the frame, whilst the left-hand half should be the reverse.

Steve A.
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Postby AncientBrit » Tue Jun 19, 2007 5:50 pm

Steve,

Your file 8RevGrey.WAV plays back well on my software.

Well done.

GL
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Postby Viewmaster » Thu Jun 21, 2007 12:24 am

Yes OK here too...it sounds good and looks good on Gary's "NBTV the big picture."
I really must get on with my machine and use your greyscales to test it out.
.........spending more time reading/writing to forums than I do
soldiering on with soldering. :)
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Excessive(?) time.

Postby Steve Anderson » Thu Jun 21, 2007 2:31 pm

Viewmaster wrote:.........spending more time reading/writing to forums than I do soldiering on with soldering. :)
Albert.


Thanks gents, good to get the feedback. It gives one a sense that the effort wasn't wasted. I'm sure Gary understands that!

The amout of time I spend each day on this forum has really eaten into the time I should really have the soldering iron on. It is amazing that today I have already spent two and a half hours here, and I doubt that will be the end of it!

I must buy a timer and set it to one hour, at the end of which when it goes 'ding', that's it for the day!

Steve A.
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Re: Excessive(?) time.

Postby Viewmaster » Thu Jun 21, 2007 6:34 pm

Steve Anderson wrote:I must buy a timer and set it to one hour, at the end of which when it goes 'ding', that's it for the day!


Get an egg timer...that will go 'ding' after 3 minutes and save you even more time, Steve! :lol:
Forums are addictive though, especially those about that mechanical television lark!
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Re: Excessive(?) time.

Postby Steve Anderson » Thu Jun 21, 2007 6:52 pm

Viewmaster wrote:Get an egg timer...that will go 'ding' after 3 minutes and save you even more time, Steve! :lol:


Yes, but with just three minutes I'll never get anything posted....some might say that's a good thing!

Steve A.

Yes, I know, I'm leaving myself wide open to all sorts of responses!
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Postby DrZarkov » Mon Sep 24, 2007 3:18 am

Aftger many month I've got a chance to try your grayscale on my "Zarkovizor Junost" (a modified MUTR kit), connected to my Amiga computer. it works, but I had some problems with getting synch. Maybe it is a problem of the outdated 8 bit "Paula" chip of the Amiga, but it thought your testcard is ideal to compare how good the Amiga and the monitor are working together.
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Postby Steve Anderson » Mon Sep 24, 2007 12:24 pm

Thanks for the feedback, it's always useful.

The waveform should be 'spot-on', within the confines of 8-bit data, I confirmed this with an oscilloscope.

I must get around to doing some more, but busy on other projects not related to NBTV. (A GPS-locked Nixie clock for example).

Steve A.
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