Strobe!

[LuTELLO]

Ok, here's my first post.

I tried making a mechanical display using the super crude method from the build your own televisor page.

http://www.youtube.com/watch?v=VnVpdCDgp4U

As you can see, I've got a long way to go.

My first important question:

Is there any place on the net I can get strobe patterns to print out or even a strobe generator program to make my own? I have one designed for turntables, but I need something for 750rpm/60hz. I'm assuming I can make my own strobe by connecting an LED to my soundcard playing a 50 or 60hz tone.

[Steve Anderson]

Hi LuTELLO, and welcome....

A very useful and well worth looking at link you provided there, thanks. I liked the idea of using a 33RPM record as the disc. It just needs to be nice and flat, Des O'Connor would be ideal.

Now, er, I hope I've done my maths right, correct me if I'm wrong. (Entirely possible, it's Christmas day and the sherry has already gotten to me).

With 50Hz power a neon or similar device will put out 100Hz pulses of light, 10ms. 750RPM = 80ms/rev, so we'll have eight pulses of light per revolution of the disc/drum.

Here's what's doing my head in....do we need four, eight or 16 sectors? The eight sector version is below, it can easily be re-drawn for either four or 16 sectors, just let me know.

If you need a higher resolution version, again just drop a note here.

Sadly 60Hz doesn't work out so easily.

Steve A.

[DrZarkov]

The most simple design for a mechanical TV is this one: http://users.pandora.be/ON1AIJ/nbtv.htm.

A stroboscope you'll find on my homepage:

. This one is made for 750 rpm at 50 Hz.

For 60 Hz it should have 25 segments, right? I'm sure I saw somewhere a stroboscope for 60 Hz NBTV, I think it was somewhere there: http://www.televisionexperimenters.com/home.html.

[Steve Anderson]

I obviously didn't get my maths right yesterday! With the way my head feels at the moment it's no suprise!

Steve A.

[Viewmaster]

You can make any strobe for any rotational speed and for either 50 or 60 hertz mains freq. using "Strobo" downloaded from this site. (Near bottom of page)
http://www.shellac.org/wams/wstrobe1.html

Notice the space 'cheating' for 60 hertz at 750RPM.

Don't get too dizzy and have a great rotational new year!

Albert.

[Klaas Robers]

And the real stroboscopic disc for 50 Hz should be in between the two shown discs. It should have 8 black sectors and 8 white sectors. There is no need to make it nice and sharp. The unsharpness due to the glowing up and down of the incandescent lamp will blur all sharpness in the pattern. A felt pen is sharp enough.

For 60 Hz this is not possible. This might be the reason that in 60 Hz countries a disc speed of 15 rev/sec or 900 rpm was commonly used. But if your disc can reach 15 rev/sec it can reach 12.5 rev/sec as well. Just a little bit slower. The important point is that you know that the disc is running almost the correct speed. The precise correct speed should be adjusted to the video signal itself. To get that playing the CD's of the NBTVA is the easiest way.

[Klaas Robers]

And the real stroboscopic disc for 50 Hz should be in between the two shown discs. It should have 8 black sectors and 8 white sectors. There is no need to make it nice and sharp. The unsharpness due to the glowing up and down of the incandescent lamp will blur all sharpness in the pattern. A felt pen is sharp enough.

For 60 Hz this is not possible. This might be the reason that in 60 Hz countries a disc speed of 15 rev/sec or 900 rpm was commonly used. But if your disc can reach 15 rev/sec it can reach 12.5 rev/sec as well. Just a little bit slower. The important point is that you know that the disc is running almost the correct speed. The precise correct speed should be adjusted to the video signal itself. To get that playing the CD's of the NBTVA is the easiest way.

[Steve Anderson]

The other thing to consider is that short-term the power supply frequency is not that stable, long-term it is, as corrections are made to ensure clocks that use the supply to keep time do so.

The cheapest crystal oscillator will maintain maintain frequency far better over a short period than the 50(60) Hz mains could ever do. Divide this down to 100Hz, drive a LED and you're in business, no matter what your power supply frequency might be.

The poorest tolerance crystal I was able to find was 100ppm (Parts Per Million), but I'm sure if you really tried you could. At 100ppm that's equal to 49.995Hz to 50.005Hz, or around 100us in a second, 8us in a complete 400Hz NBTV frame.

Here's an interesting link to show what's going on with the UK system regarding frequency short-term.

http://www.dynamicdemand.co.uk/grid.htm

Steve A.

[LuTELLO]

Thanx, all. I live in the United States of Idiot Cowboys, where we of course use 60hz. So I'm assuming I can use this stroboscope if I just connect an LED to my soundcard generating a 100hz tone?

[DrZarkov]

Yes! A good idea, indeed. Even useful in the age of normal light bulbs in a household replaced by halogen and energy saving lamps.

[Steve Anderson]

So I'm assuming I can use this stroboscope if I just connect an LED to my soundcard generating a 100hz tone?

Yes, assuming two things. Firstly the soundcard can drive some small speakers or headphones quite loud. If it can't you'll need to add some form of amplifier (small) to it.

Secondly that the soundcard is quite accurate enough in frequency, I can't imagine one that would not be suitable for this purpose, they're usually crystal controlled.

Steve A.

[gary]

Yes, assuming two things. Firstly the soundcard can drive some small speakers or headphones quite loud. If it can't you'll need to add some form of amplifier (small) to it.

Secondly that the soundcard is quite accurate enough in frequency, I can't imagine one that would not be suitable for this purpose, they're usually crystal controlled.

Steve A.

True - unless you are playing out at a different sampling rate to it's native sampling rate (usually 48kHz), otherwise the timebase can drift quite a bit over time because it does on-board resampling. This is problematic for recording (because of the cumulative effect) but probably less so for playback where the monitors electronics will generally handle the drift well, and, I think, the error would be relatively insignificant for a strobe since it is more an instantaneous thing - perhaps it will rotate very slowly in the direction of the drift.

I always try to record/playback at the card's native sample rate. This is usually 48kHz but can be different on lesser known cards/codecs.

I would recommend that, at least, the signal for a stroboscope be played out at the same sample rate as your video (If you use NBTV The Big Picture! it will be played out at 48kHz, if from CD it will be 44.1kHz).