Moderators: Dave Moll, Andrew Davie, Steve Anderson
Klaas Robers wrote:Steve, these laser diodes on their own don't give a parallel focussed beam. When you look into the specification you find the "divergence", which has a FWHM (Full Width Half Maximum) of 7 deg in one direction and 22 deg in the other direction. You can see that too in the graph a few pages further.
Laser diodes always give an oval shaped beam, where the points of origin are not identical, but are still very close to each other. In a laser pointer there is a focussing lens that tries to make the beam parallel in both perpendicular directions. Without that lens you have the oval spot.
Klaas Robers wrote:You should not modulate the laser, but switch it fast on and off. By pulse dutycycle modulation you may get an analogue modulation, without any form of gamma.
Steve Anderson wrote:Good see you back Steve...
Steve Anderson wrote:Klaas is correct, I did try to modulate a laser diode with an analogue current (not voltage) source, the problem comes from the 'lasing current' [1] below which it simply stops emitting light...around 30% of the nominal current. So you lose all the detail in the darker scenes. With colour/RGB each diode will 'give up' at a different point leading to severe colour shading in the lowlights.
Steve Anderson wrote:PWM or similar is the way to go. Very similar to what you did on a previous LED mirror-screw monitor, in fact that circuit could be adapted with a few simple mods. You may recall my laser-link in the newsletter which used the same concept (no idea of the issue number).
Steve Anderson wrote:Incorporating Gamma correction would require a modified ramp generator which drives the three comparators. You could use a 'breakpoint corrector similar to the one of mine also published in the newsletter many years ago (again, no idea of issue number). Actually I've just thought of an easier/better way to do this - let me look into it...
Steve Anderson wrote:There's a problem with going the micro route and processing three signals at 120 lines - the class of micros I'm used to simply cannot process that amount of data at the speeds required, even at a 64MHz clock speed. I can't recall what frame rate you use but even at 12.5Hz it's too much data. So it's the old analogue method...
Steve Anderson wrote:Steve, if you can pass on the lines (assumed to be 120), aspect ratio and frame rate that might help...what would be the source of signals? Your Aurora converter? Any data/timing info would help greatly...I guess you're aiming for next years NBTVA convention...hopefully by then we'll be clear of this dreaded virus thing so it can go ahead...
Panrock wrote:Finally (one can never say this enough): thanks for the help! Steve O
Panrock wrote:The next logical step would therefore seem to be to design and make a laser diode driver circuit to enable a diode to be safely fired up. Then try some examples of laser diode with the line lens....Cumbersome light screening measures might be needed. Steve O
Panrock wrote:Can you remember the switching frequency, which appears to be unchanged? The video bandwidth is 240 KHz. Steve O
Panrock wrote:You're going to a lot of trouble over this Steve A!
The BD139's could be replaced by BD135's or BD137's, the same but with lower voltage rating, but all OK for this application.
I also suggest getting hold of about 1m of screened audio cable (single), it doesn't need to be video or RF coax as the extra capacitance of the audio cable will help to reduce the chances of static or other short pulses blowing the laser diodes. The stuff that's often included with domestic audio gear.
As these diodes are quite expensive it might be worth investing in a pair of 'shorting pliers' so there's no chance of zapping the diodes as you work on them. When you close them there's a latch at the handle end that holds them firmly closed, the inner surface of the jaws are serrated ensuring good contact. Or use ordinary ones with a strong rubber band at the handle end. I guess you could get away with a 'croc clip' or even a paper clip.
I wonder if the "Left-Hand Shop" in London is still going? Everything they sold was for left-handed people...it was somewhere in the Wardour Street area... no, I'm not left-handed,
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