Dan's Data letters #99
Publication date: 9 April 2004.Last modified 03-Dec-2011.
Projectitude
I re-read your "Cheap high-res TV: Forget it." column this afternoon, and I was left wondering if the current raft of 800x600 projectors represent better value than mid-range non-HDTV big screen TVs.
Can a low end LCD projector handle the task?
Rick
Answer:
Today's cheap(ish) LCD projectors are great, provided you're happy to watch
TV in the dark.
If you use a mere 800 by 600 projector to throw a really big image then you're going to have to get over seeing the pixels (quite clearly with LCD projectors; less so with DLP - for video, just de-focussing the projector a little helps with this), but the actual image resolution will be adequate for everything up to DVD. High-res projectors (which cost a lot more) are the only even vaguely affordable genuine HD large screen display option.
The problem is simply that the darkest black any display system can show is, inescapably, the same as the colour of the unilluminated screen, since all of them add light to the unilluminated screen's colour; none of them can subtract it. CRT and LCD monitors (and rear projection screens, to some extent) can be made pretty darn black by the use of coatings that absorb ambient light and don't reflect it back towards the audience, while letting light from behind through, but front projection systems just can't be as good. They have to reflect light from the projector, which means they inescapably also reflect ambient light. There are fancy screen coatings that minimise this problem, but you can't get rid of it completely, so you have to dim or completely extinguish the lights to use a projector, unless you don't mind a washed-out picture.
For this reason, front projectors aren't suitable as an everyday TV. A combination of a front projector for movies and a cheap TV for everything else, though, can work really well.
I talk more about projectors here.
Wanted: Knob
I'd like to build a small volume control to be used between a Pioneer DJ Mixer and my powered Behringer Truth monitors.
The reason that I need this is so that I can leave the output volume of the mixer at an optimum level for recording, while still being able to have the monitors at an acceptable level (they are too loud otherwise). The mixer (a DJM-300) doesn't have a record volume control independent of the main output level, and the Truths only have a +/-6db gain trim control on them.
I can't find anything available (that's affordable) to buy to suit my needs, so I think that I should give building it a go.
I have read through the guide to potentiometers here, and it looks like it won't be too difficult to make.
I am planning on doing one box with two single gang pots. I'd like to be able to control each speaker independently so that I can balance them if required. I'll have two RCA sockets going in, and two going out.
I can't find any guides to making a simple volume control like this. Can you please suggest what sort of pots I should buy? I think that I should get "log" pots, but I don't know what sort of "kOhm" rating I need. The sound will not be amplified, and the volume control will only be passive.
I've never done any soldering before, but I'm prepared to have a bit of a go at it. Do you have any tips for me? What sort of enclosure would you suggest, and what should I solder the ground wires to?
Liam
Answer:
As you suspect, this should be pretty easy to do. Incidentally, fancier
versions of what you want to make, with multiple switchable inputs, are
commonly referred to as "passive pre-amplifiers". This name is a contradiction
in terms - a pre-amp, by definition, is an active device - but it gets the
basic idea across. A passive pre-amp does the input selection and volume
control job of a real pre-amp, but all it can do is pass through the input
signal, or attenuate it.
A one-input inline volume control such as you want to make is about as simple as circuits get - it should be a great first soldering project! (Practice on some scrap wire first, though!) The optimal potentiometer value depends on the input sensitivity of the monitors and the output level of the mixer; "line level" means different things to different companies. A 47K log pot should be fine for anything, though. Any old super-cheap log pot from your local electronics store will work, but if you want to avoid raspy noises when you turn the knobs, get something "audio grade".
If you want to get fancy, you could use an aluminium project box, and connect the shield from the cable on both sides to the box, thus maintaining shielding of the signal wire. If you paint the box, you won't get hum if something with a different earth touches the box.
In the real world, though, you're likely to be OK if you just pass the shield through a plastic project box with a separate wire. There shouldn't be any significant noise. You could even install a switch in the middle of the shield wire, allowing you to break the ground path at the box to stop earth-loop hum problems, if any arise!
Barrowload of batteries
I came across this while I was doing a bit of scratching in the dirt to find some grubs about getting a new 12" Powerbook. A poster compares using alkaline batteries versus the ones you buy for laptops nowadays, and concludes that you'd need 700 D alkalines to get the same run time as a lithium ion Powerbook battery. If this is so, why the disparity?
Alex
Answer:
Read that thread now and you'll see Alex conveying my original reply, and
the poster recanting. Apple will no doubt be
dead soon enough,
though, so I'd better post the reply here as well.
Rechargeables in general are much happier about high drain operation than alkalines, but not by this large a margin.
Say you're aiming for 2.5 hours of run time, as the poster suggests. Assume that's what you'll get from the standard 12 inch Powerbook battery. That battery has 47 watt-hour capacity, so let's assume all of the numbers are perfectly accurate and so you're drawing 18.8 watts.
18.8 watts at 10.5 volts (nominal voltage for seven 1.5V alkalines; let's ignore voltage sag under load for now) is about 1800 milliamps.
Now, let's check a handy-dandy datasheet for a current-model alkaline D cell (PDF, here).
1800mA is off the end of the scale, so let's consider 14 cells - two strings of 7 in parallel. That gives 900mA for each string, at which we should get about one hour of service, down to 1.2 volts per cell (let's assume the Powerbook would be happy with that; it might well not, but I'm enjoying this ballpark-figures thing). A bit longer, in the unlikely event that the Powerbook keeps running when the alkalines are down to 1.0V or 0.8V per cell.
OK. Now let's tack on one more string, for 21 cells. Now the cells in each string see a roughly 600mA load. Run time's now up to around four hours, to the 1.2V-per-cell point; this greatly increases the chance that the laptop will still be happy after only 2.5 hours.
So there we go. 21 D cells will probably give a very comfortable 2.5 hours of service, and likely rather more. Instead of 700.
Hey, what's a factor of 33 between friends?
Aspect rationalisation
I have a question about the resolution of a widescreen LCD vs a more conventional shaped one. The 17 inch widescreen Acer AL1751W, for instance, has a native 1280 by 768 resolution, whereas other similarly priced units such as Samsung's 172X have 1280 by 1024, but are not wide screen.
I guess I would have expected the Acer to have more pixels across its horizontal; how is it that it is so much bigger in size without having a correspondingly greater number of pixels?
Terry
Answer:
The missing piece to this puzzle is that the Acer monitor isn't bigger
than the Samsung, or various other 17 inch LCDs. It's smaller. Both monitors
have a 17 inch diagonal, which means the 16:9 aspect ratio monitor is slightly
wider than the 5:4 ratio one - because its 17 inch diagonal is closer
to lying flat - but it's considerably less tall, giving a lot less area.
Widescreen LCDs are not very desirable products. Yes, they let you view widescreen movies without black letterboxing bars above and below the picture (well, as long as the movie's in 16:9 aspect ratio; even wider aspect ratios will still be letterboxed on a 16:9 screen). But when widescreens cost the same as or more than a 4:3 or 5:4 aspect ratio screen with about the same width, the same horizontal resolution, and much more vertical resolution for normal computer images, there's really no reason to buy one.
A similar argument applies to widescreen and conventional TV sets. My advice is always to buy the squarer screen, learn to love letterboxing, and enjoy the extra screen real estate when viewing non-widescreen content.
Phone peril
I recently changed my phone plan and got a new cell phone. It's a Nokia 3200. Yeah, I know, it's a kid's phone, but it has a camera, FM radio and a pretty decent flashlight. Anyway, I know that cell phones can produce a decent amount of RF radiation. I keep my phone on my desk next to my Pile of Stuff (TM). In that pile is a thumbdrive as well as a couple of floppy disks. Is there any chance that, when my phone rings, it can put out enough of anything to affect either media?
I'm probably mixing up RF with magnetism at this point. And I know that the phone itself has memory of some kind, and obviously it doesn't affect that.
Am I totally off base here?
Rich
Answer:
There's no danger whatsoever to your data from the phone, unless its battery
shorts and sets the desk on fire. The RF from the phone is strong
enough to affect various poorly shielded circuits, but not destructively.
Many people are familiar with hearing a rhythmic buzz through their car
stereo, computer speakers or other sound system when a phone's about to
ring or just having a quick chat with the local base station, and some other
gadgets will also respond - for instance, the microprocessor-controlled
Eternalight LED flashlights will turn on if
you leave them next to an active phone. There's no chance of data loss,
though.
Old-fashioned electromagnetic-ringer landline telephones can create a considerable magnetic field when they ring; lean a floppy against one of those and you might be able to damage some data on it. No modern phone's a risk, though.
A ghost car! Hold me!
"Photo Blocker" is a spray that reflects light from a speed camera's flash back into the lens, overexposing the photo and thus obscuring your number plate. It sells for about $AU69 but is it nothing that a $AU20 can of clear spray-on gloss can't do? Does your wealth of camera and physics knowledge see flaws?
The website is one of those suspect WYSIWYG affairs where input from a marketing exec would have been beneficial.
Not that I'm a speed demon or anything, it's just that I'd like to be able to actually drive without having to glare at my speedometer.
Mark
Answer:
What this spray is supposed to do is basically the same thing that reflective
license plate covers do, or claim to do at any rate. To do this trick, you
need something reasonably retroreflective - having the quality of bouncing
light back in the direction it came - over your plate. Number plates have
retroreflective material on them already, but not on the dark parts. So
do most road signs - that's why they glow so brightly in your headlights,
and even brighter (relatively speaking) if you hold a flashlight beside
your head as you look at them.
In theory, a translucent retroreflective layer on top of the plate will, indeed, make it turn into a white rectangle from a flash camera's point of view. Regular clear lacquer won't do that.
Plate covers are likely to be illegal pretty much everywhere (this SMH piece refers to both covers and spray being illegal; covers and sprays are also illegal or restricted pretty much everywhere in the USA). The big deal about the spray, of course, is that it's not nearly as noticeable as a chunky plate cover, so you're meant to be able to get away with it. And just owning the spray may be legal; using it for its intended purpose may be the illegal part.
According to this page, Fast Fours and Rotaries tested some sprays and found they did nothing. In any case, it definitely won't work on cameras that don't use a flash; the only way to prevent non-flash photography from seeing your license plate is by permanently obscuring it somehow (also illegal, of course...), not covering it with retroreflective flash-camera-dazzling material.
And, of course, if you're pinged by cops in a car (the only way I've ever been caught speeding) then you're screwed. They're not known for saying "oh well" and giving up if they have trouble reading your plate. Also, if the spray half works, you're also screwed, running the risk of being busted for speed and for trying to obscure your plate.
From reading the gun-review-esque wealth of detail in various US articles about radar detectors and jammers and such, I've come to the conclusion that given the current state of the art in speeding car detection, just making a commitment to drive at, or trivially above, the speed limit is less trouble than trying to stop the fuzz from catching you driving faster.
Or, of course, you can just follow the great Australian tradition of Driving Really Fast, Losing Your License, Then Keeping On Driving Anyway, Only Now Really Carefully.
The only way I know of to stop yourself from running the risk of being busted for driving safely, but quickly, is to exercise your democratic right to vote for people who advocate sensible road laws.
Unfortunately, in my experience, political candidates who have sensible ideas about road laws tend to have very strange ideas about lots of other things.
