Re: Stopped down focus

achilleaslazarides@xxxxxxxxxxx <achilleaslazarides@xxxxxxxxxxx> wrote:
Wolfgang Weisselberg wrote:
No problem even for consumer electronics.

But the return trip for an object 1m away would be less than 7ns, ie 7
cycles. I suppose it is possible to emit a signal and measure the
number of cycles until something is received, but I imagine that 15
years ago this was not feasible.

Sorry, having seen that with consumer VHS video cameras back
then, I disagree.

But did they measure distance by measuring the time between emission
and reception? I don't know, I am asking.

Yes. To the best of my knowledge and memory. They would be
mis-focussing on windows, they were not hunting, the light
was not visible.

Also, I suppose you'd need to do some
processing to work out if the thing you received really is the signal
you wanted,

Send many signals and average over time (the focus motor has
a finite speed, thus doing averaging itself), and use either a
specific frequency or a specific pattern (patterns are no problems
either, how do you think IR remote controls work? And they have
been around a _long_ time.

What do you mean? If your distance measuring is done by detecting the
time elapsed between emission and reception, then you have to emit,
wait until you receive the signal, and then divide that time by the
speed of light to get the distance, and only then activate the focus
motor to focus properly.

The maximum stated range was 10 meters. So the maximum time to
wait would be 20m / c ~= 6.7 * 10^-8 seconds. If you allowed just
1/1000s before activating the focus, you'd be able to do more thas
10.000 measurements. (Not that you'd want to, it'd be draining
the NiCd battery (yep, that old), but you can cram in a few ...)

Furthermore, changing the focus has absolutely
no effect on the signal received by the IR sensor; so I fail to see how
the speed of the motor enters into the discussion.

The focus motor was not _very_ fast. So what would happen if
you had the following pattern of distance results (probably
a few dozen per seconds):

4m focus to 4m
4m still focussing
4m still focussing
....
.... arrived at focus
....
4m
4m still at focus
0.3m start focussing motor, now at 4.0m
0.3m still focussing (now at 3.95m)
4m reverse focus motor (now at 3.85m) (inertial mass!)
4m still reversing (now at 3.9m)
4m arrived at focus
....


The fact that the focus motor is slow DOES mean you average
out spurious signals. Think about this sequence. The whole
sequence probably takes half a second or less:

4.0m at 4.0
4.1m at 4.0 starting for 4.1
3.9m at 4.05 starting for 3.9
4.0m at 4.0 stop motor
3.9m at 4.0 starting for 3.9
4.1m at 3.95 starting for 4.1
4.1m at 4.0
4.0m at 4.1 starting for 4.0

You see, even with a fairly fast motor and slow updates, the
speed of the motor evens out a lot.

As for IR remotes, I don't know how quickly the TV (say) reacts, but I
suppose there is some time lapse between reception and reaction, since
you have to react differently according to the signal so some
processing is needed.

First patent for remotes: 1893, Nicola Tesla, US Patent 613809.
First remote controlled model airplane: 1932.
First remote controlled SAM missile "Wasserfall", WWII.
First wireless TV remote control "Flashmatic" 1955 (visible light)
First ultrasound TV remote control "Zenith Space Command",
1956. (4 buttons, 4 frequencies, no batteries needed!, and
6 extra tubes in the TV)
Many-Button remotes, prototypes at 1977-78.
IR-remote controls from the early 1980s.
Learing remote controls from mid 1980s.

Remember that the whole unmanned space exploration (and a
good part of the manned one as well) are using remote

Of course some lapse occurs because you have to decode the
whole signal, and probably wait for more pulses to come, but
the pauses are not that long.

and I don't know if this is possible in such short times
(since you must be ready to restart if it's the wrong signal).

No problem. Throw away signals detected a spurious, continue
sending pulses as before.

What I was trying to say is this: You emit a signal, start a stopwatch
and wait; when its reflection is detected, you stop the stopwatch, and
may calculate the distance.

Exactly. All you need is a 'counter' which can cope with a GHz
pulse and some start, stop and readout electronics. If you want
to do that digitally.

You can probably get by by discharging a capacitor over a resistor
while the light is going there and back again, and measuring the
rest voltage. And since the voltage is dropping fast at first,
you get increased accuracy with close targets. (Accumulate
by discharging over multiple bounce cycles.)

However, to do that, you have to make sure
that the received signal is the reflection of whatever you emitted, so
you need to do some processing to check.

Nope. It would be nice if you had. You can average over 10 or
100 or so measurements. You can use a well-defined frequency.
And you can say 'I don't care, it's consumer electronics anyway'.

If you process and find that
it is, then all is fine; but if it's not, then you have to wait some
more.

We are talking about times like .000000067 seconds per measurement
for the light to travel. Ok, take 1.000 times that time to
handle the stuff. We still talk about .00067 seconds. Negible.
About 1/2000s lagtime, even with generous handling time.

The problem is that while you're processing the spurious signal,
the real one might have arrived. What do you do?

You switch off or ignore the receiver once you get the first
signal. After all, you _will_ often get scatter from the
background, after the target has reflected.

I don't know if you
can process a signal while managing a queue of incoming candidates so
quickly (of the order of nanoseconds).

You are thinking digital. Try thinking analog.

I think I am misunderstanding you. Could you please specify whether you
are talking about a system that measures the elapsed time between
emission and reception (as opposed to the angle of reception, a
completely different method)?

Elapsed time.

If so, could you explain how the speed of
the focus motor has anything to do with the feasibility of doing this?

As I said, if you get a few spurious data (we are talking about
that VIDEOcamera), the focus speed can average them out.

For a photo camera you'd probably average measurements electrically.

-Wolfgang
.

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