ds2 camera overreading buy up to 17 mph
Discussion
carrying on from a thread over on peppio
If you are travelling at 47 mph then you must have covered 21.01088 meters in a second.
Now a frame count from photo evidence shows 19 frames elapsed,I believe a PAL system consists of 25 Frames per second. Though I have been informed that there is a fill frame taking the count to 50 frames per second.
The next number needed is the distance. I thought the three strips in the road are triggered by the car wheels right.Not in this photo the first clearly shows the front wheel, what looks to be 0.5 meters away from the first of the three rubber strips, and the second and last photo shows the rear wheels just over the last strip. the car has covered about a meter more than it's length 4.25m so lets say 5 meters.
someone please help me work this out as i just can not get 47 mph back as the speed.I can get 29 ish.
Incidently there are no markings on the road as this is not a permanent trap.
Thanks in advance for any help Nick.
>>> Edited by Nickccc on Saturday 30th October 13:09
If you are travelling at 47 mph then you must have covered 21.01088 meters in a second.
Now a frame count from photo evidence shows 19 frames elapsed,I believe a PAL system consists of 25 Frames per second. Though I have been informed that there is a fill frame taking the count to 50 frames per second.
The next number needed is the distance. I thought the three strips in the road are triggered by the car wheels right.Not in this photo the first clearly shows the front wheel, what looks to be 0.5 meters away from the first of the three rubber strips, and the second and last photo shows the rear wheels just over the last strip. the car has covered about a meter more than it's length 4.25m so lets say 5 meters.
someone please help me work this out as i just can not get 47 mph back as the speed.I can get 29 ish.
Incidently there are no markings on the road as this is not a permanent trap.
Thanks in advance for any help Nick.
>>> Edited by Nickccc on Saturday 30th October 13:09
Couldn't begin to guess the frame rate. Normal broadcast PAL is 25 fps and NTSC is 24 or 30, but you get double rate versions of both. For something like this where it is being used for measurement rather than video display I wouldn't be surprised if they used a custom frame rate though.
By my arithmetic 19 frames at 24 fps takes 0.79 seconds implying 3.96 ms-1 (8.9 mph), 60 fps implies 22 mph, 128 fps implies 47 mph. But your estimate of the distance, and the guessed rame rate, could be substantially out.
By my arithmetic 19 frames at 24 fps takes 0.79 seconds implying 3.96 ms-1 (8.9 mph), 60 fps implies 22 mph, 128 fps implies 47 mph. But your estimate of the distance, and the guessed rame rate, could be substantially out.
19 frames have elapsed between the first and last pic. ( only two pics) Though the time clocks are the same on both pics at 17/07/10
now in picture 2 the car which has a wheelbase of 2282mm and a total length of 4015mm is i would estimate one car length and maybe a meter more than the first pic probably 5 m. Now with the calculations I have got I get
5 meters in 19/25 of a second (5/19*25)
=6.57894736842 meters per second
*2.236936292054 (conversion)
=14.71668613193 miles per hour
based upon 25 frames per second
or29.4333722638684 based upon 50 frames per second
at 100 fps the speed would be 58.8
so 128 fps would more than 58.8, as they are claiming 47 mph it just does not add up. Even with over 7merers as the length covered.
Nick.
now in picture 2 the car which has a wheelbase of 2282mm and a total length of 4015mm is i would estimate one car length and maybe a meter more than the first pic probably 5 m. Now with the calculations I have got I get
5 meters in 19/25 of a second (5/19*25)
=6.57894736842 meters per second
*2.236936292054 (conversion)
=14.71668613193 miles per hour
based upon 25 frames per second
or29.4333722638684 based upon 50 frames per second
at 100 fps the speed would be 58.8
so 128 fps would more than 58.8, as they are claiming 47 mph it just does not add up. Even with over 7merers as the length covered.
Nick.
Oops, seems I multiplied instead of dividing! Your sums seem correct. 47 mph implies a frame rate of about 79 fps, assuming the estimated length is correct. 80 fps is a nice round number and doesn't seem unreasonable. But unless you know the actual frame rate I don't think it will be possible to draw any definite conclusion.
In the UK the video frame rate is 25 frames per second. Each frame is composed of two interlaced "fields" - this reduces flicker while not increasing the bandwidth requirement. Sometimes the "frame count" is actually a field count so it might be 25 frames per second or 50 fields per second.
It's vitually inconceivable that they use a non-standard frame rate in this equipment - if they did there would be massive problems showing evidence in court on standard equipment. Any system of conversion would reduce the evidential value.
It's vitually inconceivable that they use a non-standard frame rate in this equipment - if they did there would be massive problems showing evidence in court on standard equipment. Any system of conversion would reduce the evidential value.
Safespeed is correct. I think they would probably use some shutter on the camera, to enhance the quality of each still frame (although this makes the camera less effective in low light).
I know of no widely available camera (or CCD) that scans at anything other than 25fps for PAL or 30fps for NTSC (its 24fps in the cinema by the way, thats why films on telly are 4% faster).
>> Edited by parrot of doom on Saturday 30th October 20:24
I know of no widely available camera (or CCD) that scans at anything other than 25fps for PAL or 30fps for NTSC (its 24fps in the cinema by the way, thats why films on telly are 4% faster).
>> Edited by parrot of doom on Saturday 30th October 20:24
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