> Hi there I have a trick question and I thought I should ask it here. I ha=
ve an opportunity to record some captive nocturnal animals in a breeding en=
closure and require a way to start recording at night when motion is detect=
ed. The place I am collaborating with on have a motion sensor setup for the=
ir camera which is what they use to observe them.=C2=A0
Daniel,
Over the years I've tried to use various triggers, sound and vision. The
first problem is you really need a pre-record function, but this is rarely=
long enough to get several seconds before the wanted sound to give any sort=
of intro.
There are basically two types of motion sensor. One detects changes in a
video image and the other triggers on sound. I found video triggering
impractical as it has to be sensitive to work reliably, and every wind
movement and passing moth triggers a run. Without a suitably long
pre-record, sound triggering usually loses the interesting bit at the start=
.
The question then is how long do you set as a minimum run time, and will it=
retrigger without a break? I soon gave up on that one.
I've been using a Bushnell trail camera (with sound) to record hedgehogs an=
d
it uses an infrared heat proximity sensor (burgler alarm type) which either=
triggers too freely or misses slow movements. Also it only runs for one
minute of video which is fine for counting hedgehogs but not for recording=
behaviour.
The problem with sound triggering is how sensitive you set it, and again th=
e
run time. You need seamless retriggering or long recording periods to get
useful clean tracks.
My solution is to leave the recording running continuously, especially with=
sound. You get a lot of unwanted stuff, but I go through the recording
visually with a sound editor, and scrap stuff ruthlessly. I've got
recordings that way of wildlife I didn't know was around on the serendipity=
principle.
I record MP3 at 320 Ks/s which is good enough for my original sound sources=
.
I did some critical tests, and found that MP3 artefacts became audible with=
my background sounds at 192 Kb/s and below.
Birdsong compresses well as it is musical, but MP3 replaces random noise
with bursts of tone. At low rates, white noise makes a tinkling sound, but=
at 320Kb/s that is masked enough not to be audible in practice or so I
reckon. Double check by subtracting an original recording on WAV from an MP=
3
version. You will probably find that MP3 time-shifts some transients, but i=
n
practice it gets away with that audibly. The "tinkling" artefacts are the
problem.
David Brinicombe
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