In a message dated 20/07/2005, gcouger@.......... writes:
Chris,
Out of sequence and possible not on subject but it has some thoughts on
timing that are important. I must have come in the middle and don't understand
your set up. I thought you were recording data to disk with a computer sound
card.
I am working on a sismomenter that records several sismometers at one time
in the 0-50 HZ range on the two channels of a sound card for imaging shallow
aquifers looking for larger gravel to find better water well in a shallow
aquifer along Red River
between Oklahoma and Texas so I can afford to put in irrigation.
I have probably imposed my project on yours without realizing it.
Hi Gordon,
We were discussing sound cards and I was rejecting this as impractical
for seismic purposes. The data files are massive and I haven't found a way of
using the 24 bit soundcard ADC for 20 sps sampling.
Chuck a bit of cash at it and get rid of the your problems? Larry sells
a fairly good 8 channel serial board, which you can use with a GPS receiver.
If you want to play, fine. If you want to work, get rid of problems that
you do not need to have.
Just what rate of water rise do you get along the Red river?? Even for
marine waves, 20 sps is adequate and most geological systems will go to
100sps, some even more.
>> A GPS will give you the best time stamp you could ask for some even
have a 100 kHz clock as well a 1 second pulse that are linked to GPS clocks.
They can be used to phase lock frequency sources to for super accurate
osillators.
I am NOT asking for the best system, just one that relatively
inexpensive, low power and ADEQUATE for my purposes.
I use VLF timing signals. You can get relatively inexpensive receiver
boards with a ferrite aerial: or gut a radio clock. You get about 10 mS + the
transit time. The only folks who need it more accurate monitor volcanoes of do
reflection seismometry. I have been all along this road. I would be great if
Larry would fit the software to decode WWVB directly, but he seems
concentrate on GPS. GPS is power hungry and needs a clear line of sight to
satellites; 60 KHz VLF radio is low power and can be used most anywere. I can use it
where I can't get MW radio, even underground. Just keep it away from
striplights, radio transmitters, TVs and computer monitors. I have used it underground
near radio and TV transmitters - find a nice hole and bury it to screen it!
It is no accident that submarine communications use the VLF range of
frequencies.
If you are not interested in 1 to 20 Hz signals sound cards are pretty good
tools. If you are really serious about low frequencies and accurate timing
Windows poses a lot of problems.
0 to 10 Hz is my range of interest. I want about +/-0.1 sec accuracy, 24
hrs / day. This is why I use an ADC which has it's own on board timing and
correction. The data goes to the hard disk with a time stamp regardless of the
rubbish timing on the operating system.
First Windows is an event driven operating system and there is no assurance
when a program will run. The sound card gets around that but you have to put
a chirp on the signal to synchronize it with time you can depend on the
computer clock or the RS232 port.
Can you say that again please? I don't quite understand.
Having worked with embedded systems for the last 20 years I would choose a
real time computer. There are real time versions of Linux but I think that's
over kill for dataloging geophones.
You have the real and the software clocks on computer boards. You used
to be able to get 4.194 MHz AT crystal clocks on computers, which could be
made moderately reliable. All the boards that I have seen recently use 32 KHz
crystals which appear to be rejects from clock manufacturers. A random error of
20 sec to 2 min a day is what I call utter ****. If you had a quartz watch
which was that bad, you would take it back to the shop and demand your money
back.
A GPS for a time base, a good a/d converter and fast microcontroler would be
the way I would do it. I am working with a very fast board 60 MHz with 4 10
bit A/D channels
Does 10 bits give you enough resolution, taking into account sensor
noise? The Dataq $25 starter ADCs give you 4 Channels at 240 sps total - 60 sps /
channel.
I can't figure out at the moment why you would need a high monitoring
rate for looking at water levels?
Regards,
Chris Chapman
In a message dated 20/07/2005, gcouger@.......... writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
size=3D2>Chris,
Out of sequence and possible not on subject but it=20=
has=20
some thoughts on timing that are important. I must have come in the middle=
and=20
don't understand your set up. I thought you were recording data to disk wi=
th a=20
computer sound card.
I am working on a sismomenter that records sev=
eral=20
sismometers at one time in the 0-50 HZ range on the two channels of a soun=
d=20
card for imaging shallow aquifers looking for larger gravel to find better=
=20
water well in a shallow aquifer along Red River
between Oklahoma and T=
exas=20
so I can afford to put in irrigation.
I have probably imposed my=20
project on yours without realizing it.
Hi Gordon,
We were discussing sound cards and I was reject=
ing=20
this as impractical for seismic purposes. The data files are massive and I=20
haven't found a way of using the 24 bit soundcard ADC for 20 sps sampling.
Chuck a bit of cash at it and get rid of the yo=
ur=20
problems? Larry sells a fairly good 8 channel serial board, which you can us=
e=20
with a GPS receiver.
If you want to play, fine. If you want to work,=
get=20
rid of problems that you do not need to have.
Just what rate of water rise do you get al=
ong=20
the Red river?? Even for marine waves, 20 sps is adequate and most geologica=
l=20
systems will go to 100sps, some even more.
>> A GPS will give you the best time s=
tamp=20
you could ask for some even have a 100 kHz clock as well a 1 second pulse th=
at=20
are linked to GPS clocks. They can be used to phase lock frequency sources t=
o=20
for super accurate osillators.
I am NOT asking for the best system, just one t=
hat=20
relatively inexpensive, low power and ADEQUATE for my purposes.
I use VLF timing signals. You can get relativel=
y=20
inexpensive receiver boards with a ferrite aerial: or gut a radio clock. You=
get=20
about 10 mS + the transit time. The only folks who need it more accurate mon=
itor=20
volcanoes of do reflection seismometry. I have been all along this road. I w=
ould=20
be great if Larry would fit the software to decode WWVB directly, but he=20
seems concentrate on GPS. GPS is power hungry and needs a clear l=
ine=20
of sight to satellites; 60 KHz VLF radio is low power and can be used most=20
anywere. I can use it where I can't get MW radio, even underground. Just kee=
p it=20
away from striplights, radio transmitters, TVs and computer monitors. I have=
=20
used it underground near radio and TV transmitters - find a nice hole and bu=
ry=20
it to screen it! It is no accident that submarine communications use=20
the VLF range of frequencies.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>If you=20
are not interested in 1 to 20 Hz signals sound cards are pretty good tools=
.. If=20
you are really serious about low frequencies and accurate timing Windows p=
oses=20
a lot of problems.
0 to 10 Hz is my range of interest. I want abou=
t=20
+/-0.1 sec accuracy, 24 hrs / day. This is why I use an ADC which has it's o=
wn=20
on board timing and correction. The data goes to the hard disk with a time s=
tamp=20
regardless of the rubbish timing on the operating system.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>First=20
Windows is an event driven operating system and there is no assurance when=
a=20
program will run. The sound card gets around that but you have to put a ch=
irp=20
on the signal to synchronize it with time you can depend on the computer c=
lock=20
or the RS232 port.
Can you say that again please? I don't quite=20
understand.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Having=20
worked with embedded systems for the last 20 years I would choose a real t=
ime=20
computer. There are real time versions of Linux but I think that's over ki=
ll=20
for dataloging geophones.
You have the real and the software clocks on=20
computer boards. You used to be able to get 4.194 MHz AT crystal clocks on=20
computers, which could be made moderately reliable. All the boards that I ha=
ve=20
seen recently use 32 KHz crystals which appear to be rejects from clock=20
manufacturers. A random error of 20 sec to 2 min a day is what I call utter=20
****. If you had a quartz watch which was that bad, you would take it back t=
o=20
the shop and demand your money back.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>A GPS=20
for a time base, a good a/d converter and fast microcontroler would be the=
way=20
I would do it. I am working with a very fast board 60 MHz with 4 10 bit A/=
D=20
channels
Does 10 bits give you enough resolution, taking=
=20
into account sensor noise? The Dataq $25 starter ADCs give you 4 Channels at=
240=20
sps total - 60 sps / channel.
I can't figure out at the moment why you wo=
uld=20
need a high monitoring rate for looking at water levels?
Regards,
Chris Chapman
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