data recording with analog NTSC

[Mike]

Hello everyone,

I have need for an analog data recorder.

What I'd like to do is make encoder / decoder circuits that interface
to the
video in/out connections of an NTSC VCR, allowing a time-domain analog
signal to be recorded to VHS tape.

The signals that I'm interested in will be between +/-10V with a
frequency range of DC-40kHz .. though for the video channel, I believe
I can get more bandwidth and I'd very much like to, for future use.

I'll probably use the stereo audio channels as well.

Any help with a circuit to do the required encoding to NTSC-compatible
video signaling would be very helpful. I'm good with a soldering iron,
but I don't know much about video applications.

A reference (preferrably web) for the required info on NTSC would be
welcome too.

Mike

 

[Tim Shoppa]

The signals that I'm interested in will be between +/-10V with a

frequency range of DC-40kHz ..

Let's say 16-bit resolution, and a sampling rate of 100kHz. You could
double both of those and it still wouldn't change the conclusion.

That's a storage requirement of 720 Mbytes/hour.

A 160 GByte hard drive is $80 and will store more than 222 hours of
data.

Storing 222 hours of data on 6-hour videotapes will take about 40
tapes, which will cost more than that little hard drive (never mind the
handling/loading/unloading issues or the cost of the mechanism) by a
factor of several.

If the year were 1986 then what you're asking for would make sense to
videotape (and indeed there were products on the market to do that).
But today it doesn't make sense, it's just so much easier to dump it to
a hard drive in digital form.

Analysis of data after-the-fact is way way faster from a hard drive too
(by a factor of hundreds).

If you have legal requirements for archiving etc. these usually drive
the solution rather than anything technical.

Tim.

 

[[email protected]]

A VHS video-tape recorder is an analog recorder. You'd need to scale
your signal down to video levels, and add in frame and line synch
signals - though the video recorder won't pay any attention to the line
synch signals, except to extract the "black level" - the most negative
possible signal excursion, nominally 0V for video - from the "back
porch" on the line synch.

Around 1978 we used a standard consumer VHS recorder to record medical
ultrasound scans, where the line period was about 300usec rather than
64usec and the frame rate about 5Hz (so we added extra framing
information to the basic 50Hz/60Hz frame rate).

The video recorder was perfectly happy with the funny scan rate - in
fact consumer video recorders happily recognise very crude
approximations to the rather complicated frame signals specified in the
video standards, which vastly simpliies the logic that you need to
generate adequate frame signals.

 

[Mike]

A VHS video-tape recorder is an analog recorder.

Yes. Thats why I want to use a VHS recorder, rather than a digital
system as another poster suggested. As I stated originally, I need an
analog recorder.

You'd need to scale your signal down to video levels

Yeah, I figured this would be necessary..

and add in frame and line synch signals - though the video recorder won't
pay any attention to the >line synch signals, except to extract the "black >level" - the most negative possible signal excursion, nominally 0V for video - >from the "back porch" on the line synch.

Ok, now this is what I need to read about, and this response allows me
to ask a few pointed questions:

a) Can the video signal be bipolar? Or must I add an appropriate DC
offset?

b) The voltage range obviously needs to be scaled. What is the allowed
range?

c) What are / where can I read about "frame" and "line synch" signals?

 

[Matthias Melcher]

Whatever you are planning, use a harddrive or memory card interface
instead. It will cause you a lot less pain.

That said:

a) Can the video signal be bipolar? Or must I add an appropriate DC
offset?

No, video signals are stored in a VCR using composite signals. A
composite signal transmits luminance (Y) in the lower 4MHz and color
information on a kind of chrom wheel at around 2MHz (Yr, Yc). A VHS VCR
lowers those rates considerably and writes that to tape, using the sync
signals to drive the drum with its heads. The Y signal can be recorded
reliably between 0 and 0.7 V

Details are here: http://en.wikipedia.org/wiki/Composite_video
and some more stuff here
http://www.tek.com/Measurement/cgi-...Video_Msmt/colorbars.html&FrameSet=television
and in many many other places on the web.

b) The voltage range obviously needs to be scaled. What is the allowed
range?

The Y signal can be recorded reliably between 0 and 0.7 V.

c) What are / where can I read about "frame" and "line synch" signals?

Google.

You need to create at minimum horizontal and vertical sync signals and
possibly a color burst. You have to fit all that into a tight timing
scheme and you can't record any values during sync and retrace phases as
well as the first and last few lines of a picture.

I should alos mention that, due to the helical scan recording of VHS,
you can not slow down or speed up the tape transport beyon the usual SP
and LP settings.

 

[Mike]

Whatever you are planning, use a harddrive or memory card interface
instead. It will cause you a lot less pain.

That said:


No, video signals are stored in a VCR using composite signals. A
composite signal transmits luminance (Y) in the lower 4MHz and color
information on a kind of chrom wheel at around 2MHz (Yr, Yc). A VHS VCR
lowers those rates considerably and writes that to tape, using the sync
signals to drive the drum with its heads. The Y signal can be recorded
reliably between 0 and 0.7 V

Details are here: http://en.wikipedia.org/wiki/Composite_video
and some more stuff here
http://www.tek.com/Measurement/cgi-...Video_Msmt/colorbars.html&FrameSet=television
and in many many other places on the web.


The Y signal can be recorded reliably between 0 and 0.7 V.


Google.

You need to create at minimum horizontal and vertical sync signals and
possibly a color burst. You have to fit all that into a tight timing
scheme and you can't record any values during sync and retrace phases as
well as the first and last few lines of a picture.

I should alos mention that, due to the helical scan recording of VHS,
you can not slow down or speed up the tape transport beyon the usual SP
and LP settings.

What about dropout? It seems that could be major problem.

 

[[email protected]]

The video recorder couldn't care less about the colour burst - it just
uses to the frame synchs to check that the drum the tape wraps around
is spinning at the right speed.

 

[martin griffith]

Hello everyone,

I have need for an analog data recorder.

What I'd like to do is make encoder / decoder circuits that interface
to the
video in/out connections of an NTSC VCR, allowing a time-domain analog
signal to be recorded to VHS tape.

The signals that I'm interested in will be between +/-10V with a
frequency range of DC-40kHz .. though for the video channel, I believe
I can get more bandwidth and I'd very much like to, for future use.

I'll probably use the stereo audio channels as well.

Any help with a circuit to do the required encoding to NTSC-compatible
video signaling would be very helpful. I'm good with a soldering iron,
but I don't know much about video applications.

A reference (preferrably web) for the required info on NTSC would be
welcome too.

Mike

I'm confused, basically video recorders are analogue devices that
record analogue, but they have digital gaps where nothing can be
recorded, ie analogue with gaps in it.
I cannot see an easy way without digitizing your signal and a
buffering FIFO. the first 16 or so lines are more or less unusable(
from memory), as the record head is not touching the tape, problem.
The sony audio F1 system, 1970's digitised the analogue and spread it
around the tape, for error correction purposes.

I dont think you have a hope in hell of directly recording 40khz on
vhs without digitsing first. As others have pointed out, dropouts will
be a very significant problem.


martin

 

[[email protected]]

Alesis ADAT XT, modified for DC coupling. We use them for record and
playback of laser show control signals with 30 Khz content.

Steve Roberts

 

[Tim Shoppa]

Yes. Thats why I want to use a VHS recorder, rather than a digital
system as another poster suggested. As I stated originally, I need an
analog recorder.

That's a rather arbitrary decision considering that you've
made no statements about resolution or noise. Or maybe it's
only because you can make no statements about resolution or noise?

In the 60's and 70's your analog recorder would be an "instrumentation
recorder". To get bandwidth down to DC you FM-modulate a carrier. To
get 40kHz on your data you'd probably have to run the tape at 30IPS or
faster.
The instrumentation recorder from HP probably cost $10K (in 1960's
dollars)
and tape was another $10 a reel for a few minutes of data. Scaling
and bias are done by knobs on the modulator (and hopefully matching
knobs on the demodulator).

If you have no confidence in sampling your data and insist on using
an NTSC VCR and analog recording then you're simply going
to have to put up with horizontal sync/retrace at 15kHz and vertical
sync at 60Hz and the random dropouts.

I suspect we could give you much more useful advice if you could
parameterize your requirements and stop insisting that you know the
solution. If I wanted vague requirements with mandated solutions
that'll never work, I'd be at my day job instead of on Usenet!

Tim.

 

[Mike]

That's a rather arbitrary decision considering that you've
made no statements about resolution or noise. Or maybe it's
only because you can make no statements about resolution or noise?

Noise is exactly what I'm investigating, and in fact for my purposes I
need it. I'm making a GPAC. Its storage will be magnetic tape (hence my
desire to use a VHS -- because it is cheap. I'd consider other
solutions, but it MUST be entirely analog).

Several systems of differential equations will be solved by this
machine, and its qualitative results will be compared to the results
obtained by a digital machine.

An all-analog system capable of solving differential equations, storing
the results, and replaying those results as input to other equations is
a requirement here.

I have the computation circuits nearly entirely built (I still need to
wire up a few precision rectifiers). Its now time for me to consider
the storage side.

Testing thus far has shown that my computation circuits are capable of
reliable operation from DC to 40kHz, so I'd really like a storage
system that is reliable over that range. I can definately trade
bandwidth for dollars, though.

Instrumentation recorders are an option, for sure -- I have
occasionally seen untested devices on ebay for a few tens of dollars.
Tested/known-working items for much more.

I'd like to keep costs to a minimum, hence my desire to use a VCR.

I suspect we could give you much more useful advice if you could
parameterize your requirements and stop insisting that you know the
solution. If I wanted vague requirements with mandated solutions
that'll never work, I'd be at my day job instead of on Usenet!

Thats funny. Honestly though, does ANY of the above actually HELP you
in answering my question? I doubt it. You don't really need to know my
application, or why I need analog.

My question was how to make some interface circuits that will allow me
to record an arbitrary analog wave onto the video track of a VCR. I
appreciate the digital proselytizing -- but really, in this instance, I
have to be a pagan.

Mike

 

[Robert Scott]

..My question was how to make some interface circuits that will allow me
to record an arbitrary analog wave onto the video track of a VCR...

Do you realize that the VCR requires a signal in the video format,
including sync pulses? You cannot record a continuous arbitrary
analog signal on a VCR without packaging it with some H-sync and
V-sync pulses. These will interrupt your analog signal, unless you
have some way of buffering the signal, which almost certainly involves
digital conversion. However the general-purpose data acquisition tape
decks will do what you want because they were designed for exactly
that purpose.


-Robert Scott
Ypsilanti, Michigan

 

[Tim Shoppa]

I doubt it. You don't really need to know my

application, or why I need analog.

At the same time, you should realize that an analog recorder like a VCR
will reproduce amplitudes to an accuracy of a few percent at best (and
this includes tweaking knobs to get it aligned to this point - the
accuracy will be worse than 10% without tweaking.)

If you don't want 16-bit digital recording because it has 0.0015%
quantization error/noise but are willing to put up with a few percent
error on an analog system, that sets off all sorts of alarm bells in my
head.

I do share your interest in analog computers etc. (see my posts from
earlier this year about building an integrator for the Lorentz
equations) but you have to actually specify resolution and noise
requirements before you can choose a solution.

Tim.

 

[Mike]

At the same time, you should realize that an analog recorder like a VCR
will reproduce amplitudes to an accuracy of a few percent at best (and
this includes tweaking knobs to get it aligned to this point - the
accuracy will be worse than 10% without tweaking.)

Yeah, I am aware of this. I am hoping to get 5% replay accuracy.

Are you saying that a few percent is all I can get from _any_ magnetic
analog recording technology, or are you restricting your comment to
VCRs?

If you don't want 16-bit digital recording because it has 0.0015%
quantization error/noise but are willing to put up with a few percent
error on an analog system, that sets off all sorts of alarm bells in my
head.

Well, thats exactly what I'm saying. If I were strictly concerned with
the precision of my solutions, I would certainly go with a digital
recording method. Then again, I'd probably be doing digital computation
too.

What I'm interested in is solving a certain set of equations with
purely analog technology. I will also find the solution digitally.
Comparison of the two is what I'm ultimately interested in. If the
analog recording media increases my error by 10, or even 20% .. then
thats life.

My hypothesis is that there are equations for which digital solutions
are hard to find, and prone to error. The quantitative accuracy of the
digital solution will degrade AND its qualitative behavior will vary
from the actual solution.
OTOH, the analog solution -- while having substantially less
quantitative precision, will qualitatively behave similarly
(analogously) to the actual solution.

That is why I am looking for an analog recording solution, but from
what I've read so far, it doesn't appear as if a modified VCR is going
to do the trick.

I do share your interest in analog computers etc. (see my posts from
earlier this year about building an integrator for the Lorentz
equations) but you have to actually specify resolution and noise
requirements before you can choose a solution.

I'm afraid my perspective differs. I'm more interested in price than
the noise contribution. I'd really like a cheap analog storage device,
and a VCR with some signal conditioning circuitry seemed like the best
bet.

Mike

 

[Glenn Gundlach]

Yes. Thats why I want to use a VHS recorder, rather than a digital
system as another poster suggested. As I stated originally, I need an
analog recorder.


Yeah, I figured this would be necessary..


Ok, now this is what I need to read about, and this response allows me
to ask a few pointed questions:

a) Can the video signal be bipolar? Or must I add an appropriate DC
offset?

b) The voltage range obviously needs to be scaled. What is the allowed
range?

c) What are / where can I read about "frame" and "line synch" signals?

Since you seem hell-bent on using a VCR, are you aware of the Sony
PCM-F1?

http://www.proaudioreview.com/may00/SonyPCM-F1RetroReviewWeb.shtml

It was a digital audio to NTSC (for a VCR) before CD but used the same
sample rate. It had the error correction, dropout correction and
metering, the whole nine yards. Right now, for 4 more days, there is
one on eBay for $35 (paid $1000 for my PCM701, the line powered
version, in 1983). It might be possible to alter the analog section for
DC coupling. I KNOW its possible to intercept the digital data stream
because I did it in '83 to make a digital audio fader using a TRW
hardware multiplier and EPROM log (for dB) lookup table to fade in 1/16
dB/step. Crude but it worked.

GG

 

[Richard Crowley]

My question was how to make some interface circuits
that will allow me to record an arbitrary analog wave
onto the video track of a VCR. I appreciate the digital
proselytizing -- but really, in this instance, I have to be
a pagan.

You are asking for the imossible. No VCR records a continuous
signal. They all require horizontal and vertical sync intervals
which consume a significant time period.

If you really must have analog, you are likely stuck with
FM recording on an industrial data recorder. You have
specified an impossible set of requirements if you insist
on any kind of video recording device.

 

[Tim Shoppa]

Are you saying that a few percent is all I can get

from _any_ magnetic analog recording technology

There are tricks like recording pilot tones that are filtered out on
playback to let you get better amplitude calibration. And of course FM
lets you go down to DC. But to say that the absolute error is less
than a few percent full scale across 0 to 40kHz is going to be hard
even with an instrumentation recorder.

My hypothesis is that there are equations for which
digital solutions are hard to find, and prone to error.

Oh, yeah, lots of stiff differential equations out there. But solving
them with error-prone analog circuitry won't help you any.

For many stiff PDE's there are numerous tricks that let you solve the
problem through relaxation (especially when you know the values at
extremes).

But all this stuff is first-semester numerical analysis and applies
whether your computer is analog or digital.

I'm afraid my perspective differs. I'm more interested
in price than the noise contribution. I'd
really like a cheap analog storage device

Get rid of your DC to 40kHz requirement and make it 100Hz to 3kHz and a
plain old cassette recorder will do OK (assuming you can tolerate the
phase shifts, dropouts, etc.)

Make it DC to a kHz and a FM-modulated carrier on plain old cassette
tape ought to do great (well, until you start noticing the wow and
flutter, but you already told us that you're impervious to these
because your soul is pure and intentions are good.)

Tim.

 

[Don Lancaster]

At the same time, you should realize that an analog recorder like a VCR
will reproduce amplitudes to an accuracy of a few percent at best (and
this includes tweaking knobs to get it aligned to this point - the
accuracy will be worse than 10% without tweaking.)

If you don't want 16-bit digital recording because it has 0.0015%
quantization error/noise but are willing to put up with a few percent
error on an analog system, that sets off all sorts of alarm bells in my
head.

I do share your interest in analog computers etc. (see my posts from
earlier this year about building an integrator for the Lorentz
equations) but you have to actually specify resolution and noise
requirements before you can choose a solution.

Tim.

The analog system will almost certainly be measuring its own
limitations, rather than what you think you are recording.

The error rate of VHS is totally unacceptable for any reasonable data
recording app.

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

What I'm interested in is solving a certain set of equations with
purely analog technology. I will also find the solution digitally.
Comparison of the two is what I'm ultimately interested in. If the
analog recording media increases my error by 10, or even 20% .. then
thats life.

Mike

Odds are overwhelming that your "comparison" will only show up the gross
deficiencies of analog data recording.

And have nothing whatsoever to do with whatever it is you are trying to
measure.

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

Odds are overwhelming that your "comparison" will only show up the gross
deficiencies of analog data recording.

And have nothing whatsoever to do with whatever it is you are trying to
measure.

Also, the ---> standard <--- method of solving complex difficult
differential equations IS digital. And HAS BEEN SO FOR THREE CENTURIES!

It is called "Newton's Method".

If an analog solution attempt produces a different result, it is wrong.

Tutorial at http://www.tinaja.com/glib/stalac.pdf

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com