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Shigemi Numazawa & Japan Planetarium Lab inc. Home Page


about Video Field and Flame




The Video recorder record and playback the image on tape by 2 rotating heads.
One of 2 heads is working to record and to playback the odd field.
Other is working to record and to playback the even field.


When we see the image using stop motion, the tape stops.
And the head scan only the signal of an alternative field on tape, not both field.
This is the reason why we can not see the frame.

Then if we use Jog dial and move 1 step, the tape go ahead.
But we can see just 30 frame a second. (This is the point that Ron suggested)
Since the control signal to move the tape is written every 2 fields, it is happen.
When we use stop motion, each format is decided whether the video recorder read the odd field or the even field.
I will say again. When we use stop motion, we can see the field, not frame. But when we use Jog dial, the step is 1/30second.

The field that we can see using consumer machine is 2 kinds.
One is single field.
The other is false frame that combine one field twice.
But in either case we can see the field signal.

Beta cam adopts a component signal system that record an chominance signal and a brightness signal separately. Moreover it has 6 heads and record 2 lines for one field on the tape. The control signal is different with VHS or 8mm video. And it has the TBC time base corrector built-in. So it has many functions to meet high level require for professional.
But among of Beta cam machine, using BVW-70,60,50 etc., we can select the odd field or the even field. Every kinds of Beta cam machine can not do that.

Jul.10 1998 Numazawa


record signal on the video tape

A head and B head record each field individually on a each row.
A run control signal is recorded each 2 field.


When we use stop motion

A head and B head trace only 1 truck.
But the azimuth of A head and B head are different, so only one head of them can read the signal. This is that some field image can read out only and other field image can not.

Step by step mode

When we use the step by step mode, the video tape run followed the control signal, so we can see only some field, odd field or even field.
We are not able to see the other field.
But in the case of the Beta cam SP, due to a circuit of TBC etc., run control of 1/2 step is available.
At this time, we have to record ?? signal.

Ordinal play mode

The tape run with a certain speed, and A head and B head trace A track and B track correctly.
With each 2 field run control signal, the run precision of each 1 field can be kept.



Test of field


That is the frame If we can see 30 step a second, that is the field If we can see 60 step a second. Ron said.
This definition is wrong.
To understand this thing, we have to consider what is frame and what is field.
When we use stop motion mode, we can see the field.
The evidence of this is understood when we observe the monitor image carefully.
At that time, we must to watch a tilt and a clear line especially.



if we can see fine dark lines in the image, we see A field or B field alternatively.
Thus we see only single field!
If there is no fine dark line but we can see the tilt line as ridges as stairs,
A field or B field is read out twice and complement empty row.
So this is a false frame and not true frame.




Digital VTR

The digital VTR has a different record system from analog system.
On the DV format, the signal of 1 frame is recorded using 10 track of the DV tape.



All signal pass through the memory circuit, and is made up and is revised.
So we use capture board, then we can select to see the following freely.

A field
B field
A+B=frame
Ax2 field=false frame
Bx2 field=other false frame
etc.





At this time, you first can see a real frame image even though you use stop motion mode or step by step mode.
When we capture a moving subject using frame, a blurring is yielded.
This shows the time lag between A field and B field, so it is the proof that is frame.




The knowledge of them is able to see in books about TV or VTR.
I have many books, but they are written in Japanese unfortunately.
I guess there are many books similar of them in U.S.
I recommend Ron to read some of them.
This is the fundamental knowledge of VTR.

Jul.21 1998 Numazawa



Observation of occultation

So far I have observed occultation of Spica or grazing occultation few times using the video and have analyzed. To know the reliability of the data, I have used Beta cam SP and Hi8 video recorders simultaneously, but at different places.

The timing of the occultation is not received the influence of seeing, since it happen the world without atmosphere.
So the timing of disappear is in a moment.

But the star image decreased in 2-3 frames gradually when I observed.
First, I thought that this decreasing might be due to influence of companions of Spica.
That was one of purpose of my study whether we could catch Spica's companions from the occultation data.
But the reason of this decreasing is not clear yet.
The patterns of the appear and the disappear are completely different.

At that time, I learned many things.
In the case that the brightness changed in few sequence field, we have to point out what time?
When the video recorder make the odd field and the even field, A row of pixel on CCD to make image is decided. I guess that there is the possibility to change the brightness due to this system.iAs the focal length is shorter, the influence is bigger.)
As Ron say, the star may be not disappear, but I think that the brightness may change.
Not depend on the number of CCD pixel, the resolution of lengthwise of the field is 262.5 lines.
We can not say it is fine enough.

And there is a video camera that has progressive CCD.
This makes image using all pixel of the CCD. Then it slice one image and makes the odd field and the even field. In this case, we can see each field but the resolution of time is 1/30second.

In the case of the occultation, I said that the accuracy is 1/30second before.
That is not precise, I made mistake. That may be worst. Please consider carefully whether we should believe the accuracy of 1/60second at least.

I have worked concerned with a video for about 15 years. I have used the Beta cam, U-matic, 1-inch video, the latest many kind of digital VTR, Hi-vision VTR and consumer machines.
But the video system is very complicated and there remains many questions. Even engineers who are specialist for video have different opinions.
I feel that I should study about video harder.

I understand that Ron's article on S&T produces great results. I will introduce his article on Tenmon Guide magazine.

Jul.10 1998 Numazawa


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