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.
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.
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
