Re: Morecambe and wise 1971 Christmas show

On Sat, 27 Dec 2008 16:22:42 +0000, Gareth
<gareth-see-sig-block@xxxxxxxxx> wrote:

<snip>

By "exposure latitude" I think you mean "dynamic range". This is going
back a long time, but I think you generally needed about 4 stops of
overload before the comet tails appeared on non-ACT cameras (unless the
beam current had been wound right down).

The standard line-up for EMI 2001s in London involved winding the beam
current up until the peak white square on the line-up chart was
discharged and then adding one extra step of the control for Y and two
steps for the colours. (The beam current control in the EMI 2001 is a
multi-position rotary switch).

Memory fades a bit (and I can't be bothered to get the handbook out),
but I think the increment of beam current was 6dB per step, so the
above would imply between 1 and 2 stops of headroom for Y and between
2 and 3 for the colours. Too much Y beam current caused a loss of
resolution, as you would expect.

If I've got this hugely wrong, Rod or Dave Pitt or another Studio Eng.
bod will be along in a minute to put me right.

The really objectionable problem with comet-tails in drama always
seemed to me to be if they were a funny colour. Candles, for example,
seldom leave green or blue trails in real life, whereas a red trail is
subjectively much less objectionable (within reason, clearly). This
could be achieved with suitable on-the-fly tweaking of beam current
settings on problematic scenes. Watch the rehearsal, spot the
problems. On the recording, put a notch of beam into whichever colour
needed it to cope (at a suitable moment when the camera was off shot,
obviously), and go back to the "correct" setting afterwards.

With regard to very dark elements of the pictures, a problem in the blacks
with plumbicons was something called 'lowlight beam lag'. Basically, if
not enough photons had landed on an area of the target, the electron beam
could feel somewhat disinclined to land there.

As I understand it, the effect arises because electrons are emitted
from the cathode with a spread of initial energies. Typically,
low-energy electrons are relatively abundant in the emission and
high-energy electrons relatively scarce. In a tube with a
control-grid, many of the lower-energy electrons are retained in the
space charge due to the action of the electric field due to the
potential on the control-grid. The beam therefore consists mainly of
electrons with medium and high initial energies, the former being
relatively more plentiful than the latter.

Ignoring target leakage, in a tube without bias light, black is
represented by zero signal current. This implies that no electrons are
being landed on the target. This condition will occur when the target
potential is sufficiently below cathode to prevent even the most
energetic electrons from landing.

Consider a dark object moving in front of a truly black background.
Beam is landing on those areas of the target representing the dark
object. As the object moves, some parts of the target which used to
represent the object will now represent background: the beam must land
electrons on these areas of the target so as to charge them back to
the point where no more electrons can land i.e. to a potential such
that even the most energetic electrons cannot land. The problem is
that high-energy electrons are scarce in the beam and there are not
enough of them available to do this in one pass. The moving dark
object is therefore followed by a diminishing dark trail as several
fields of scan are required to land enough charge on the target.

In a bias-lit tube, black in the scene is no longer represented by
zero signal current because the bias light is always falling on the
target. If the bias light is adjusted correctly, black is represented
by a target potential which the beam can achieve in a single pass
because it is close enough to cathode potential that the relatively
plentiful medium-energy electrons are landed: hence, no trails.

(n.b. for completeness, the above applies to tubes with pure intrinsic
lead monoxide targets only).

(also n.b. for completeness, since the action of the bias light is to
establish a definite minimum level of illumination on the target, even
for black areas of the scene, the true picture signal current sits on
top of that due to the bias light, and arrangements must be made
elsewhere in the camera signal chain to counteract this).

The LDK5's I was familiar
with had bias lights so there was always a minimum illumination level of
the target, but I don't think the EMI's in use in Studios had them.

The last generation of 30mm non-ACT plumbicons and leddicons (P8400
series) did have bias lights built into them, run from the tube heater
supply, adjusted by a miniature pot in series with the lamp filament
and mounted inside the tube base. You could get some cracking pictures
out of a 2001 with bias-lit tubes if they were lined-up carefully and
correctly.
Remove 2001. for email address.
.

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