Hi everyone, I have been reading Alan Pemberton's pages, hosted on this site, but I'm a bit confused.
I'm talking about the 625/50 monochrome system here, on a screen with aspect 4:3.
To keep things simple I think the units of measure, (number of lines per frame) by (number of micro-seconds per line), will suffice.
Thus a complete frame, syncs and all, is 625 lines of 64 micro-seconds duration.
I always thought a testcard was larger than normal video content. The many pictures of TVs displaying testcards invariably cut of some, if not all, of the castellations. So is it the part of the card inside the castellations that is the often cited 576 lines of 52 micro-seconds?
What size is the testcard castellations and all?
As far as I can find, the minimum horizontal sync period is 8 microseconds and minimum for vertical syncs is 15 lines per frame, so there is plenty of room for a testcard to be oversize.
What is the size of actual video content?
I seem to recall that in the early 60s the active picture included a narrow black border, if so the actual video would be even smaller than the active screen area. Did transmission of the testcard omit
this border and have castellations instead?
Thank you for any clarification.
On the later test cards I think the centre castellation had arrows to depict the viewing area, so the very edge of the castellations would be over scanned.
For practical purposes a bit of overscan was always worthwhile, if the mains voltage decreased before the days of stabilised power supplies the picture would have a black border and a call back to fix it.
There was the difference in aspect ratio to take into consideration as well with some CRT’s. Usual was to get the size approx correct then get the circle round or near a possible. Adjust height and width to correct any error.
Photo showing arrows in the castellations, note the round corners of many CRT’s will cut off the corner of the test card.
Frank
Hi Nuvista,
I'm aware of the complications of CRT aspect ratio.
I've read that the arrows were introduced to assit displaying 4:3 (20:15) transmissions on different aspect CRTs, especially the 5:4 (20:16) CRTs.
This pic is supposed to show a correctly adjusted 5:4 screen
The implication being that with its lesser height a 4:3 would not display any castellations. This makes sense to me as the castellations do not add anything to the visual clarity, at least on the monochrome cards, but are there to test the oscillators for spurious triggering.
But how much larger were the testcards than the transmission of normal content?
Hi Nuvista,
But how much larger were the testcards than the transmission of normal content?
Sorry can’t help with that, perhaps someone who was in the broadcasting side would know.
I presume there couldn’t be a lot of difference or having a test card wouldn’t have been worth while. The set up of a domestic TV 50 years ago involved quite a bit of compromise.
Frank
Maybe cutting off the castellations was considered acceptable overscan, or seeing the castellations required underscan (is that a term?).
Testcards were not designed for the consumer, they were used to test the system from camera to receiver. The frequency gratings exceed anything you could resolve on a TV.
Perhaps someone who had access to a CRO in the early 60's would have noticed if there were differences in the sizes, by observing their relationship to the syncs.
But how much larger were the testcards than the transmission of normal content?
Zero percent!
Both the test cards and programme material occupied the entire active area of the field. There was absolutely no difference. Several points have been raised in this thread: I'll attempt to deal with them in no particular order.
The arrows which were added to the test card slides were purely to assist in accurately mounting the slide in the scanner so that the tip of the arrows was coincident with the edges of the scanned area. These were not needed on later electronically generated cards, as can be seen on the many examples on Alan Pemberton's articles but seem to have been retained for Test Card F and later variants.
Early CRTs were round and could have been masked off to show the entire active screen period but this would have resulted in a very small picture so, instead, the tube was over scanned so that the picture height was much closer to the top and bottom of the screen which meant that much of the sides of the picture was beyond the edges of the tube. The reason that early CRTs were round was simply that it ensured that the tubes were relatively safe from implosion - but a plate glass implosion screen was fitted in front of the viewing surface, just in case.
Glass technology improved so that, by the early 50s it was possible to produce rectangular 14" tubes, such as the MW36-24, as used in sets like the Pye V4/VT4 and Bush TV53. However, in the 17" version of these tubes, there was a definite curve to the edges of the tube to provide extra strength and the later 21" tubes were very curvy indeed!
It took until the mid 60s to develop an implosion proof tube using a metal rim band and, at long last, the implosion screen could be dispensed with.
The important thing to remember, though, is that all of these tubes were over scanned in the horizontal direction, to a greater or lesser extent.
The usual way to set up these sets was to adjust the height to fill the screen, check and adjust the linearity, if necessary, then finally set the height to slightly overscan so that about half the castellations were off the screen then set the width (and horizontal linearity, if necessary) for the correct aspect ratio i.e.: a round circle.
As for the black border round the edge of the tube, this would have been the glass of the tube walls viewed edge-on around the active screen area due to the push-through masks used.
When all else fails, read the instructions
Very interesting, I've never seen a roundish TV CRT.
We didn't get TV until the 2nd half of the 50's and even in the early 60's they were few and far between. I remember people gathering at TV shops to watch the moon landing in glorious monochrome, although they were quite common by then they were by no means ubiquitous.
So all the CRT's I have seen were bulging rectangles (pin cushions). The cabinets only masked off an inch or so off the maximum bulge. The front, of course, was rounded to keep the beam length about the same.
The cabinet mask was 4:3, the transmission was 4:3 so overscan would need to be the same horizontally and vertically?
Sorry, I meant 'barrel' not 'pin cushion'.
Don't know where my head was.
Very interesting, I've never seen a roundish TV CRT.
As I said in my previous post, this had to do with the ability of the glass industry to produce a large glass envelope strong enough to resist the forces pressing on it. No sharp corners, just gentle flowing curves. Pre-war tubes had narrow deflection angles so that the length of a 12" tube was measured in feet rather than inches! To house these, they were mounted vertically with the screen viewed via an angled mirror and the tube base was just a couple of inches above the floor.
Take a tube and estimate how many square inches there are on the surface, taking into account the screen, the body and the neck. Then multiply by 14.7 (nominal air pressure in pounds per square inch at sea level). Just the face plate of a 12" tube has around 1700 lbs pressing on it - around two thirds of a ton!
Improvements in glass technology enabled shorter, rectangular tubes to be produced from the early 50s which explains why you never saw any round tubes because, unlike many other areas of the country, there would have been no older sets around.
When all else fails, read the instructions
The cabinet mask was 4:3, the transmission was 4:3 so overscan would need to be the same horizontally and vertically?
No, the transmission was 4:3, as you say, but not the tubes. Taking a popular 19" tube from the mid 60s as an example, the A47-11W screen has a useful width of 384mm and a useful height of 305mm, a ratio of 1.26:1 which is equivalent to 5:4 so, with only a slight vertical overscan to take into account any slight variations in height, the horizontal overscan would need to be of the order of 8 or 9 percent.
When all else fails, read the instructions
We didn't get TV until the 2nd half of the 50's ...
Sorry, not very clear was it?
I'm in Australia, so I was talking about the whole country.
Didn't get colour until 75!
We never had 5:4 transmissions or 405 lines.
I think we made our own CRTs.
I remember people gathering at TV shops to watch the moon landing in glorious monochrome ...
Only two people saw it in colour - Neil Armstrong and Buzz Aldrin - because it was a monochrome camera that they took with them!
RCA developed a special miniature colour camera for the next landing but, from what I saw, it got pointed at the sun by mistake while it was being carried down the steps of the lunar lander - and that is instant death to a vidicon tube which was the only thing small enough for the purpose in those days.
By sheer coincidence, I've just read this post on the: uk.tech.digital-tv newsgroup:
My grandson was asked, "Who was the first man on the moon". Quick as a flash, he replied "Louis Armstrong".
However, it was the reply to this that I found particularly interesting:
It is to my eternal shame that once I'd got the idea of Louis Armstrong being "the first man on the moon" implanted in my brain, I had to think for about 10 seconds before I got Neil Armstrong's first name. If you'd asked me without first making me think of Louis I could have told you immediately.
I had exactly the same experience!
When all else fails, read the instructions
I just assumed everyone else got colour.
I always feel sorry for Collins, an epic journey and no one remembers his name.
Coincidentally I was just reading about the moon transmission. We got the raw feed before it was sent to NASA, so we got it 6 and 1/3 seconds earlier than the rest of the world.
"I think to myself, what a wonderful moon"
My Dad has a Colemanballs book which mentions Louis Armstrong being given as an answer on a TV quiz.
IIRC one of the later moon missions took a colour camera with a rotating filter disc, which actually survived long enough to send some pictures back.
Hi.
I had a chuckle at the title of this thread. I "was" going to reply, "the size of the test card is dependent on the size of the TV's CRT" Oh well perhaps not so funny.
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
If it made you smile Trevor, that’s good enough.
Frank
From my memory of a visit to Crystal Palace in 1963, Test Card C was the size of a 2" slide!
When all else fails, read the instructions
A test card generator, check all nine pages.
https://tvcameramuseum.org/marconi/bd665/p-1.html
Frank
Yes, in fairness, the slide I saw at CP was not the normal source as it was broadcast on the network, so I don't don't know what the origin was. The slide was there just as a backup.
There were other slides in the scanner. One very formally headed 'Crystal Palace BBC Transmitting Station' , very unusually for BBC TV in the Times New Roman font contained an apology which could be used if there was a major failure. Apparently it was used once when someone dug up and severed both main and back-up feeds! (Normal practice is to route both feeds via different routes but I think this happened where both feeds crossed the Thames on the same bridge.)
This would have been a 405-line scanner so I don't know what was available at CP in April of the following year for the non-opening night of BBC2!
When all else fails, read the instructions
I don’t know but I presume test card F couldn’t have been monoscope.
Frank
