625 Scan Lines and 30 FPS
 

If the NTSC folks had given a bit more thought to it, more than half a century ago, that's what the specs would be. They could have allocated 8 MHz per broadcast channel, instead of 6 MHz and the TV system could have had the 625 scan lines of PAL and the 30 frames per second of NTSC. Better resolution for NTSC and no flicker for PAL. There wouldn't need to have been the two different major systems, in the first place. One worldwide TV system could have been developed. Of course, there would have been a few less channels, but after the UHF band was activated, there should have been enough to go around. Too bad for everybody, all these years. I wonder how wide an SD channel would have to be, to support 625/30P? Wouldn't that have been something? Would we have ever thought we needed HDTV, if we'd had that?

Steve McDonald

Barbados and Bolivia are sometimes listed as using 625-line NTSC-N. But information on the web is contradictory in this respect:
http://www.camerasunderwater.co.uk/i...stds_list.html

Since we are on the subject of such ideas, there is a widescreen analog PAL standard that has not caught on much but is in use in Germany. Near-HD quality with SD broadcast compatibilty. PALplus is the product of a big co-operative project undertaken by many of the major European broadcasters. They started in 1990 with the view of providing (by 1995) an enhanced definition television system (EDTV) which would be compatable with existing receivers. In this, they seem to have succeeded admirably, and PALplus has in fact been transmitted by some broadcasters since at least mid 1994. More info on PALplus:
http://www.wordiq.com/definition/PALplus

Steve,

The main reason for the NTSC 525 line system is due to the existing power line frequency.

The US is based on 60hz while Europe is based on 50hz. As television was being developed, the engineers opted for a system which could use the incoming power frequency as a sync base if you will. Channel bandwidth wasn't really the issue then.

Hence, in the original B/W interlaced NTSC you have

60hz x 525= 31500
31500/2 = 15750 which is the horizontal sync frequency(per field)

When color came along, it had to be backwards compatible with B/W sets and so the horizontal and vertical frequencies were reduced just enough to allow for the 8 to 11 cycles of chroma reference signal to ride on the back porch of the sync pulse. The frequency was close enough (59.97) to allow the older sets to 'lock in' on the sync pulses and thus compatibility was maintained.

By the way, people with exceptional hearing could sometimes hear the 15750hz horizontal oscillator in old tv sets that were mainly tube based.

Just to add my .02 worth...

The 60/50Hz issue may be one reason why the frame rates of NTSC and PAL were originally set as they are. But, that doesn't explain why so few scanning lines and a 6 MHz bandwidth were used for NTSC. It might also be asked why PAL wasn't given just 525 scanning lines, and so requiring just 5 MHz for each channel.

Obviously, they weren't giving as much thought as they might have to bandwidth, the number of lines and the resulting resolution, in those days. But, we have all been stuck with their decree ever since. Didn't they realize that the standards they imposed on a system that at that time had no means of electronic recording, would persist as long as NTSC existed?

I wonder if all the PAL and SECAM countries had 50Hz power systems in the beginning and all those who adopted NTSC used 60Hz? Have any countries switched from one power frequency to another and developed a way to not have to change their television systems?

How many know that the human eyes and brain use a 25Hz still-frame acquisition system and a binary code to transmit the signal along the optic nerves? We don't see moving images, but a rapid series of still pictures, very much like our video cameras.

Steve McDonald

<<<-- How many know that the human eyes and brain use a 25Hz still-frame acquisition system and a binary code to transmit the signal along the optic nerves? We don't see moving images, but a rapid series of still pictures, very much like our video cameras. Steve McDonald -->>>

Steve,

Where did you hear this? Please cite your reference that describes how the human eye/brain operate at 25Hz. Enquiring minds (and eyes) want to know if this is just another urban myth or fact.

Patrick, the details I gave about human vision, came from a year-long series of classes I took in college, that were focused on visual, audial and tactile perception. I also had several other courses in Biology and Bio-mechanics, that covered the human sensory system. Rather than my going into the attic and digging out some old textbooks for reference, why don't you go online and find the very latest information about this on Search? Keyword: Human Vision.

The human visual system has capabilities of processing, color and drop-out correction and other functions that are awe-inspiring. It doesn't take long to figure where electronic imaging got its
starting blueprint. Or, was it just disconnected parallel evolution in both cases?

Steve McDonald

> Have any countries switched from one power frequency to another and
> developed a way to not have to change their television systems?

I think most TV sets, even"modern" black and white ones, do not sync to their power input at all.

The country I live in has 50Hz power and uses NTSC. We also use european telecom standars like basic ISDN service which is 64Kbps and E1 (as opposed to T1) which is 2 Mbits/sec instead of 1.5 Mbits/sec.

We get flicker when we try to use flourescent mains-powered lighting with video aquisition. The cumbersome solution to this problem is to use a 60Hz AC diesel generator (the same trick can be used at 48Hz when film is the aquisition format).

There is another trick which involves driving groups of flourescent lamps with different mains circuits at different phases, but I don't know the technical details involved.

Ignacio,

Modern TV sets do not need to sync to the power line, but early ones did. Oscillator circuits had to be made as simple as possible and the power line frequency made a great 'tap' if you will.

Steve,

Regarding bandwidth. In the late '50s when the NTSC system was developed, modulating a circuit 'affordably' beyond 6mhz was difficult because it was all vacuum tubes. Tubes have inherent frequency limitations due to the size and spacing of the tube elements. Bandwidth is a function of modulation frequency. In NTSC, this is an amplitude modulated carrier for video, and a frequency modulated subcarrier for audio. How fast the electron gun is switched on or off (or somewhere in between) as it scans across your picture tube, determines how crisp the picture can be reproduced. The number of scan lines is a limit of how precisely the circuit can place the position of the beam at the beginning of each scan line. In the late 50's, this was also a major consideration.

Sorry to get so long winded. Anyway, my sig doesn't show it but my knowledge comes from a broadcast background. I held an FCC First Class Radiotelephone license, as well as General Class ham radio license.

Believe me, television was nothing short of a radio miracle when it was invented. It still requires a great deal of electronic precision to this day.

Best regards,

Greg's correct in everything but the time frame. the first televisions for the general public were introduced after the war (WWII). The National Television Systems Committee was developing the standards in the the '40's and '50's.

<<<-- Originally posted by Steve McDonald : Patrick, the details I gave about human vision, came from a year-long series of classes I took in college, that were focused on visual, audial and tactile perception. I also had several other courses in Biology and Bio-mechanics, that covered the human sensory system. Rather than my going into the attic and digging out some old textbooks for reference, why don't you go online and find the very latest information about this on Search? Keyword: Human Vision. Steve McDonald -->>>

Steve, Those textbooks you've got in the attic are a little dated if they mention 25Hz as the temporal resolution of the human eye; they probably mention the Victorian era myth of 'persistence of vision' too. I spent the day researching this online since you couldn't give me a reference and even spent the afternoon at the Army Aviation Aeromedical Research Laboratory (Tech Library and talking to the research scientists whom I work with frequently). The current best scientific evidence is that the limit of temporal resolution for the human eye is at least above 72Hz and at or below 100Hz (92Hz quoted to two separate recent studies). That's why we whine if we have to work on a old 60Hz monitor and not a newer, faster one; we can percieve the temporal differences displayed at 60Hz. The threshold is also dependant on spatial resolution (the size the image subtends on the retina; larger image higher threshold - many report less flicker with widescreen imagery), the tonal resolution (color quality, poorer quality lower threshold), and the luminance (poorer brightness, lower threshold).

Its also untrue that "the human eyes and brain use a 25Hz still-frame acquisition system and a binary code to transmit the signal along the optic nerves". In addition to the 25Hz threshold being wrong by a factor of three or four, the optic nerve transmits impulses of different magnitude which are currently thought to be second order data conduits and not mere binary code. Impulse duration, frequency, stimulation of nearby cones, impulse data from surrounding rods and other factors constitute the 'quality' information the brain uses to evaluate retinal stimulation in the fovea centralis. 25Hz is however recognized as just above the upper limit of frequency that causes Flicker Vertigo in some individuals, but it isn't the "still-frame acquisition" frequency of the human eye. In fact the human eye purposefully interlaces imagery. When photons strike a cone cell, it transmits data, but not all cones fire at once, they sequence their firing. While the ones that just fired recharge (rebuild iodopsin levels), others are firing. This staggered sequence has no recurring, measurable frequency. Just thought you'd want to know.

Patrick, I didn't provide any older references for you, for precisely the reason I stated, that the very latest research you could find, would be more
advanced.

Your summary was generally good, but didn't describe the means by which impulse duration and frequency is varied. This is by done by different combinations of "on" and "off" nerve pulses being strung together. In other words, a binary code, consisting of a pulse or no pulse, is used as the building block for very complex optical messages. A coded impulse might consist of several pulses together, followed by some blank spaces, then more pulses. These give the durations and intervals in the longer impulse segments.

Regardless of how complex these messages are or whether they are first order from the retina or second order from an intermediate processing center in the eye, they are constructed from this code.

As my brief mention of human vision in my previous message was just a sidenote, I didn't touch on many of the functions you described. One that might be further explained, is what you mentioned in the phrase, "stimulation of nearby cones and data from surrounding rods". The first part of that refers to a sympathetic response, similar to that seen in other parts of the nervous system. When the retinal cells that are the primary receptors of an optical image, stimulate nearby cells to fire duplicates of the impulse, a very weak or tiny optical image can be strengthened and enlarged.

Electronic versions of this have been developed for video image enhancement in sophisticated surveillance equipment. If you approach an old barn at night, that has narrow cracks between its wall boards, this sympathetic retinal response can be observed. If an interior light shines out between the cracks, the spaces may have an apparent width of up to half an inch. If you measure them from inside, they may actually be no wider than 1/16-inch. I independently discovered this by accident once, then later found it had been recognized for centuries. This is just one of the marvellous capabilities of human vision, to which I gave a generalized mention.

From the different "refresh rates" your research disclosed, from 72Hz to 100Hz and perhaps 92Hz, it appears that the human eye may have a variable rate for sending image impulses. Possibly, this could depend on the intensity or size of the optical stimulation, as you mentioned. A larger, brighter and more complex image might cause more frequent
impulses to be sent, than a small, simple and darker image. Since there are millions of pathways in the optical nerves, perhaps not all of a retina's acquisition has to be sent at the same instant. These ideas are just speculations, but such things can lead to
discovery and confirmation and are important factors in scientific methodology.

And, I might add, it's encouraging to see someone take such an intense interest in this subject and follow it up with so much research, including person-to-person contact. I learned something from it, as I'm sure has anyone else reading the message.

Steve McDonald

Steve,

After a late-evening summary of yesterday's study, it seems that my answer was in response to a topic on another thread as much as in response to your post. Having read the thread on the 'filmic' quality of 24p/25p and then read your assertion that the human eye/brain connection uses a 25Hz 'still-frame acquisition' system, I was truly wondering if there was some physiological reason that humans might prefer 24p/25p.

So my discovery that the eye/brain connection uses a continuous data flow that is sensitive to changes all the way up to 90 something Hz burst my bubble in trying to link the two. The only info I came across that supports 'filmic' rates being preffered is circumstantial in that the 24/25Hz is the lowest frequency most humans can view without some discomfort from Flicker Vertigo (4-20Hz; generally only a problem if this frequency is viewed covering most or all of the visual field). So if it isn't making folks blow grits, it must be good (of course this logic does not prove the link).

So I think I missed the intent of your post in my quest for the 'holy grail' of film rates. If anyone else has studies concerning the human perception of certain visual frequencies, I'd be interested in reviewing them.

what about video games redrawing upto 100 "frames per second"?