04-04-2017, 02:41 AM
Another thought – if one could provide 405-line and 625-line video feeds (to a dual-standard monochrome monitor with say a normal (23 inch) screen size) in which just one active line was at or near peak white (but anyway the same for each case) and all others were blanked, would the single 405-line appear to be brighter than the single 625-line?
If so, and let’s say for argument’s sake that the single 625 line was roundly two thirds as bright as the single 405 line, then repeat the experiment with two consecutive 405 lines at peak white, the rest blanked, as compared with three consecutive 625 lines (as near as possible on the same place on the screen) at peak white, the rest blanked. From a reasonable viewing distance, would the 405 and 625 line sets look to be of similar brightness? And so on in a few coarse steps up to a full raster.
Again if the individual 405 lines are brighter than the individual 625 lines, does this exacerbate the lininess issue? One might expect that a brighter line will make an adjacent “black” space more apparent than a less bright line. If that holds, then the perceived lininess would appear to increase in a non-linear manner (maybe approximately second order) as the number of lines is progressively decreased below the threshold minimum number required to produce a reasonably flat field. Fewer lines means brighter lines and wider black spaces, both of which would contribute to increased perceived lininess.
Lininess, with bright individual lines abutting visible black spaces, might also contribute to an impression of greater contrast than for a picture with the same overall contrast but with minimal lininess.
And yet another thought: to that same monitor, let’s apply switchable spot wobble to the 405 scan, such that with spot wobble switched in, the 405 field has about the same flatness as the 625 field. Then repeat the above experiment. What would be the expected outcomes?
I have also pondered the situation where the transmitted signal had “wrong” black and peak white levels. Then let’s assume a good receiving system with high-gain black-level agc, a contrast control as a gain control in the video amplifier, and black level clamping. So the black level at the tube should be correct, and could be adjusted in the normal way with the brightness control. Incorrect white level would show up as deviation of the peak white video signal amplitude from its nominal value. “Low” white level could be offset by turning up the contrast, assuming that there was enough spare video gain. So the transmission errors would not appear to be fundamentally inimical to obtaining a “full contrast” picture, at least with a properly specified receiving system. But with domestic receivers built to a lesser standard, perhaps there would be problems. And if, because of transmission errors, 625 typically required a higher video gain setting than 405, how many users adjusted the contrast control accordingly when changing from one to the other? (But then, back in the monochrome days, how often did one see any domestic receiver with properly adjusted brightness and contrast controls? (Even though in those days there were plenty of test card transmission hours when it could be done quite easily.)
The French case is interesting. Certainly the choice of positive/AM for its 625-line system L was made to align with system E, with a view to simplifying the design of dual-standard receivers. I don’t think that made any difference to the performance achievable with full specification receivers, but it might have made a given performance level easier and less costly to reach. Bear in mind that the French setmakers had several years experience with “Strasbourg” and other border-area multi-standard receivers that had to deal with both positive/AM and negative/FM systems. (And there were one or two models that also covered System A.) And of course four-standard receivers (B, C, E and F) had been the norm in Belgium since 1953. Then with the arrival of French UHF transmissions, these moved to five-standard (B/G/H, C, E, F and L). It is reasonable to assume that the vast majority of the basic problems associated with valve-based multi-standard domestic receivers had been well-addressed by the European setmakers by the time that the UK requirement for dual-standard receivers arrived. To what extent those lessons were known or applied by the UK setmakers is another question, though.
Perhaps one advantage in the French case is that their wideband IF strips designed for System E would have provided more than enough gain for System L. I understand that in the 819-line only days, video bandwidth was a parameter that was often quoted for domestic receivers, with 9 MHz and 10 MHz being common numbers. In the dual-standard era, there is some evidence that there were two basic approaches, one which used the same bandwidth (6 MHz or near to it) for both systems, and the other with 6 MHz for 625 and 9 or 10 MHz for 819.
My impression is that the UK setmakers generally settled for 625 video bandwidths that were on the low side. Pye’s prototype dual-standard receiver (with 38.9 MHz IF) had a 4.25 MHz bandwidth on 625. I suspect that other makers also chose a basic dual-Nyquist IF curve (with additional traps for 405) which would have allowed only a 4.85 MHz bandwidth with the 39.5 MHz IF. On the other hand, full (3 MHz) bandwidth for 405 would have been readily obtainable. So such receivers did a much better job on 405 than they did on 625.
That brings in the question as to what, if any effect did horizontal definition have on the perception of picture contrast. Lower-than-desirable video bandwidth would result in somewhat smeared horizontal transitions, without say visibly sharp black to white changes. Would the existence of sharp changes create the impression of greater contrast? If so, then 625 was at a perceptual disadvantage as compared with 405 on some/many UK receivers.
Cheers,
Steve
If so, and let’s say for argument’s sake that the single 625 line was roundly two thirds as bright as the single 405 line, then repeat the experiment with two consecutive 405 lines at peak white, the rest blanked, as compared with three consecutive 625 lines (as near as possible on the same place on the screen) at peak white, the rest blanked. From a reasonable viewing distance, would the 405 and 625 line sets look to be of similar brightness? And so on in a few coarse steps up to a full raster.
Again if the individual 405 lines are brighter than the individual 625 lines, does this exacerbate the lininess issue? One might expect that a brighter line will make an adjacent “black” space more apparent than a less bright line. If that holds, then the perceived lininess would appear to increase in a non-linear manner (maybe approximately second order) as the number of lines is progressively decreased below the threshold minimum number required to produce a reasonably flat field. Fewer lines means brighter lines and wider black spaces, both of which would contribute to increased perceived lininess.
Lininess, with bright individual lines abutting visible black spaces, might also contribute to an impression of greater contrast than for a picture with the same overall contrast but with minimal lininess.
And yet another thought: to that same monitor, let’s apply switchable spot wobble to the 405 scan, such that with spot wobble switched in, the 405 field has about the same flatness as the 625 field. Then repeat the above experiment. What would be the expected outcomes?
I have also pondered the situation where the transmitted signal had “wrong” black and peak white levels. Then let’s assume a good receiving system with high-gain black-level agc, a contrast control as a gain control in the video amplifier, and black level clamping. So the black level at the tube should be correct, and could be adjusted in the normal way with the brightness control. Incorrect white level would show up as deviation of the peak white video signal amplitude from its nominal value. “Low” white level could be offset by turning up the contrast, assuming that there was enough spare video gain. So the transmission errors would not appear to be fundamentally inimical to obtaining a “full contrast” picture, at least with a properly specified receiving system. But with domestic receivers built to a lesser standard, perhaps there would be problems. And if, because of transmission errors, 625 typically required a higher video gain setting than 405, how many users adjusted the contrast control accordingly when changing from one to the other? (But then, back in the monochrome days, how often did one see any domestic receiver with properly adjusted brightness and contrast controls? (Even though in those days there were plenty of test card transmission hours when it could be done quite easily.)
The French case is interesting. Certainly the choice of positive/AM for its 625-line system L was made to align with system E, with a view to simplifying the design of dual-standard receivers. I don’t think that made any difference to the performance achievable with full specification receivers, but it might have made a given performance level easier and less costly to reach. Bear in mind that the French setmakers had several years experience with “Strasbourg” and other border-area multi-standard receivers that had to deal with both positive/AM and negative/FM systems. (And there were one or two models that also covered System A.) And of course four-standard receivers (B, C, E and F) had been the norm in Belgium since 1953. Then with the arrival of French UHF transmissions, these moved to five-standard (B/G/H, C, E, F and L). It is reasonable to assume that the vast majority of the basic problems associated with valve-based multi-standard domestic receivers had been well-addressed by the European setmakers by the time that the UK requirement for dual-standard receivers arrived. To what extent those lessons were known or applied by the UK setmakers is another question, though.
Perhaps one advantage in the French case is that their wideband IF strips designed for System E would have provided more than enough gain for System L. I understand that in the 819-line only days, video bandwidth was a parameter that was often quoted for domestic receivers, with 9 MHz and 10 MHz being common numbers. In the dual-standard era, there is some evidence that there were two basic approaches, one which used the same bandwidth (6 MHz or near to it) for both systems, and the other with 6 MHz for 625 and 9 or 10 MHz for 819.
My impression is that the UK setmakers generally settled for 625 video bandwidths that were on the low side. Pye’s prototype dual-standard receiver (with 38.9 MHz IF) had a 4.25 MHz bandwidth on 625. I suspect that other makers also chose a basic dual-Nyquist IF curve (with additional traps for 405) which would have allowed only a 4.85 MHz bandwidth with the 39.5 MHz IF. On the other hand, full (3 MHz) bandwidth for 405 would have been readily obtainable. So such receivers did a much better job on 405 than they did on 625.
That brings in the question as to what, if any effect did horizontal definition have on the perception of picture contrast. Lower-than-desirable video bandwidth would result in somewhat smeared horizontal transitions, without say visibly sharp black to white changes. Would the existence of sharp changes create the impression of greater contrast? If so, then 625 was at a perceptual disadvantage as compared with 405 on some/many UK receivers.
Cheers,
Steve
