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System A Vision Bandwidth

 
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System A Vision Bandwidth

Post by Synchrodyne » Sun Dec 27, 2015 7:09 am

I recently came across an item, copy attached, in the 1949 July “Foreign News” section of the American magazine “Radio-Electronics” that referred to the then-new UK Band I TV channel allocations. In referring to the vestigial sideband format that was to be used in channels 2 through 5, it was noted that the lower (full) sideband was 2.75 MHz wide.

RE 194907 p.58 UK TV Frequencies.gif
RE 194907 p.59 UK TV Frequencies.gif


This is less than the 3 MHz number usually quoted for System A. So at some stage a change was made from 2.75 to 3 MHz.

The 3 MHz number was quoted in an article in Wireless World (WW) for 1952 August (attached), which was essentially a précis of what came out of the CCIR 1951 meeting.

WW 195208 p.296.png
WW 195208 p.296.png


So somewhere between say mid-1949 and mid-1951, the bandwidth change was made. Thus we have at least a coarse fix on the “when”, which leaves the “why” to be addressed.

A plausible theory is that it coincided with the change of aspect ratio from 5:4 to 4:3. The latter was announced in WW 1950 March.

WW 195003 p.99 BBC Aspect Ratio Change.gif




If one runs the numbers, then to maintain the same relative horizontal definition as provided by a 2.75 MHz bandwidth with the 5:4 ratio would require 2.93 MHz with the 4:3 ratio, and possibly 2.93 was rounded up to 3 MHz for convenience.

Somewhere though, the exact when and why of the bandwidth change will have been recorded; at least one hopes so.

That 3 MHz was the bandwidth in use by 1952 is confirmed in a BBC letter in WW 1952 November.

WW 195211 p.457 TV Vision Bandwidth.gif


The BBC letter was in response to a 1952 October letter in which the correspondent had been advised otherwise by the setmakers.

WW 195210 p.420 TV Vision Bandwidth.gif




Returning to the TV system data shown in the WW 1952 article:

NTSC (525/60, later System M) bandwidth was shown as 4 MHz, the original 1941 number. That was later increased to 4.2 MHz, either before or coincident with the release of the NTSC colour system in 1953.

CCIR (625/50, later System B) was shown as having equalizing pulses in groups of 6, covering 3 lines. That was later changed to groups of 5, covering 2.5 lines, but just when that happened is not apparent.

The original channel dimensions were shown for the French 819/50 system (later System E). The slightly later tête-bêche channelling system retained the same basic numbers, but introduced a reversed bêche channel, and also spaced the vision carriers for like channels at 13.15 MHz, thus using up the 0.85 MHz of guard band, comprised 0.75 MHz beyond the vestigial sideband to the nominal channel edge and 0.1 MHz between the channel edge and the sound carrier. The tête-bêche system would appear to date from at least the 1952 Stockholm ITU meeting.

In 1951, the CCIR evidently ignored the Russian (OIRT) version of 625/50 in its report, perhaps reflecting the “cold war” politics of the time, and the Argentinean 625/50 version was missing. As the start data in Argentina was 1951 October, this was probably after the 1951 CCIR meeting. But the 6 MHz channel 625/50 variant was not new. The BBC RD Report 1949/33 “International TV Convention in Milan” included notes on a visit to Radio Turin, which had in operation a 625/50 transmitter in a 6 MHz channel, with vision at 83.25 MHz and sound at 87.75 MHz, which frequencies corresponded to American channel A6. The sound channel was FM at ±25 kHz deviation, as in the NTSC 525/60 system. Quite possibly the transmitter was of American origin. Italian channel C, 82.25/87.75 MHz, appears to have been derived from channel A6.

Another aspect though is that having very recently accepted the Gerber compromise of a 7 MHz channel for 625/50, the CCIR may not have been too keen on mentioning the other contenders. From the available literature, it is not clear whether, prior to the 7 MHz option, the greater weight of opinion was with the 6 MHz channel, thus aligning with North American practice, or with the 8 MHz channel, technically probably the “right” answer. The 6 MHz channel must have seemed attractive to some, and Philips developed its 567/50 system as the best fit for a 50 Hz system in a 6 MHz channel, but that I think would have been dead-on-arrival.

Cheers,

Steve

 
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Re: System A Vision Bandwidth

Post by Till Eulenspiegel » Sun Dec 27, 2015 1:20 pm

I believe the original System A specification allowed 2.75Mhz to be the minimum video bandwidth for a good detailed picture, this assumes the active video part of the transmitted waveform is 83 microseconds.
However, because since the late sixties almost all 405 line transmissions were originated from the 625 line standard the active video part of the waveform became 79 to 80 microseconds, thus 3 Mhz would be the ideal bandwidth to maintain the same horizontal detail.
The filter network at the output of the BBC digital 625 to 405 standards converter set the video bandwidth limit to 3.1Mhz.

Till Eulenspiegel.

 
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Re: System A Vision Bandwidth

Post by Till Eulenspiegel » Sun Dec 27, 2015 1:27 pm

I'm sure I'm not dreaming this but didn't Cossor suggest a 525/50 system for the post war TV system in the UK? The line timebase frequency would be 13,125c/s and whole waveform could be accommodated in the existing channel bandwidth. Max video being just over 3Mhz.

Till Eulenspiegel.

 
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Re: System A Vision Bandwidth

Post by Cathovisor » Tue Dec 29, 2015 4:19 pm

There's no reason for there to have ever been a change in the system bandwidth. The active line period remained the same: as far as I can tell the change in aspect ratio was simply because latterly more lines were blanked in the field interval - the Pye manual for the model 16 television points out that field blanking increased from about ten to fourteen lines per field. This would account for the change in aspect ratio.

 
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Re: System A Vision Bandwidth

Post by nuvistor » Tue Dec 29, 2015 5:26 pm

From the set makers point of view, a slight reduction in bandwidth would slightly increase the gain and most viewers would not notice the difference.
The TV manufacturers in the USA seem to have reduced the vision bandwidth before the early 50's, adverts from after that date boast the fact, quote" New Westinghouse Broadband TV gives you 22% more detail" quote from the book Window to the Future by Steve Kosareff, the book is filled with USA TV set adverts from the 1930's to 1970's

It is also possible that the sets needed to have the wider bandwidth to accomplish inter carrier sound which was also starting to be used by then.

Frank

 
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Re: System A Vision Bandwidth

Post by occiput » Tue Dec 29, 2015 5:57 pm

From Birkenshaw:

"Lastly, the greatest problem of all is that the modulator must show a linear frequency and phase characteristic between 0 and 3 mc/s." (October, 1939, so well in the 5:4 era).

Mindful of McLean's letter, and applying Occam's Razor, perhaps the most pertinent question might be: "Why did Hallows quote the wrong number in 1949?"

 
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Re: System A Vision Bandwidth

Post by Briancuff » Tue Dec 29, 2015 7:23 pm

The old cinema aspect ratio was 4 x 3 and I assume that as TV became more popular and more films were transmitted, it was a natural thing to change to the aspect ratio that everyone was used to from the cinema.

 
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Re: System A Vision Bandwidth

Post by Till Eulenspiegel » Tue Dec 29, 2015 10:52 pm

The converted 405 line system: The reason why the number of lines blanked out was increased to fourteen each field was again to accommodate the characteristics of the input 625 line standard. 405 blanking period = 1400 microseconds. The 625 blanks out twenty-five lines per field which is 1600 microseconds.

Interesting to note that the 441 line system transmitted from the Eiffel Tower actually displayed 383 active lines, only six more than the converted 405 line picture. Twenty-nine lines were blanked out each picture field = 2.6mS.
The field blanking commenced six lines before the eight broad field sync pulse train.
About the 441 line system: https://en.wikipedia.org/wiki/441-line_ ... ion_system

Till Eulenspiegel.

 
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Re: System A Vision Bandwidth

Post by occiput » Tue Dec 29, 2015 11:35 pm

Till Eulenspiegel wrote:The converted 405 line system: The reason why the number of lines blanked out was increased to fourteen each field was again to accommodate the characteristics of the input 625 line standard.


Not so. I have a waveform diagram dated January, 1950 in my copy of Birkinshaw showing 14 lines per field blanking interval. The change was made, as Cathovisor states, for the adoption of the 4:3 aspect ratio.

 
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Re: System A Vision Bandwidth

Post by Cathovisor » Tue Dec 29, 2015 11:40 pm

Till Eulenspiegel wrote:The converted 405 line system: The reason why the number of lines blanked out was increased to fourteen each field was again to accommodate the characteristics of the input 625 line standard. 405 blanking period = 1400 microseconds. The 625 blanks out twenty-five lines per field which is 1600 microseconds.

No, it was not. This change happened in 1950 - long before electronic standards conversion was even possible. If you look at the relevant pages of Birkinshaw in the library it clearly shows the frame blanking period as 14 lines, and those pages are dated January 1950. Plus the growing usage of film and the 'Academy' aspect ratio pretty much dictated it.

Oops - post crossed with Occiput's.

 
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Re: System A Vision Bandwidth

Post by Synchrodyne » Wed Dec 30, 2015 3:04 am

occiput wrote:From Birkenshaw:

"Lastly, the greatest problem of all is that the modulator must show a linear frequency and phase characteristic between 0 and 3 mc/s." (October, 1939, so well in the 5:4 era).

Mindful of McLean's letter, and applying Occam's Razor, perhaps the most pertinent question might be: "Why did Hallows quote the wrong number in 1949?"


Furtther searching has produced additional evidence in support of the 2.75 MHz number, although not unequivocally so.

In the BBC Research Report 1949/9 “Adjacent Channel Interference in Television Broadcasting” (http://downloads.bbc.co.uk/rd/pubs/reports/1949-09.pdf), it was stated: “As an alternative means of increasing the number of stations which can be operated in a given total bandwidth, a proposal was initially considered, in which the upper vision sideband of each channel overlapped the sound carrier of an adjacent channel. The proposed frequency separation between the respective vision and sound carriers was 2.65 Mc/s, so that slight overlapping would occur in the case of existing standards, for which in theory the vision sidebands spread to a limit of ± 2.75 Mc/s above and below the carrier.”

Later in the same report was this: “The decision was therefore made to adopt a vestigial sideband system of transmission for vision, similar to that which has been standardised in America. Whilst such a System must introduce some degree of distortion, for a vision signal, and in particular when positive modulation is used, the distortion is only just perceptible.”

And in the Conclusion section:

“The investigations have shown that it would be quite practicable to operate adjacent television channels with the vision carriers spaced at 5.0 Mc/s without introducing any attenuation to the transmitted vision sidebands, provided that the receiver is designed to have a vestigial sideband characteristic as shown in Fig. 1(b).

“Whilst such an arrangement would provide an acceptable service, it has been, however, considered desirable to include a relatively simple filter at the vision transmitter, the characteristic of which is approximately flat, with negligible attenuation, over the band extending from 3.0 Mc/s below the vision carrier frequency to 0.75 Mc/s above the carrier frequency and with attenuation not less than 10 db over the band from 1.5 Mc/s to 3.0 Mc/s above the carrier frequency.

“With such a characteristic, the interference would be imperceptible even in the worst case of equal received fields from the wanted and interfering stations.”


Then here is the opening sattement from BBC Research report 1947/36 “Interim Report on Sutton Coldfield Television Aerial” (http://downloads.bbc.co.uk/rd/pubs/reports/1947-36.pdf).

“This report constitutes an interim statement on the progress so far made on the design of a television aerial for the Sutton Coldfield Station; the sound channel will be on a frequency of 59.95 Mc/s, and vision in the band 60.75 - 66.15 Mc/s.”

At that time (1947), it would appear that Sutton Coldfield was envisaged as a double sideband transmitter with ±2.7 MHz vision bandwidth and positioned towards the upper end of Band I.

Then if one looks at BBC Research Report 1950/2 “Final report on Sutton Coldfield Television Aerial” (http://downloads.bbc.co.uk/rd/pubs/reports/1950-02.pdf), the graphs at the end of the report show the “vision band” as extending from 59.0 to 62.5 MHz, with the vision carrier at 61.75 MHz, implying a lower sideband 2.75 MHz wide.

It would be interesting to see how Wireless World magazine reported the announced Band I channelling scheme. But the mid-1949 issues, wherein one might expect to find the report, are not yet available at the customary resource, http://www.americanradiohistory.com/Wir ... gazine.htm.

Kerkhof & Werner reported a 3 MHz vision bandwidth and 4:3 aspect ratio for the British 405-line system. But a 4.12:3 aspect ratio was reported for the French 819-line system. Evidently this had been changed to 4:3 by the time of the 1951 CCIR standardization report. The English translation of Kerkhof & Werner was dated 1952, so the original was probably written somewhat earlier, evidently after the BBC aspect ratio change (and putative vision bandwidth change) and before the French aspect ratio change.

The 4:3 ratio in TV goes back at least to the NTSC system of 1941. Fink reported that this was largely based upon cinematic practice. See: http://www.earlytelevision.org/pdf/tele ... dards.pdf; chapter 8.

Cheers,

Steve P.

 
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Re: System A Vision Bandwidth

Post by Synchrodyne » Wed Dec 30, 2015 6:01 am

Till Eulenspiegel wrote:Interesting to note that the 441 line system transmitted from the Eiffel Tower actually displayed 383 active lines, only six more than the converted 405 line picture. Twenty-nine lines were blanked out each picture field = 2.6mS.
The field blanking commenced six lines before the eight broad field sync pulse train.
About the 441 line system: https://en.wikipedia.org/wiki/441-line_ ... ion_system.


There doesn’t seem to be all that much information available about the French 441-line system. Here is a small amount that I found some time back. Vision bandwidth seems to have been in the vicinity of 3.5 MHz, with vision-to-sound separation of 4.0 MHz.

French 441-Line A.png
French 441-Line B.png


Cheers,

Steve

 
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Re: System A Vision Bandwidth

Post by occiput » Wed Dec 30, 2015 12:41 pm

Synchrodyne wrote:
Furtther searching has produced additional evidence in support of the 2.75 MHz number, although not unequivocally so.



All proposals, though. No evidence so far that any of these schemes were actually adopted. Indeed, one thing we know for sure is that no proposal involving a move away from the nominal 3.5MHz sound-to-vision carrier separation was implemented.

The fundamental frequency of the finest grating on TCC was notionally 3MHz. This seems a curious thing to do if there had ever been a serious intention to band-limit the radiated signal to 2.75MHz or some similar figure.

Perhaps Hallows was simply reporting as established fact what were, in fact, no more than proposals. This is, after all, one of the traps into which many technical authors have fallen over the years.

 
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Re: System A Vision Bandwidth

Post by Cathovisor » Wed Dec 30, 2015 2:38 pm

occiput wrote:
Synchrodyne wrote:
Furtther searching has produced additional evidence in support of the 2.75 MHz number, although not unequivocally so.



All proposals, though. No evidence so far that any of these schemes were actually adopted. Indeed, one thing we know for sure is that no proposal involving a move away from the nominal 3.5MHz sound-to-vision carrier separation was implemented.

Seconded.

I sat and read through your post Steve (before going back to bed, it has to be said), and likewise all I could see were nothing more than proposals in the scanned texts.

 
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Re: System A Vision Bandwidth

Post by Synchrodyne » Wed Dec 30, 2015 11:15 pm

I am not sure that the final report (BBC RDR 1950/2) on the Sutton Coldfield TV aerial could be described as a proposal. To me it looks more like a post facto report, including measurements, on the installed item. Most of the measurements are presented in graphical form at the end of the report. In all cases the vision band is shown as extending from 58.0 to 62.5 MHz, with a lower sideband 2.75 MHz wide. I have extracted and attached one of the graphs.

BBC RDR 1950-02 p.15.gif


Quite how one should interpret this I do not know. But its does seem to be evidence that the 2.75 MHz number existed as more than a proposal. If not, why did the Sutton Coldfield aerial measurements not extend to 3 MHz lower sideband bandwidth.

Perhaps there was a duality, in that the 3.0 MHz number was aspirational, with as much equipment as reasonably possible designed to provide this vision bandwidth, but with a drop back to 2.75 MHz allowed where it would have been difficult or excessively costly to get to 3.0 MHz.

Also, how does one interpret this statement from BBC RDR 1949/9: “The proposed frequency separation between the respective vision and sound carriers was 2.65 Mc/s, so that slight overlapping would occur in the case of existing standards, for which in theory the vision sidebands spread to a limit of ± 2.75 Mc/s above and below the carrier.”

The idea of DSB transmission with overlapping channels, with 2.65 MHz between the vision carrier of one channel and the sound carrier of the next higher channel was surely just a proposal, and in the event a dead-end proposal, and thus may be set aside. But the ±2.75 MHz vision sideband range was a reference to existing standards, and not simply a proposal. Or is there another way to interpret that part of the statement that I have not seen.

So on the basis of the evidence that has so far come to light, I don’t think that we may easily renormalize the situation, as it were, to dismiss that 2.75 MHz number as being merely a proposal.

That does not mean that Hallows was correct in his reporting. But to prove that he was wrong, I think that we need more and persuasive evidence that justifies the setting aside of that pesky 2.75 MHz number as being irrelevant to the actuality at the time.

occiput wrote: Indeed, one thing we know for sure is that no proposal involving a move away from the nominal 3.5MHz sound-to-vision carrier separation was implemented.


And as far as I know, that was never proposed. What BBC RDR 1949/9 referred to was the possibility of 2.65 MHz separation between the vision carrier of one channel and the sound carrier of the next higher channel, which with DSB operation, implied a minor overlap of the vision upper sideband with the next-above sound carrier.

Cheers,
Attachments
BBC RDR 1950-02 p.15.png

 
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Re: System A Vision Bandwidth

Post by Synchrodyne » Thu Dec 31, 2015 12:03 am

occiput wrote:The fundamental frequency of the finest grating on TCC was notionally 3MHz. This seems a curious thing to do if there had ever been a serious intention to band-limit the radiated signal to 2.75MHz or some similar figure.


But effectively the question in play is: was there a change in transmitted video bandwidth from 2.75 MHz to 3.0 MHz, or was the number 3.0 MHz all along. Nothing has been said about a change to 2.75 MHz from a previous higher number.

Either way, the inclusion of 3 MHz gratings on Test Card C was logical. If say the transmitted video bandwidth had been 2.75 MHz when TCC was introduced (in 1948, I think), but it was planned to increase this to 3.0 MHz when feasible, it made sense to allow for this in the new test card. In fact I think Test Card A also had 3 MHz gratings, which does suggest an original intent to get to 3.0 MHz in due course, even if it wasn't quite done initially.

Cheers,

Steve

 
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Re: System A Vision Bandwidth

Post by occiput » Thu Dec 31, 2015 12:55 am

Synchrodyne wrote:
occiput wrote:The fundamental frequency of the finest grating on TCC was notionally 3MHz. This seems a curious thing to do if there had ever been a serious intention to band-limit the radiated signal to 2.75MHz or some similar figure.


But effectively the question in play is: was there a change in transmitted video bandwidth from 2.75 MHz to 3.0 MHz, or was the number 3.0 MHz all along. Nothing has been said about a change to 2.75 MHz from a previous higher number.

Either way, the inclusion of 3 MHz gratings on Test Card C was logical. If say the transmitted video bandwidth had been 2.75 MHz when TCC was introduced (in 1948, I think), but it was planned to increase this to 3.0 MHz when feasible, it made sense to allow for this in the new test card. In fact I think Test Card A also had 3 MHz gratings, which does suggest an original intent to get to 3.0 MHz in due course, even if it wasn't quite done initially.


In 1948, the only transmitter operating was the DSB one at AP. We know this radiated sidebands to 3MHz in 1939: Birkinshaw says it did. The same equipment remained in service until CP opened in 1956.

 
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Re: System A Vision Bandwidth

Post by colly0410 » Thu Dec 31, 2015 2:41 pm

Would any viewers out there in the wild ever notice the 1/4 Mhz difference with all the degradation's from ghosting, snow, spark plug, tram, trolley bus, & as European stations fired up sporadic E interference? I'm thinking about the Belgian 819 system that only went up to around 5 Mhz where as the french went up to around 10/11 Mhz, did Mr,Mrs,Miss Belgian Joe/Joan Blogs notice any difference?

 
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Re: System A Vision Bandwidth

Post by occiput » Thu Dec 31, 2015 3:30 pm

colly0410 wrote:Would any viewers out there in the wild ever notice the 1/4 Mhz difference with all the degradation's from ghosting, snow, spark plug, tram, trolley bus, & as European stations fired up sporadic E interference? I'm thinking about the Belgian 819 system that only went up to around 5 Mhz where as the french went up to around 10/11 Mhz, did Mr,Mrs,Miss Belgian Joe/Joan Blogs notice any difference?


The 0.25MHz: never mind the ghosting, snow etc etc, I'd be surprised if the difference was noticeable on anything other than a test signal.

Having seen full-bandwidth 819-line pictures off-air, I imagine the Belgian band-limited version would have looked rather odd since the vertical resolution would have been much better than the horizontal resolution (assuming the source was offering the full vertical resolution, of course).

 
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Re: System A Vision Bandwidth

Post by nuvistor » Thu Dec 31, 2015 3:45 pm

Even with a good noise/ghost free picture I don't think most viewers would have noticed. I was much too young to give an objective opinion of the picture but I remember watching the Coronation in 53, most of the family came round to watch. Also remember the Watch with Mother programs in I think the mornings if I was off school ill.

From memory the picture was always a bit soft on any highlights, probably the CRT giving out. The Bush replacement in around 1958 gave a much improved picture.

I think the main thing is the program content, many including myself would rather have a picture not 100% if the content is enjoyable, I just forget about the imperfections and concentrate on the content. Perhaps that's why with all the near perfect HD pictures available today I watch very little TV, the programs just are not to my taste.

That's my ramble for today :zx:

Frank

 
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Re: System A Vision Bandwidth

Post by Synchrodyne » Fri Jan 22, 2016 12:53 am

The visibility or otherwise of reduced video bandwidth is a hard item to call. I can recall that here in NZ, in the late 1960s, and before the full microwave link was completed, we had some network programs from Wellington that got to Auckland in several steps, including one or two off-air links. These definitely suffered from reduced horizontal definition, easily apparent on regular (monochrome) receivers that I suspect did not typically have the full 5 MHz (System B) video bandwidth, but probably rolled off at around 4.5 MHz, perhaps even before that. Possibly the interim link was limited to something like 4 MHz.

When I was living in the US, I noticed a distinct difference in picture sharpness between using the TV receiver’s own tuner, and feeding the output from a high-quality outboard tuner (Luxman T407) into the monitor input. I think that the Luxman was specified to the full NTSC video bandwidth of 4.2 MHz.

Still, one could say that Systems B and M were a bit short on vision bandwidth as they stood, such that their chosen numbers were still on the sloping part of the perceptibility vs. bandwidth curves, which in turn would have made reductions from nominal more visible. System A was, relatively speaking, much more generously specified, so that 3 MHz was probably on or close to the flat part of the perceptibility vs. bandwidth curve, leaving some margin for reduction with negligible effect on picture quality.

I suspect that the Belgian 819-line system, with 5 MHz video bandwidth, did not look too good. It was very much a compromise, required, as I understand it, so that all transmitters could handle both 625- and 819-line incoming programmes, even though they were nominally assigned to one or the other. This required alignment of the channel details. I think that Belgium abandoned its System F around 1968, standardizing on System C (625-lines) for the VHF channels. By then of course standards conversion was well established and routine, something that was not the case when the network was first planned.

In France in the late 1950s, video bandwidth was a published number for TV receivers, with 9 MHz being usual and 10 MHz also found. (See Wireless World 1959 October page 456.) System E had a nominal 10.4 MHz bandwidth. So one assumes that the difference between 9 and 10 MHz was perceptible to some viewers at least and so worth doing for the upper end of the market. Against that the degradation caused by the Belgian 5 MHz bandwidth would likely have been quite noticeable.

Belgian receivers were usually four-standard (Systems B,C,E,F) from the start. I am not sure about this, but I think that not much effort was made to realize greater video bandwidth on System E than on Systems B, C and F. Presumably the sound carrier trap required for Systems B, C and F could have been switched out for System E, and that would have allowed some bandwidth extension, but probably not all that much. The rationale might have been that as the primary (local) programmes were anyway narrow band, so could the secondary (French) programmes be received in narrow band.

The French also had some multistandard (“Strasbourg”) receivers for the border areas, but I’m guessing that these would have treated System E a lot better, and so would have clearly shown the difference between E and F.

I understand that the UK TAC, in its deliberations on 625 lines, said that 5 MHz video bandwidth was not enough, and noted that the planned 8 MHz UHF channels allowed 6 MHz. Then after further study it determined that the best trade-off in terms of overall picture quality was 5.5 MHz with the extra 0.5 MHz allocated to a wider vestigial sideband. Having lived in both System B areas (NZ and Australia) and System I areas (UK and RSA), my impression was that on typical receivers, System I pictures were typically a bit better, that is a bit sharper and with fewer artefacts, “cleaner”, one might say.

Returning to the original issue, namely early System A bandwidth, it seems as though the BBC provided some conflicting information during the late 1940s. One may see how the setmakers and others, if they read the Sutton Coldfield aerial reports, could infer that the provincial transmitters had lower bandwidth (2.75 MHz) than did Alexandra Palace (3 MHz).

As well as the Sutton Coldfield data, there was also this from Research Department Report 1947/3, “Television Interference: Effect of Aircraft Radiating “G.H.” Pulses”:

“Regarding the vision channel, the result obtained depended, as would be expected, on the actual-pass band of the receiver used. The ideal
receiver, however, should accept without. attenuation a frequency band from 42.3 to 47.7 Mc/s, but would be required to attenuate by 40 db or more over the band occupied-by the pulses.”


Or in other words, the ideal receiver bandwidth was seen as being 2.7 MHz, even though the transmitted bandwidth from Alexandra Palace was 3 MHz.

From BBC Research Report 1949/9, one gains the impression that the BBC was somewhat reluctant to adopt vestigial sideband transmission because of the distortion it introduced, and only did so when alternatives, such as channel overlapping, were deemed to be unsatisfactory. The VSB distortion aspect was also mentioned in BBC Monograph 39 of 1961 October, “Twenty-five years of BBC Television”. Therein it was said: “The total band (1.5 Mc/s) occupied by the ‘Nyquist flank’ in the receiver characteristic is a larger fraction of the video bandwidth than in any of the other television systems now in use, and distortion due to vestigial sideband working is therefore lower.”

VSB to main sideband ratios for the various systems were:

A 405-line 0.25
H 625-line 0.25
I 625-line 0.227
K’,L 625-line 0.208
E 819-line 0.192
M 525-line 0.179 (originally 0.188)
N 625-line 0.179 (originally might have been 0.188)
B,C,G 625-line 0.15
F 819-line 0.15
D,K 625-line 0.125

Cheers,

Steve

 
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Re: System A Vision Bandwidth

Post by Synchrodyne » Sat Apr 02, 2016 11:21 pm

Synchrodyne wrote:It would be interesting to see how Wireless World magazine reported the announced Band I channelling scheme. But the mid-1949 issues, wherein one might expect to find the report, are not yet available at the customary resource, http://www.americanradiohistory.com/Wir ... gazine.htm.


Those 1949 WW issues are now available, and the UK Band I TV channelling scheme was announced in the May issue, p.175, copy attached.

WW 194905 p.175.gif


Therein the bandwidth for the vision chain including transmitters was said to be 2.75 MHz.

In view of what has gone before in this thread, that leaves me somewhat puzzled.

Back in 1939, the “working” number for vision bandwidth was clearly 3 MHz.

In the late 1940s, “published” number dropped to 2.75 MHz.

Then around 1950 or so, the “published” number went back to 3 MHz.

I wonder if 2.75 MHz was a conservative number, chosen on the “underpromise but overdeliver” principle, before those involved were sure that the 3 MHz number achieved in a combined studio/transmitter installation was replicable in a network situation.

Cheers,

Steve

 
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Re: System A Vision Bandwidth

Post by colly0410 » Sun Apr 03, 2016 4:33 pm

Brilliant how you keep posting these magazine clips from years ago, thanks I love reading them..

 
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Re: System A Vision Bandwidth

Post by Synchrodyne » Sun Apr 17, 2016 11:52 pm

Thanks for the feedback and no problem!

The interesting thing is that this is all relatively recent history, yet it is not always easy to find robust answers to the questions, and when one does, they sometimes go against conventional wisdom.

In a simplistic way, you could say that the generalized question is – who did what, when did they do it, why did they do it and how did they do it. One expects to have difficulties with the “why” absent first-hand accounts. But “when” is not an easy find, and sometimes the “who” can be difficult.

And then as this thread shows, searches sometimes turn up conflicting answers from equally reputable sources. If no resolution is to hand, my usual approach is to leave it be and look at something else for a while. Then sooner or later, and often quite accidentally, one will trip across another key piece of the puzzle or make an obscure connection that allows one to go forward another few steps. Having a few open-ended threads running on this and other forums is a spur to keeping a look out for new datapoints and then thinking their consequences through enough to write a coherent post.

Cheers,

Steve

 
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Re: System A Vision Bandwidth

Post by Synchrodyne » Tue Jul 12, 2016 5:23 am

“Wireless World” (WW) for 1948 December (http://www.americanradiohistory.com/Arc ... 948-12.pdf) was significantly devoted to TV matters, particularly the British 405-line system, for which a subject supplement was provided.

There was a section on British TV Standards, supplement page 4, attached.

WW 194812 S.4 British TV Standards.gif


Therein it was clearly stated the Alexandra Palace transmitter vision bandwidth was 3 MHz. And it was also implied that Sutton Coldfield would be the same, except for vestigial sideband operation, with 0.75 MHz upper sideband.

Case solved then? Yes, but...

That was followed by an article on “Why the 405 Standard is Best”. On the second page of that article (attached) it was stated that the BBC vision bandwidth was 2.7 MHz.

WW 194812 S.6 405 Lines.gif


Then another article on international TV quotes 2.735 MHz. (Highly improbable precision, though, and the aspect ratio was not 4:3 at that time; it was still 5:4.)

WW 194812 S.13 International TV.gif


Still, one wonders how the WW editorial staff allowed conflicting information to be published. But then as the BBC itself published conflicting numbers, as shown in earlier postings, I guess that one could excuse WW for doing the same. And the BBC appeared to have used 2.75 MHz as the design basis for the Sutton Coldfield aerials.


Cheers,

Steve


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