Origins of System I

[Andy Green]

While reading through some old Parctical Television Magazines from the late 50's to about 1964, it is interesting to see the evolution of progress towards 625 lines. Initially, the consensus was that most likely the UK would adopt a system with negative vision modulation, intercarrier sound with FM and 5.5 MHz spacing, which is referred to as the CCIR standard (there is no reference to system letters then that I can see)
Howver, around 1963, the planned system is announced as having the above standards, but with 6 MHz sound spacing.

I wonder why this decision was made, and bearing in mind that Ireland adopted this standard (hence system I) in 1961 when TV started here.

Can anyone shed any light on why this decision was made?

[Michael Maurice]

From what I remember being tolds to me, the decision was made so that the vision bandwidth could be extended up to 5.5Mhz thus giving slightly better resolution.

The fact that most TV's of the day and many more modern ones couldn't resolve the full vision bandwidth was neither here nor there.

[paulsherwin]

Originally Posted by Andy Green

I wonder why this decision was made, and bearing in mind that Ireland adopted this standard (hence system I) in 1961 when TV started here.

I don't think the standard was called PAL I because the Irish used it The various standards were lettered sequentially starting with VHF and then UHF. The Irish don't actually use proper PAL I as this is a UHF standard, and would probably have used PAL B/G if it wasn't for the Brits and particularly the existence of Northern Ireland.

These spec differences were largely down to protectionism by the big European nations. That's why France adopted SECAM with AM sound.

The early 60s editions of Radio & TV Servicing have introductory articles on the ongoing negotiations about common European TV standards, including colour standards. There was a big lobby in the UK for NTSC, particularly from the manufacturers as it would have kept costs down. The commercial broadcasters had lobbied hard for NTSC on 405 lines in the late 50s and also supported NTSC.

Paul

[Heatercathodeshort]

I think the UK's preference in the use of 6mcs sound was something to do with the i.f. frequency used [38mcs] and the very close proximity to BBC1 channel 1 [41.5mcs] I believe there would have been patterning problems if a slightly different i.f. frequency had been used. Channel 1 was only used in the U.K. I used to work within close range of the Crystal palace transmitter and I can remember the odd display when the local oscillator packed up in the vhf [405 line]tuner. BBC1 would come through at almost watchable quality due to the i.f. strip acting as a TRF amplifier! The Thorn 3000 had provision for the fitting of filters on the i.f. board to eliminate channel 1 breakthrough. The picture would be covered in patterns in the Wimbledon area without retro fitting these filters. Regards, John.

[ppppenguin]

I'm sure the answer to this will be found in BBC Research reports. http://www.bbc.co.uk/rd/pubs/reports/index.shtml

The service planning guys at BBC Research did all the broadcast band and transmitter planning in the UK. In the 1960s they devised the plan that gave us 4 channels per TX site along with almost complete national coverage on UHF.

A quick glance at the list of reports shows several that might have useful information. It's also possible that the report showing the actual findings was kept confidential. It may have been jointly issued with ITA and BREMA.

[Andy Green]

Originally Posted by paulsherwin

I don't think the standard was called PAL I because the Irish used it The various standards were lettered sequentially starting with VHF and then UHF. The Irish don't actually use proper PAL I as this is a UHF standard, and would probably have used PAL B/G if it wasn't for the Brits and particularly the existence of Northern Ireland.Paul

Well, I don't think this could be the case, as Irish TV started in 1961 using 625 lines and what is now known as system I (6Mhz intercarrier sound) - the UK didn't start using it until 1964 - Indeed if you look at contemporary reports of tests on UHF 625 lines from Crystal Palace, many of them used 5.5 MHz spacing - as for the PAL colour standard - this wasn't even invented back then.
Ireland did also radiate 405 lines system A (standards converted from 625 lines, either optically or electronically), but only in areas which were close to (and had reception of) UK TV

[paulsherwin]

Originally Posted by Andy Green

Well, I don't think this could be the case, as Irish TV started in 1961 using 625 lines and what is now known as system I

Well, yes, but it wasn't called PAL-I then, as that is a colour standard. RTE and the BBC had a lot of contact and they both probably agreed to use the same UHF monochrome standard for obvious reasons. Theoretically they could have ended up using different colour systems (Ireland on PAL and Britain on NTSC/50 for example) but this would obviously not have been a sensible development.

Paul

[Ray Cooper]

Choice of 6MHz sound/vision spacing was a welcome example of thinking ahead a while, bearing in mind that colour hadn't even been introduced at that time.

I think that a couple of the factors that might have influenced this thinking wwere as follows:-

a) With a 6MHz S/V spacing, it becomes feasible for the colour subcarrier information to be transmitted completely double-sideband. The colour-difference information (R-Y, B-Y) is nominally generated with a 1MHz bandwidth, so colour information sidebands will occupy space between 3.43 and 5.43 MHz. This somewhat eases decoder design. The American NTSC system didn't use R-Y and B-Y of course, it used I and Q signals. Due to the positioning of colour subcarrier at 3.58MHz, one of these signals (can't remember which) had to be transmitted vestigial sideband: the other was so narrow-band that it fitted in anyway. A marvellous example of cunning design, but not applicable to PAL where both colour-difference signals had ther same bandwidth.

b) Placing the colour subcarrier further away from the sound carrier meant that there'd be less problem controlling group delay (GD) across that part of the spectrum. GD is introduced by any component having a sharpish frequency-amplitude falloff, and the sound/vision combining unit is a prime example of this, causing GD anomalies (think of them as phase disturbances, if you like) across the upper end of ther passband which have to be pre-corrected for. This pre-correction becomes easier the further the sound carrier is from the colour subcarrier.

So there.

[ppppenguin]

ISTR that System B/G uses group delay pre-distortion at the TX to compensate for the difficult sound/vision separation filters at the RX. The 8MHz channel for Sytem I also allows a wider VSB (1.25MHz vs 0.75MHz) which again made receiver design easier. Not a problem with a modern synchronous demodulator.

Systems D,K,L use 6.5MHz sound spacing which must make the filter designers life even easier. They used 0.75MHz VSB to fit in an 8MHz channel.

The wideband I, narrowband Q in NTSC was a clever idea to fit the perception of the human eye. In practice most coders and receivers used narrow bandwidth for both and worked on the U/V axes rather than the 57 degree offset of I/Q.

None of this really answers the question "Why I?" but it's useful background info and shows that System I was a good compromise provided you had 8MHz channels. This may have been less feasible in areas where a 7MHz channel plan was already in use. This problem was severe in Argentina where 625/50 was used with narrow (5.5MHz?) channels appropriate to 525/60. When colour came they had to use PAL-N with a very low SC frequency.

[Mr Hoover]

Reading issues of Television magazine from 1957/58 the initial
Crystal Palace Uhf tests around ch 44 used 6.5 MHz
sound vision spacing.Presumably 405 & 625 lines were used together with
some NTSC tests??
Think after 1958 Crystal Palace Uhf "went quiet" till 1961-62 though Uhf
was used for some OB links

[irishtv]

I disagree about System I being purely UHF - ISTR a 1972 edition of the WRTH stating that the RoI used System I at a time when RTE did not use UHF. All RTE 625 transmissions were on VHF at that time.

I met with Paddy Clarke, the curator of the then RTE museum in 1984, who told me that Ireland had pioneered System I - on VHF - ahead of the UK, along with standards conversion of 625 to 405.

The System I VHF channels were also deliberately assigned letters (A-C, D-I) to avoid confusion with the 405 line VHF channels.

[FERNSEH]

Pye was involved with the installation of the 405 line transmitter and equipment at the Dublin Station. Did the same firm install the 625 line transmitter?
The first electronic 625 to 405 standards converter was designed by the BBC and Pye constructed converters to the BBC design and sold them to other broadcasters. Was the Pye converter used by RTE?

I'd consider France to be the pioneer country of standards converter design.
After the introduction of the 819 line system a converter was required to maintain the 441 line standard.

DFWB.

[irishtv]

The original 625 line transmitter at Kippure was a Pye transmitter.

[John Robson]

Originally Posted by ppppenguin

I'm sure the answer to this will be found in BBC Research reports. http://www.bbc.co.uk/rd/pubs/reports/index.shtml

The service planning guys at BBC Research did all the broadcast band and transmitter planning in the UK. In the 1960s they devised the plan that gave us 4 channels per TX site along with almost complete national coverage on UHF.

A quick glance at the list of reports shows several that might have useful information. It's also possible that the report showing the actual findings was kept confidential. It may have been jointly issued with ITA and BREMA.

Planning was and still is European matter, alongside the development of PAL
and the 625 line system. Part of a EBU effort, just as modern DVB-T (DTT).

The plan in Europe was for three UHF TV channels, while the UK wanted
four channels.

The two bands 4 and 5. Are broken up into three groups of 13 channels
(Allowing for N+5,N+9 IF problems) which normal are sufficent for grouping
the 4 main TV stations together across the country.

The bandplan works well except in the south east of england were Heathfield,
Dover, Midhurst, Hannington and Bluebell Hill share adjacent channels for
channel 4 which under normal conditions isn't a problem, except Hannington
and Dover need to share the same channel (66), this can cause problems
for some viewers in the direction of Newbury, whose aerial is pointing at
Hannington and in the same direction as Dover.

[paulsherwin]

Originally Posted by irishtv

I disagree about System I being purely UHF - ISTR a 1972 edition of the WRTH stating that the RoI used System I at a time when RTE did not use UHF. All RTE 625 transmissions were on VHF at that time.

I met with Paddy Clarke, the curator of the then RTE museum in 1984, who told me that Ireland had pioneered System I - on VHF - ahead of the UK, along with standards conversion of 625 to 405.

The System I VHF channels were also deliberately assigned letters (A-C, D-I) to avoid confusion with the 405 line VHF channels.

It's certainly true that RTE started 625 line transmissions on that standard before the UK, so in that sense they pioneered it. The RTE 405 service was only to support people who had bought UK spec sets to watch the BBC from Holme Moss or Divis. I believe the initial 625-405 conversion was done using the 'tv camera pointing at a monitor' method but RTE later bough a Pye standards converter similar to the ones then being used in the UK. This soldiered on until the mid 80s when it was again replaced by a camera/monitor until the end of Irish 405.

Paul