TV system differences?
 

How did the vintage 405 Marconi system differ from the later analogue 625 PAL and today's digital system in general?

Re: TV system differences?
 

Have a read of this Wikipedia article, answers some of your questions.

https://en.m.wikipedia.org/wiki/Broa...vision_systems

Re: TV system differences?
 

Hi well that's a pretty big question with so many elements.

You could do worse than starting here.

https://en.wikipedia.org/wiki/405-li...evision_system

Also PAL refers to the method of encoding colour information, there are other standards for this such as NTSC.

There are also other line standards.

See here:

https://en.wikipedia.org/wiki/Broadc...vision_systems

Crossed with Frank

Cheers

Mike T

Re: TV system differences?
 

If you check the specs on the video signal itself, the main difference is the line rate which in the 625 system requires a 15.625 kHz line (horizontal) oscillator and in the 405 requires a 10.125kHz oscillator. The horizontal flyback time I think is a littlle longer in the 405 system too. The field rate is the same in both cases at 50Hz, so the vertical oscillator in the TV is the same, but there are some differences with equalizing pulses in the vertical sync block of the video signal with 625 system to improve the interlace.

Because of that different timing though, and the the way the line scanning current builds up with time, converting a set or making a set with dual horizontal scanning frequencies is not as simple as just altering the line oscillator frequency, the width & linearity have to be corrected as well.

On the other hand American 525 line , 60Hz field rate monochrome TV sets are easy to convert to 625 line/50 Hz sets as often the H osc will just pull into range and the vertical oscillator timing capacitor is easily reduced in value. (These are also FM sound sets with the same modulation system and 4.5MHz sound easily reset to anywhere between 5 & 6 MHz)

Also in the 405 system, the modulation of the transmitted video carriers was such that the sync reduced the carrier level. Opposite to the system later used in 625 line sets and in America. Also in the era of the 405 line system, the video carrier 45 MHz was transmitted 3.5 MHz above the sound carrier on 41.5Mhz, later the video carrier was transmitted on higher range VHF frequencies and 5, 5.5 or 6 MHz below the sound carrier depending on the country. And of course in the 625 system , by then, the sound was FM not AM too.

It is possible to make a vintage 405 line TV run on the modern 625 system (now obsolete) the steps that are required:

1) modifiy the line osc an line deflection circuit for 625 operation (Surprisingly this is relatively easy with sets with self oscillating line output stages, by increasing the core gap in the Lopty) In other sets the H oscillator is changed and the width & linearity compensated for with various methods.

2) Reverse the video detector diode

3) replace the audio detector with a discriminator or Ratio detector

4) Re-tune the RF circuits for a wider gap, say 46MHz video and 40.5MHz sound, if the sound-vision difference was say 5.5 MHz, I think its 6MHz UK , its 5.5 in NZ & AU.

5) add a VHF turret tuner to the front end (connect it to the antenna input) to down-convert the higher VHF channel frequencies (This also flips the vision carrier above the sound carrier)

I did this with a vintage 405 line set in NZ and it worked very well on the 625 line system there.

And thats just for monochrome TV.

As for digital TV, that's a whole other story.

Re: TV system differences?
 

Digital systems are completely different. The details vary, but generally speaking a number of different bitstreams are multiplexed together and transmitted on a single frequency. The digital TV or digibox decodes the multiplex to extract the desired signal, and then processes the bitstream to produce video and audio according to the standard in use. Digital TVs and digiboxes are essentially computers.

Re: TV system differences?
 

.. sorry should have said the vertical timing capacitor is easily increased, in value to get a 60 Hz field rate set to run at 50Hz, but many cases it comes in on the vertical hold control anyway when a USA made set is used in the UK or AU. Also an interesting bit of information, the American line rate was initially 15.750 KHz, but when color (NTSC) came it was slightly reduced to 15.734 kHz, and the vertical rate increased a fraction above 60Hz I think, but existing monochrome sets in the USA locked to these small changes ok.

Re: TV system differences?
 

29.97 Hz vertical rate, NTSC 525 line system.

Re: TV system differences?
 

Another thing about 405 vs 625 in the United Kingdom, is all the 405 transmitters operated at VHF and all the 625 transmitters were on UHF, so when the 405 line system was eventually switched off in 1985, the VHF allocations were not re-engineered for 625 as you might have expected, VHF was abandoned as far as TV was concerned.

Unlike NTSC in the USA , PAL does not define the 625 line monochrome signal. As transmitted in the UK, it should be referred to by its CCIR designation letter I. 405 lines was system A (naturally).

Colour was never transmitted on 405/VHF apart from a few BBC engineering tests, not intended for public consumption, and I think they were NTSC rather than PAL

Re: TV system differences?
 

I didn't know that. So UK made 625 line TV's would not have had VHF tuners in them. All the early NZ ones did and UHF came later. Surprisingly NZ was still running VHF & UHF TV analog when I was last there some months ago. Analog TV transmissions have been gone in AU for some time.

Re: TV system differences?
 

Making the vertical osc frequency or field rate just under 60Hz, rather than just above as I suggested in post #6.

Re: TV system differences?
 

Sadly that was usually the case. Problem was some areas had cabled systems with 625 at VHF (Stevenage was one), and blocks of flats were often like this as well. I had an argument with Philips over this when I wanted a set for such a situation. Didn't help though and I had to source a set from a continental manufacturer. Towards the end of the analogue era manufacturers, including Philips, started fitting all band tuners and still do to this day (except Panasonic).

Re: TV system differences?
 

The original post didn't ask about converting a set, or say in which direction.

The 405 line TV system was a monochrome system introduced in the UK in 1936. 50 fields per second with a half line offset creating 25 frames per second as the fields interlace together. This was an anti-flicker measure. Of the 405 lines, 377 are in the visible picture, the remainder are blanked in the retrace periods. The line frequency is 10.125 kHz.

405 signals were transmitted with positive modulation (white gives max transmitter output, sync gives mimimum) AM. All UK 405 transmitters were on VHF channels. Accompanying sound was transmitted as AM, offset 3.5 MHz above the vision carrier.

625 line PAL is not so easy, it covers a group of standards. Different countries chose different bandwidths and sound offsets, and radio frequency bands. All were colour. Sound standards varied. The standards were indicated by a suffix letter like PAL-I.

The 625 monochrome system introduced in the UK used negative modulation (peak transmitter power on the sync pulse) AM, and the sound was FM. These signals were only broadcast on the new UHF channels in the UK. The 50 fields/25 frames only remained. Line frequency became 15.625kHz

PAL colour signals were designed to be compatible with the monochrome ones, so monochrome TVs would give a monochrome rendition. PAL was derived from the earlier American NTSC system with several improvements, some of them quite complex. To understand PAL, it's easiest to study NTSC first so you can see why some things were done.

If you just want to understand the two systems, there is plenty of reading and wikipedia is a reasonable starting point. If you are aiming to convert signals, or convert a set, you have to decide on which 625 PAL standard.

David

Re: TV system differences?
 

Given that the OP lives in Trinidad and Tobago, I wonder if he is enquiring about the legacy systems used there rather than in the UK?

Re: TV system differences?
 

It may be PAL-M

David

Re: TV system differences?
 

The main difference is that 625 line TV, and digital, is that they were "watchable".

The UK 405 programs I saw were absolutely dreadful.

Re: TV system differences?
 

Do you mean that UK 405 productions looked bad on the Australian system? That's because they would have been 16mm telerecordings. The 405 system gave very good results on the 405 TVs of the day. It was abandoned for reasons of standardisation within Europe, not because it performed poorly.

Re: TV system differences?
 

405 line compared to 626 = not good, hence body filler attempts like spot wobble.

Not to mention the budgie killer line whistle8-o

Lawrence.

Re: TV system differences?
 

Very interesting, and very strange given that Philips were marketing standard all-band and VHF/UHF equipped sets in Ireland from the 60s onwards - what era are we talking about?

Rgds
John

Re: TV system differences?
 

Another minor difference between 405 and 625 is that transmissions of the former were very largely vertically polarised whereas the latter were very largely horizontal. I say very largely because some of the smaller local stations broke the general rule.

Peter

Re: TV system differences?
 

UK dual standard TV sets had both VHF and UHF tuners in them, along with the need for two antennae (usually opposite polarisations) and downleads.

There was no point in a VHF tuner in a 625 line single standard set. There were no suitable transmissions.

David