Why did 405 look so good?

[Murphyv310]

Some of the early portables were pretty grim, cost cutting comes to mind.
One of the worst on 405 oddly enough is the Sony 990UB it's low frequency response is appalling with streaking and banding on some scenes, on 625 it is actually very good.

[peter scott]

http://www.vintage-radio.net/forum/showt...php?t=2464

[Murphyv310]

The link Peter raises a smile. I particularly hate the 990 especially as i worked for Sony. I dreaded spring time when the caravanners brought them in, in droves, time for my annual nervous breakdown. At least 405 had gone by then and most of the stripdowns were noisy controls and 12v socket issues.

[Geordie McBoyne]

I must introduce my all transistor Pye TT1 into the discussion.  Introduced in 1960 this 405 line mains and battery portable was one the first UK made transistor TVs to have a full size CRT, a 14" rectangular Cathodeon C14-23 which until recently I believed to be a special CRT made for portable TVs.  In fact it is the equivalent to the more common AW36-20.

Geordie McBoyne.

[Terry]

Just spotted the closed thread on UKVRR site from 1910, I must have missed it as I can't remember it. .... 

I think we can all be forgiven for suffering from memory loss after 107 years ...

  

[Refugee]

That post must have been stored on a mechanical adding machine.

[colly0410]

Oops, why did I put 1910? Brain fade, lol.... The Elazabethan T12 was very sensitive & always gave a picture of some sorts wherever it was used. Was on a caravan/camping site at Watermouth Cove North Devon & it gave almost perfect reception (apart from wandering black level) on it's loop aerial from Wevoe in South Wales. I'm wondering if it uses mean level AGC & AC coupling? Did any single standard 625 sets used mean level AGC? Mam got it free when she bought a Sony KV 1800 telly..

[colly0410]

Read somewhere (another TV tech site, practical TV mag?) that 3 MHz bandwidth for 405 was too much & it could have got away with 2.5 or even 2 MHz without anyone noticing. IMO 2 MHz is a bit low & would have been noticed but I think they could have got away with 2.5 MHz without anyone noticing. Maybe we could have crammed another channel in band 1 = channel 5A , maybe?

[Geordie McBoyne]

In the early days of 405 line TV a maximum video bandwidth of 2.7Mc/s was the ideal figure. The bandwidth figure was arrived at from an aspect ratio of 5:4 and an active video of 17/20 of the total line period, about 83microseconds. When the aspect ratio was changed to 4:3 more bandwidth was required so the figure is nearer 3Mc/s. Since the mid sixties 405 line television was sourced from a converted 625 waveform and to accommodate the characteristics of the 625 system the active video of a 405 line signal was reduced to about 80 to 81 Mc/s. Again slightly more video bandwidth was called for. The BBC digital 625 to 405 converter has a output video bandwidth of nearly 3.1Mc/s.

Geordie McBoyne.

[ppppenguin]

The "correct" bandwidth for a given number of lines and aspect ratio is a bit of a moveable feast. Nominally there's equal H and V resolution, nowadays expressed in pixels across against lines down. Simple calculation but....

Especially in interlaced systems the apparent vertical resolution is less than it might appear from the number of lines. This is the so-called Kell Factor, named after Raymond Kell: https://en.wikipedia.org/wiki/Kell_factor This article isn't well written and seems to contain serious errors (I don't think Mertz and Gray claimed a Kell Factor of 0.53 for example) but at least it introduces the subject.

If you allow for the Kell effect you can get away with less bandwidth than needed for equal H and V resolution. Hence modern TV systems do this. But 405 "System A" did not. It has always been seen as having rather more BW than strictly necessary.