Dawn of Television and 405 vs 625 vs 525

How and why particular line rates were chosen

Because an interlaced system requires accurate positioning of scanning lines it is important to make sure that the horizontal and vertical timebases are in a precise ratio. This is accomplished by passing the one through a series of electronic divider circuits to produce the other. Each division is by a prime number. Therefore there has to be a straightforward mathematical relationship between the line and field frequencies, the latter being derived by dividing down from the former. Technology constraints of the 1930s meant that this division process could only be done using small integers, preferably no greater than 7, for good stability. The number of lines was odd because of 2:1 interlace. The British 405 line system for example used a vertical frequency of 50 Hz (Standard AC mains supply frequency in Britain) and a horizontal one of 10,125Hz (50 × 405 ÷ 2) Interlacing is a method of displaying images on a raster-scanned display device, such as a cathode ray tube (CRT). ... In mathematics, a prime number (or a prime) is a natural number which has exactly two distinct natural number divisors: 1 and itself. ... Hz or hz may mean: Herero language (ISO 639 alpha-2, hz) Hertz, unit of frequency This is a disambiguation page — a list of articles associated with the same title. ...

* 2 × 3 × 3 × 5 Gives 90 (not interlaced)
* 2 × 2 × 2 × 2 × 2 × 3 Gives 96 (not interlaced)
* 2 × 2 × 3 × 3 × 5 Gives 180 (not interlaced)
* 2 × 2 × 2 × 2 × 3 × 5 Gives 240 (Used for the experimental Baird transmissions in England -not interlaced)
* 3 × 3 × 3 × 3 × 3 Gives 243
* 7 × 7 × 7 Gives 343 (Early North American system also used in Poland before WW2)
* 3 × 5 × 5 × 5 Gives 375
* 3 × 3 × 3 × 3 × 5 Gives 405 (Used in Britain Ireland and Hong Kong before 1985)
* 2 × 2 × 2 × 5 × 11 Gives 440 (not interlaced)
* 3 × 3 × 7 × 7 Gives 441 (Used by RCA in North America America before the 525 standard was adopted and widely used before WW2 in Continental Europe with different frame rates )
* 2 × 3 × 3 × 5 × 5 Gives 450
* 5 × 7 × 13 Gives 455 (Used in France before WW2)
* 3 × 5 × 5 × 7 Gives 525 (A compromise between the RCA and Philco systems Still used today In America and parts of Asia)
* 3 × 3 × 3 × 3 × 7 produces 567 (used for a while after WW2 in the Netherlands)
* 5 × 11 × 11 Gives 605 (Used experimentally by Philco in North America before the 525 standard was adopted)
* 5 × 5 × 5 × 5 gives 625 (Still used today in most parts of the world)
* 3 × 3 × 7 × 13 Gives 819 (Used in France in the 1950’s)

Even some digital TV line rates (i.e. 480 and 576) are based on the number of active lines in the 525 and 625 analogue video standards which preceded them.

How and why particular frame/field rates were chosen

Since the mid-1930s it has been standard practice to use a field frequency equal to or a submultiple of the AC mains electric supply frequency 50 Hz in most countries (60 Hz in others) because studio lighting generally uses alternating current lamps and if these were not synchronized with the field frequency, an unwelcome strobe effect could appear on TV pictures. Secondly the smoothing (filtering) of power supply circuits in early TV receivers was rather poor and ripple superimposed on the DC could cause visual interference. If the picture was locked to the mains frequency, this interference would at least be static on the screen and thus less obtrusive. By the 1930s most countries had standardized their public electricity supplies on either 50Hz or 60Hz and this heavily influenced the choice of frame rate (12.5 25 or 30 fps)


Other considerations

The bandwidth required to transmit a television signal increases as the field rate and particularly as the line rate increases. In the interests of economic use of the available radio frequency spectrum, it is important that the bandwidth of the TV signals does not become too large; a more pressing consideration in the 1930s was that given the limits of the technology of the day, wide-bandwidth signals were very difficult to handle within transmitter, receiver and antenna circuitry. Thus while it may have been desirable to maximize picture quality by using very high line and frame rates, in practice there were limits to what could be used. This article does not cite any references or sources. ...


The THEORETICAL bandwidth requirements of the different standards are as follows

* (number of lines * frame rate) * (number of lines * aspect ratio) /2
* 405 line (405 * 25) * (405 * 4/3) /2 = 2733750 Hz or 2.75 MHz (5:4 aspect ratio pre-1950)
* 525 line (525 * 30) * (525 * 4/3) /2 = 5512500 Hz or 5.52 MHz (29.98 Hz field rate with Colour NTSC)
* 625 line (625 * 25) * (625 * 4/3) /2 = 6510417 Hz or 6.51 MHz
* 819 line (819 * 25) * (819 * 4/3) /2 = 11179350 Hz or 11.2 MHz

In practice most broadcast TV systems use slightly less video bandwidth sacrificing some horizontal resolution in the process.

The 1946 timing is confirmed in a comment I have since found at:

http://www.bvws.org.uk/405alive/faq/405_hist.html

The rest of Europe opted for 625 lines, a system devised in 1946 by two German engineers, Möller and Urtel (it appears that the Russians came up independently with a very similar system and if you had set anyone else the problem - to Europeanise the American 525-line standard - they would have come up with something pretty similar). In Geneva a Mr W. Gerber proposed this as a European system and it has remained in use until the present day.

It seems, then, that the Russian version of the 625 line system was more the result of synchronicity than imitation. And that the Russian choice of 6 MHz video bandwidth could simply have been an engineering decision, there being no precisely "right" number for this parameter.

Mike 1972 wrote: »

Ireland and South Africa but where else ?

TV broadcast system parameters

Total    Active  Vertical      Aspect    Horizontal     Frate rate
lines    lines   resolution    ratio     resolution

525       484       242         4/3         427         30 or 29.94
625       575       290         4/3         425            25


Sys-  Lines Frames Lines Ch bw Vid bw Vid-Aud   Vestig Video    Color      Sound    Country/
tem           /s     /s   MHz   MHz   sep, MHz  sb MHz  mod      MHz     mod  swing organization
                                                                               kHz

(A)    405    25   10125   5    3       -3.5     0.75  C3Fpos     --     A3E   --   England
 B     625    25   15625   7    5       +5.5     0.75  C3Fneg  4.433618  F3E   50   EBU
(C)    625    25   15625   7    5       +5.5     0.75  C3Fpos     --     A3E   --   Belgium
 D     625    25   15625   8    6       +6.5     0.75  C3Fneg  4.433618  F3E   50   Soviet Union
(E)    819    25   20475  14   10     +-11.15    2     C3Fpos   (8.37)   A3E   --   France
(F)    819    25   20475   7    5       +5.5     0.75  C3Fpos            A3E   --   
 G     625    25   15625   8    5       +5.5     0.75  C3Fneg  4.433618  F3E   50   EBU
 H     625    25   15625   8    5       +5.5     1.25  C3Fneg  4.433618  F3E   50   Belgium
 I     625    25   15625   8    5.5     +5.996   1.25  C3Fneg  4.433618  F3E   50   England
 K     625    25   15625   8    6       +6.5     0.75  C3Fneg    4.3     F3E   50   Soviet Union
(K')   625    25   15625   8    6       +6.5     1.25  C3Fneg    4.3     F3E   50   
 L     625    25   15625   8    6       +6.5     1.25  C3Fpos    4.3     A3E   --   France
M/b&w  525    30   15750   6    4.2     +4.5     0.75  C3Fneg     --     F3E   25   USA/Japan
M/NTSC 525   29.97 15734   6    4.2     +4.5     0.75  C3Fneg  3.579545  F3E   25   USA/Japan
M/PAL  525   29.97 15734   6    4.2     +4.5     0.75  C3Fneg  3.575611  F3E   25   Brazil
 N     625    25   15625   6    4.2     +4.5     0.75  C3Fneg  3.582056  F3E   25   Argentina


System   Band   Video     Colour    Video   Audio       Band    Band
         lower  lower   freq  lvl   upper   freq lvl    upper   width

M,N     -1.25   -0.75   3.58  -7    4.2     4.5  -7     4.75    6.0

B       -1.25   -0.75   4.43 -16    5.0     5.5 -10     5.75    7.0

L       -1.25   -0.75   4.43 -16    6.0     6.5 -10     6.75    8.0

G       -1.25   -0.75   4.43 -16    5.0     5.5 -13     6.75    8.0
 dual sound                                 5.74 -20


Colour TV standard parameters


System            NTSC      ----------------------- PAL -------------------------       SECAM
CCIR               M           B,G,H          I            M            N            B,D,G,H,K,L

Luminance    ----------------------------- Y = 0.299 R + 0.587 G + 0.114 B -----------------------
signal

Chrominance  I=-0.27(B-Y)   ------------------- U=0.493(B-Y) ---------------------   D(B)=1.505(B-Y)
               +0.74(R-Y)   ------------------- V=0.877(R-Y) ---------------------   D(R)=-1.902(R-Y)
signals      Q=+0.41(B-Y)
               +0.48(R-Y)

Colour
subcarrier

Modulation   ------------------ QAM - Quadrature Amplitude Modulation ------------        FM

Frequency     3.579545 MHz   4.43361875   4.43361875   3.57561149   3.58205625 MHz   f(B)=4.25000 MHz
                 +-10 Hz        +-5 Hz      +-5 Hz       +-10 Hz       +-10 Hz       f(R)=4.40625 MHz
                                                                                         +- 2 kHz

Colour burst      min 8        10 +- 1      10 +- 1       9 +- 1       9 +- 1             none
duration
(periods)



Systems A, C, E, F and K' are obsolete and no longer used. These obsolete systems were used in:
A: England, Ireland: discontinued in 1986.
C: Belgium
E: France, Monaco: discontinued in 1986
F:
K': French overseas territories

The engineering compromise is vestigial sideband modulation. In vestigial sideband the full upper sideband of bandwidth W2 = 4 MHz (USA) is transmitted, but only W1 = 1.25 MHz of the lower sideband is transmitted, along with a carrier. This effectively makes the system AM at low modulation frequencies and SSB at high modulation frequencies. The absence of the lower sideband components at high frequencies must be compensated for, and this is done by the RF and IF filters.

Mike 1972 wrote: »

How long before (BTW it was 1964 when 625 started 1967 was the date for commencment of regular colour broadcasts although there had been tests since the mid-late fifties)