The aspect ratio of the television picture shall be four units horizontally to three units vertically.
Scanning sequence
During active scanning intervals the scene shall be scanned from left to right horizontally, and from top to bottom vertically, at uniform velocities.
Number of lines and interlace
The number of scanning lines per picture shall be 625 interlaced two to one.
Picture and field frequencies
The nominal picture and field frequencies shall be 25 Hz and 50 Hz respectively, and equal to the appropriate sub‑multiples (625 and 312.5 respectively) of the line frequency.
Colour sub‑carrier and line frequencies
The colour sub‑carrier frequency (sc) shall be 4433618.75 Hz ±5 Hz and the relationship between sc and the line frequency (H) shall be:
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Video signal
The video signal comprises the synchronising information occurring alternately on a time division basis with the picture signal as shown in figure A3.1. The picture signal comprises the luminance component together with the chrominance components if present. The nominal synchronising pulse amplitude shall be 3/7 of the blanking to reference white amplitude. Table A3.2 specifies the timing intervals of the video signal and figure A3.2 illustrates this relationship.
Colour picture signal
Coding of the colour information shall be in accordance with the Phase Alternation Line (PAL) system. In this system the colour picture signal, EM, contains a luminance component E’Y, and also simultaneous chrominance components E’U sin(2sct) and E’V cos(2sct) with the phase of the latter component being changed by 180° on alternate lines. The equation of the colour picture signal is:
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where:
(a) the luminance component
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(b) the chrominance modulation signals
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(c) the colour difference signals
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E’R, E’G, E’B are, respectively, the red, green and blue gamma corrected voltages required for tristimulus matching. The value of the exponent for this correction shall be related to a display tube gamma of 2.8, and shall be consistent with optimum colour fidelity.
The sign of the term E’V cos(2sct) is positive during odd lines of the first and second fields and during even lines of the third and fourth fields (see figure A3.4).
As a consequence of the choice of colour difference signals the chrominance components both disappear in the absence of colour.
The resulting video signal for fully saturated primary and complementary colours at full luminance is shown in figure A3.2.
Bandwidth of colour difference signals
The colour difference signals shall be limited in bandwidth prior to modulation of the colour sub‑carriers by a circuit having an approximate Gaussian response, with an attenuation of not more than 3 dB at 1.3 MHz, but greater than 20 dB at and above 4.0 MHz.
Colour synchronisation
A burst of 10 cycles of the colour sub‑carrier frequency shall occur during each line blanking period, commencing nominally 5.6 microseconds after the half amplitude point of the leading edge of the line synchronising pulse. At the point of encoding the peak to peak amplitude of the sub‑carrier burst shall be 3/7 of the blanking to reference white signal amplitude and thereafter the amplitude and phase of the burst shall be preserved relative to the chrominance signal. The envelope of the sub‑carrier burst signal shall have a build‑up time consistent with the bandwidth restriction of the colour difference signals.
The sub‑carrier burst shall be omitted for nine consecutive lines starting at or before the beginning of each field blanking interval in a four field sequence as illustrated by figure A3.4. The phase sequence of the colour sub‑carrier burst is also detailed in figure A3.4.
The phase of the sub‑carrier burst relative to the +E’U signal reference phase axis shall be +135° on odd lines of the first and second fields and on even lines of the third and fourth fields and –135° on even lines of the first and second fields and on odd lines of the third and fourth fields (see figure A3.3).
During the transmission of monochrome signals the colour burst may be suppressed.
Special signals in the vertical blanking interval
Lines 10 to 22 and lines 323 to 335 may be transmitted for such purposes as control of equipment, source identification, data transmission and circuit performance testing.
Lines 17, 18, 330 and 331 are preferred for circuit performance testing purposes.
Teletext signals on lines 21 and 334 shall be restricted to closed caption services.
Note Closed caption signals are transmitted on page 801 of the teletext magazine.
Lines 22 and 335 are preferred for the measurement of noise, ie. only blanking level should be transmitted on these lines.
Detailed specifications relevant to Teletext and closed caption data transmissions are given in the Teletext System Standard, issued by the former Department of Transport and Communications.
Note Current Australian teletext and closed caption transmissions conform with the full presentation level 1 facilities defined in the Enhanced Teletext Specification given in European Standard (Telecommunications Series) ETSI EN 300 706 V1.2.1 (2003‑04).
The line numbering sequence is detailed in figure A3.4.