Burst phase is the first ten
cycles of
colorburst in the "porch" of the synchronising
pulse in the
PAL (Phase Alternation Line)
broadcast television systems
Broadcast television systems (or terrestrial television systems outside the US and Canada) are the encoding or formatting systems for the transmission and reception of terrestrial television signals.
Analog television systems were standardized ...
format. The frequency of this burst is 4.43361875 MHz; it is precise to 0.5 Hz, and is used as the reference frequency to synchronise the
local oscillator
In electronics, the term local oscillator (LO) refers to an electronic oscillator when used in conjunction with a Frequency mixer, mixer to change the frequency of a signal. This frequency conversion process, also called Heterodyne, heterodyning ...
s of the colour decoder in a PAL
television set
A television set or television receiver (more commonly called TV, TV set, television, telly, or tele) is an electronic device for viewing and hearing television broadcasts, or as a computer monitor. It combines a tuner, display, and loudspeake ...
.
This colorburst is sometimes called a "swinging burst", since it swings plus or minus 45 degrees line by line (hence the expression "phase alternating line"). This swing is used to set the centre frequency of the colour reference oscillator in the decoder. The swing of the burst phase distinguishes PAL from non-PAL lines, and produces the IDENT signal at 7.8 kHz half the line scan of 15,625 kHz.
As in the NTSC system, U and V are used to modulate the color subcarrier using two balanced modulators operating in phase quadrature: one modulator is driven by the subcarrier at sine phase; the other modulator is driven by the subcarrier at cosine phase. The outputs of the modulators are added together to form the modulated chrominance signal:
C=Usin ωt±Vω=2πFSC
FSC=4.43361875 MHz(±5 Hz) for (B, D, G, H, I, N) PALFSC=3.58205625 MHz (±5 Hz) for (NC)PALFSC=3.57561143 MHz(±10 Hz) for (M)PAL
In PAL, the phase of V is reversed every other line. V was chosen for the reversal process since it has a lower gain factor than U and therefore is less susceptible to a one-half FH switching rate imbalance. The result of alternating the V phase at the line rate is that any color subcarrier phase errors produce complementary errors, allowing line-to-line averaging at the receiver to cancel the errors and generate the correct hue with slightly reduced saturation. This technique requires the PAL receiver to be able to determine the correct V phase. This is done using a technique known as AB sync, PAL sync, PAL switch, or swinging burst, consisting of alternating the phase of the color burst ten cycles long, by ±45° at the line rate hence colour phase errors are reduced or not evident watching TV pictures.
References
Television technology
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