Burst phase

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 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 oscillators of the colour decoder in a PAL television set.

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.