VDP 313xY
ADVANCE INFORMATION
2.4.2. Demodulator
2.4.4. Frequency Demodulator
The entire signal (which might still contain luminance)
is now quadrature-mixed to the baseband. The mixing
frequency is equal to the subcarrier for PAL and NTSC,
thus achieving the chrominance demodulation. For
SECAM, the mixing frequency is 4.286 MHz giving the
quadrature baseband components of the FM modu-
lated chrominance. After the mixer, a lowpass filter
selects the chrominance components; a downsam-
pling stage converts the color difference signals to a
multiplexed half rate data stream.
The frequency demodulator for demodulating the
SECAM signal is implemented as a CORDIC-struc-
ture. It calculates the phase and magnitude of the
quadrature components by coordinate rotation.
The phase output of the CORDIC processor is differ-
entiated to obtain the demodulated frequency. After
the deemphasis filter, the Dr and Db signals are scaled
to standard CRCB amplitudes and fed to the cross-
over-switch.
The subcarrier frequency in the demodulator is gener-
ated by direct digital synthesis; therefore, substan-
dards such as PAL 3.58 or NTSC 4.43 can also be
demodulated.
2.4.5. Burst Detection / Saturation Control
In the PAL/NTSC-system the burst is the reference for
the color signal. The phase and magnitude outputs of
the CORDIC are gated with the color key and used for
controlling the phase-lock-loop (APC) of the demodu-
lator and the automatic color control (ACC) in PAL/
NTSC.
2.4.3. Chrominance Filter
The demodulation is followed by a lowpass filter for the
color difference signals for PAL/NTSC. SECAM
requires a modified lowpass function with bell-filter
characteristic. At the output of the lowpass filter, all
luminance information is eliminated.
The ACC has a control range of +30...−6 dB.
Color saturation can be selected once for all color
standards. In PAL/NTSC it is used as reference for the
ACC. In SECAM the necessary gains are calculated
automatically.
The lowpass filters are calculated in time multiplex for
the two color signals. Three bandwidth settings (nar-
row, normal, broad) are available for each standard.
For PAL/NTSC, a wide band chrominance filter can be
selected. This filter is intended for high bandwidth
chrominance signals, e.g. a nonstandard wide band-
width S-VHS signal.
For SECAM decoding, the frequency of the burst is
measured. Thus, the current chrominance carrier fre-
quency can be identified and is used to control the
SECAM processing. The burst measurements also
control the color killer operation; they are used for
automatic standard detection as well.
2.4.6. Color Killer Operation
The color killer uses the burst-phase/burst-frequency
measurement to identify a PAL/NTSC or SECAM color
signal. For PAL/NTSC, the color is switched off (killed)
as long as the color subcarrier PLL is not locked. For
SECAM, the killer is controlled by the toggle of the
burst frequency. The burst amplitude measurement is
used to switch-off the color if the burst amplitude is
below a programmable threshold. Thus, color will be
killed for very noisy signals. The color amplitude killer
has a programmable hysteresis.
PAL/NTSC
SECAM
Fig. 2–4: Frequency response of chrominance filters
10
Micronas
MICRONAS [ MICRONAS ]
