The growing popularity and availability of high-definition television (HDTV)
is creating a small revolution in the video industry. New video systems must
be capable of handling the standard National Television System Committee (NTSC)
composite signal as well as high-definition signals. Also, low-cost and low-power
concerns drive system designers to find the simplest solutions. Here's a one-transistor
network that lets a single video sync separator operate for both HDTV and NTSC
systems.
In the sample NTSC signal shown in Figure 1a,
the color burst and color subcarriers are identified. A "slice level" is drawn
halfway down the drop for horizontal synchronization. Variations in color burst
or dark blues within the subcarrier can dip below the slice level, causing false
sync pulses in addition to the 15-kHz horizontal sync signal. With high-definition
(HD) signals, the color information is carried separately (Fig.
1b). So, there's no color burst or subcarrier to cause false sync pulses.
However, note that the horizontal sync pulse is shorter and higher in frequency
(20 kHz).
It's advantageous if a single sync separator will operate with both HD and
NTSC signals. Because false triggers can occur with NTSC signals, adding a filter
in the sync-separator path can reduce the height of the color burst and subcarrier
signals. This filter cannot be included during HD detection, though, because
the shorter sync pulse also would be attenuated, causing missed triggers.
The ISL59885 is a sync separator that features both HD and NTSC detection.
An output, labeled HD, responds to the type of input—high for NTSC and
low for HD. This external pin can be used to insert a low-pass filter into the
sync separator path, preventing false sync pulses in composite video. The circuit
is shown in Figure 2.
When a composite signal (NTSC/PAL) is detected, the filter is enabled by applying a logic high to the base of the transistor. When component signal (HD) is detected, the filter is disabled by having the HD pin at a logic low state. Although the transistor is disabled during HD, a low-pass filter (RF-CF) is still present to filter out any noise present at the input.
The cutoff frequency for the NTSC filter is:
1/\[2π(RF)(CF + CF-HD)\] = 2.79 MHz
Given that RF = 100 Ω and CF + CF-HD = 570 pF, the 3.58-MHz color burst is
attenuated to 60% of the original 190-mV size while passing 15-kHz sync signals
without appreciable attenuation (Fig. 3a).
This is enough headroom to prevent false triggering because the slice level
is now ?70 mV and the new color-burst amplitude is 116 mV (Fig.
3b). Note that the lowest color subcarrier level in this example was
also attenuated from ?120 mV to ?8 mV. If we use a smaller CF-HD, the color
burst will attenuate less. Ideally, we want to maximize CF-HD to attenuate more
of the signal. But the cost of a bigger capacitor causes an increase in the
propagation delay and rounded falling edges on the sync pulses.
