Electronic Components Datasheet Search |
|
GS1881-ITA Datasheet(PDF) 7 Page - Gennum Corporation |
|
GS1881-ITA Datasheet(HTML) 7 Page - Gennum Corporation |
7 / 14 page 7 520 - 23 - 03 BACK PORCH OUTPUT (pin 5) In an NTSC composite video signal, horizontal sync pulses are followed by the back porch interval. The device generates a negative going pulse on pin 5 during this time. It is delayed typically 500 ns from the rising edge of sync and has a typical width of 2.5 µs. Both of these times are set by the external R SET resistor. During the pre-equalizing, vertical sync, and post-equalizing periods, composite sync doubles in frequency. The GS4881 and GS4981 maintain the back porch output at the horizontal rate due to Back Porch Enable (BPEN), generated by the internal windowing circuit, which forces back porch to be asserted at the horizontal rate. This gating circuit is also the reason for the excellent impulse noise immunity of the back porch output as shown in Figure 14. The GS1881 does not gate the Back Porch which allows for total pin compatibility with the LM1881. VERTICAL SYNC OUTPUT (pin 3) The vertical sync interval is detected by integrating the composite sync pulses. The first broad vertical sync pulse causes an internal capacitor to charge past a fixed threshold and raises an internal vertical flag. Once the vertical flag is raised, the positive edge of the next serration clocks out the vertical output. When the vertical sync interval ends, the first post equalizing pulse is unable to charge the capacitor sufficiently, causing the internal vertical flag to go high. The rising edge of the second post-equalizing pulse then clocks out the high flag to end the vertical sync pulse. The vertical output is clocked in and out and therefore is a fixed width of 197.7 µs (3H + 4.7 µs + 2.3 µs). In the case of a non-standard vertical interval that has no serrations, a second internal capacitor is charged and clocks the vertical pulse out after typically 65 µs. In this case the end of the vertical pulse will still be the rising edge of the second post-equalizing pulse. As the vertical detector is designed as a true integrator, it provides improved noise immunity. VIDEO INPUT IMPULSE NOISE COMPOSITE SYNC RECOVERY TIME without INCREASED DISCHARGE CURRENT (LM1881) RECOVERY TIME T1 COMPOSITE SYNC RECOVERY TIME with INCREASED DISCHARGE CURRENT (GS1881, GS4881, GS4981) RECOVERY TIME T1 / 10 Fig. 14 Back Porch Noise Immunity Back Porch Output GS4881 GS4981 Video Input Impulse Noise CIRCUIT DESPCRIPTION The block diagrams for the GS1881, GS4881 and GS4981, are shown in Figures 17 through 19, with timing diagrams for the devices shown in Figure 20. When stimulated by a composite input signal, the GS1881 and GS4881 sync separators output composite sync, vertical sync, back porch, and odd/even field information. The GS4981 substitutes the odd/even output of the GS4881 with a horizontal output. An external resistor on pin 6 is used to define internal currents allowing the devices to accommodate horizontal scan rates from 15 kHz to 130 kHz. COMPOSITE VIDEO INPUT (pin 2) and COMPOSITE SYNC OUTPUT (pin 1) Composite video is AC coupled via an external coupling capacitor to pin 2. The device clamps the sync tip of the input video to 1.5 V ( V clamp ) and then slices at 77 mV above the clamp voltage ( V slice ). The resultant signal, provided at pin 1, is a reproduction of the input signal with the active video portion removed. As V clamp and V slice are supply and input signal independent, for 0.5 V p-p signals (sync height of 143 mV) slicing will occur at just above the 50% point and for 2 V p-p signals (sync height of 572 mV) slicing will occur at approximately 13% of sync height. The video signal path and composite sync slicing circuitry have been optimized and compensated to achieve a low propagation delay that is stable over temperature. The typical delay is 60 ns with less than 3 ns drift over the commercial temperature range. The typical input clamp discharge current is 11 µA. This current is optimal under normal operating circumstances but needs to be increased when the clamp is trying to recover from negative going impulse noise. The device improves the recovery time by raising a NOSYNC flag when there has not been a sync pulse for approximately 11/ 2 horizontal lines. When this flag is raised the discharge current is increased by 85 µA so that the recovery time is sped up by nearly 10 times. Figure 13 shows a comparison between the recovery times with and without the increased discharge current. Fig. 13 Impulse Noise: Recovery Time Comparison |
Similar Part No. - GS1881-ITA |
|
Similar Description - GS1881-ITA |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |