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AD7183 Fiches technique(PDF) 10 Page - Analog Devices |
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AD7183 Fiches technique(HTML) 10 Page - Analog Devices |
10 / 40 page REV. 0 –10– ADV7183 In S-video mode there are two clamp controllers used to sepa- rately control the luminance clamping and the chrominance clamping. Also in YCrCb component input mode there are two clamp controllers used to control the luminance clamping and the CrCb clamping separately; there are, however, individual current clamps on the Cr and Cb inputs. User programmability is built into the clamp controllers which enable the current and digital clamp controllers to be set up to user-defined conditions. Refer to analog clamp control register (14H), digital clamp control register (15H), and digital color clamp offset register (15H and 16H) for control settings. ANALOG-TO-DIGITAL CONVERTERS Two 10-bit ADCs are used in the ADV7183, and they run from a 27 MHz input clock. An integrated band gap generates the required reference voltages for the converters. If the decoder is configured in CVBS mode, the second ADC can be switched off to reduce power consumption, see PSC[1:0]. AUTOMATIC GAIN CONTROL The AGC control block on the ADV7183 is a digitally based system. This controller ensures that the input video signal (CVBS, S-video, or YCrCb) is scaled to its correct value such that the YCrCb digital output data matches the correct gain of the video signal. The AGC has an analog input video range of 0.5 V p-p to 2.0 V p-p, which gives a –6 dB to +6 dB gain range. Figure 6 demonstrates this range. This AGC range will compensate for video signals that have been incorrectly terminated or have been attenuated due to cable loss or other factors. There are two main control blocks: one for the luminance chan- nel and one for the chrominance channel. The luminance automatic gain control has eight modes of operation: 1. Manual AGC mode where gain for the luminance path is set manually using LGM[11:0]. 2. Blank level to sync tip is used to set the luminance gain; manual MIRE[2:0] controls the maximum value through the luminance channel. There is no override of this mode when white peak mode is detected. 3. Blank level to sync tip is used to set luminance gain; manual MIRE[2:0] controls the maximum value through luminance channel. There is override of this mode when white peak mode is detected. White peak mode is activated when the input video exceeds the maximum luminance range for long periods; this mode is designed to prevent clipping of the input video signal. 4. Blank level to sync tip is used to set luminance gain; MIRE[2:0] is automatically controlled to set the maximum value through the luminance channel. There is no override of this mode when white peak mode is detected. 5. Blank level to sync tip is used to set luminance gain; manual MIRE[2:0] is automatically controlled to set the maximum value through the luminance channel. There is override of this mode when white peak mode is detected. White peak mode is activated when the input video exceeds the maxi- mum luminance range for long periods; this mode is designed to prevent clipping of the input video signal. 6. Based on active video peak white. PW_UPD sets the gain update frequency (once per field). 7. Based on average active video. PW_RES sets what lines are used; only relevant if the signal conforms to PAL 625 line standard. 8. The luminance channel gain is frozen at its present value. 6 0 –6 0 RANGE = 12dB MAXIMUM MINIMUM Figure 6. Analog Input Range The chrominance automatic gain control has four modes of operation: 1. Manual AGC mode where gain for chrominance path is set manually using CGM[11:0]. 2. Luminance gain used for chrominance channel. 3. Chrominance automatic gain based on color burst amplitude. 4. Chrominance gain frozen at its present setting. Both the luminance and chrominance AGC controllers have a programmable time constant that allows the AGC to operate in four modes: slow, medium, fast, and video quality controlled. The maximum IRE (MIRE[2:0]) control can be used to set the maximum input video range that can be decoded. Table I shows the selectable range. Table I. MIRE Control Function MIRE[2:0] PAL (IRE) NTSC (IRE) 0 0 0 133 122 0 0 1 125 115 0 1 0 120 110 0 1 1 115 105 1 0 0 110 100 1 0 1 105 100 1 1 0 100 100 1 1 1 100 100 |
Numéro de pièce similaire - AD7183 |
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Description similaire - AD7183 |
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