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ADV7185 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADV7185 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 40 page REV. 0 –10– ADV7185 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 12-bit ADCs are used in the ADV7185, 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 ADV7185 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 termi- nated or have been attenuated due to cable loss, or other factors. There are two main control blocks: one for the luminance channel and one for the chrominance channel. The luminance automatic gain control has eight modes of operation: 1. Manual AGC mode where gain for luminance path is set manually using LGM[11:0]. 2. Blank level to sync tip is used to set luminance gain; manual MIRE[2:0] controls the maximum value through 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 the active video peak white. PW_UPD sets the gain update frequency (once per video per field). 7. Based on the 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) 000 133 122 001 125 115 010 120 110 011 115 105 100 110 100 101 105 100 110 100 100 111 100 100 |
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