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CP-850/F Service Manual
5.3.1.2 Input Selector
The analog CVBS or SVHS luma signal are fed to the inputs CVBS1...7 of VSP94x2A (amplitude 0.5...1.5 V pp ). One
signal is selected and fed to the first ADC. A second signal (SVHS Chroma) can be selected and fed to the other ADC.
After clamping to the back porch both signals are AD-converted with an amplitude resolution of 9 bit. The AD
conversion is done using a 20.25 MHz free-running stable crystal clock. Before the A to D conversion the signals are
lowpass filtered to avoid antialias effects. One input is looped back to output CVBSO1(#63). A signal addition is
performed to output a CVBS signal even when separate Y/C signals are used at input.
5.3.1.3 Signal Levels And Gain Control To adjust to different CVBS input voltages a digitally working automatic gain
control is implemented. Input voltages in the range between 0.6 to 1.8 V pp can be applied to the CVBS inputs.
5.3.1.4 Synchronization
After elimination of the high frequent components of the CVBS signal by a low pass filter, horizontal and vertical sync
pulses are separated. Horizontal sync pulses are generated by a digital phase locked loop. The time constant is
adjusted between fast and slow behaviour to accommodate different input sources (e.g. VCR).
5.3.1.5 Chroma Decoder
The digital multistandard chroma decoder is able to decode NTSC and PAL signals with a subcarrier frequency of 3.58
MHz and 4.43 MHz as well as SECAM signals with automatic standard detection. The TV controller software has
configured the colour decoder to operate in automatic detection mode. When the signal source comes from the tuner,
only SECAM and PAL (50Hz) standard are enabled. In AV mode or when program number 0 is selected the following
standards are also enabled : NTSC M, NTSC 4.43 and PAL 60. The demodulation is done with a regenerated colour-
carrier.
5.3.1.6 Luminance Processing
A luminance notch filter is implemented to separate the chroma information from the luminance. Depending on the
colour standard, one out of three different notch characteristics is chosen ('PAL', 'NTSC', 'SECAM') automatically.
5.3.1.7 RGB Front-End
An analogue RGB input port for an external RGB source is available. The incoming signal is clamped to the back porch
by a clamping pulse. This input as an overlay input (soft mix). The RGB signal must then be synchronised to the main
CVBS/YC signal.
5.3.1.8 Signal Processing
5.3.1.8.1 Horizontal Prescaler
The main application of the horizontal prescaler is the conversion of the number of pixels coming form the 40.5/20.25
MHz pixel clock domain down to the number of pixels stored in the memory (factor 2/3). Generally the number of
incoming pixels can be decimated by a factor between 1 and 64 in a granularity of 2 output pixels. The horizontal scaler
reduces the number of incoming pixels by subsampling. To prevent the introduction of alias distortion low pass filters
are used for luminance and chrominance processing. The horizontal prescaler consists of two main subsampling
stages. The first stage is a scaler for rational decimation factors in a range of 1 to 2. The second stage decimates in
integer steps (1,2,3,4...32).
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