Samsung S5PC110 Manual page 1534

S5PC110_UM 7 6BTVOUT & VIDEO DAC
7.14 APPENDIX
7.14.1 VERTICAL BAR PHEOMENON
The goal of oversampling filter in TVOUT module is to obtain 54MHz sample-rate data for DAC from 13.5MHz/
27MHz source sample-rate. The quality of interpolation result using FIR filter depends on the number of FIR filter
taps. Unfortunately, ideal interpolation filter needs infinite number of taps, which cannot be implemented. Thus,
practical consideration into the trade-off between interpolation quality and computational complexity is needed.
In TV application, it is the sub-carrier waveform that is the most sensitive to the quality of interpolation filter.
Source rate of 13.5MHz for CVBS output is too coarse to represent the shape of sub-carrier and the quality of
interpolation highly affects the shape of resulting 54 MHz sub-carriers. The error of sub-carrier induced by non-
ideal interpolation filter is basically implies distortions in chrominance components. However, in the case of CVBS
signal, the chrominance component and the luminance component are together to be mixed and transmitted via
one channel. Then, parts of the interpolation error appears in luminance parts if the CVBS signal is separated into
luminance component and chrominance components by the comb filter of TV decoder. According to an analysis
on this interpolation error, the error pattern is repeated with a frequency of 102.3 kHz and the error spectrum is
highly concentrated around 102.3 kHz. Thus, most of interpolation error is classified into luminance part by TV
decoder and this error is shown in the shape of brightness distortion in TV monitor.
The oversampling filter in S5PC110 adopts 95-tap interpolation for CVBS signal in order to minimize the vertical
bar phenomenon. Compared to 47-tap interpolation of legacy TVOUT version, the peak of vertical bar reduces by
about 0.1 IRE assuming the use of 6th order Butterworth analog interpolation filter.
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