11/14

Notes on video line driving

TA0310

Reconstruction Filtering:

The output stage of an MPEG decoder is a DAC (digital analog converter). This DAC makes an analog

video signal from the digital one used for video treatments on previous stages. During this conversion the

sampling frequency will be unfortunately in the video spectrum as a parasitic frequency which causes

distortion and noise in the video band. A low-pass filter (called a reconstruction filter) is used to remove

this parasitic frequency. The cut-off frequency of this filter fits with the useful video bandwidth (6MHz,

12MHz or 30MHz). The order is directly linked to the value of the sampling frequency compared to the

video band. An increasing of the sampling frequency allows a lower order of the filter. Some of the newer

designs of DAC target a higher frequency sampling in order to remove the filtering on the future solutions.

Examples:

Figure 13: Filtering of the analog output

Figure 14: STi5518 (set-top-box chip). Fs=27MHz for SD outputs. 6MHz LPF to limit the 27MHz

Rg

video line

75

Ω

75

Ω

Rfb

+

-

+Vcc

TSH

-Vcc

LPF

Video Chip

DAC

Fs

Digital-to-Analog

Conversion

Filtering

(Reconstruction)

Buffering

Matching and Line

6MHz

1Vp-p

Amplitude

Frequency

27MHz

(STi5518)

Parasitic frequency due

to sampling of the DAC

21MHz

Filter