參數(shù)資料
型號(hào)： TMC2491A
廠商： Fairchild Semiconductor Corporation
英文描述： Multistandard Digital Video Encoder(多標(biāo)準(zhǔn)數(shù)字視頻編碼器)
中文描述： 多標(biāo)準(zhǔn)數(shù)字視頻編碼器（多標(biāo)準(zhǔn)數(shù)字視頻編碼器）
文件頁數(shù)： 21/36頁
文件大?。?/td> 208K
代理商： TMC2491A
PRODUCT SPECIFICATION
TMC2490A/TMC2491A
21
Filtering Within the TMC2490(1)A
The TMC2490(1)A incorporates internal digital filters to
establish appropriate bandwidths and simplify external ana-
log reconstruction filter designs.
The chroma portion of the incoming digital video is band-
limited to reduce edge effect and other distortions of the
image compression process. Chrominance bandwidth is
selected by CHRBW. When LOW, the chrominance pass-
band attenuation is <3 dB within
±
650 kHz from f
SC
. The
stopband rejection is >26 dB outside f
SC
±
2 MHz. When
HIGH, the chrominance passband attenuation is <3 dB
within
±
1.3 MHz from f
SC
. The stopband rejection is
>33 dB outside f
SC
±
4 MHz.
Figure 9. Color-Difference Low-Pass Filter Response
The Chroma Modulator output and the luminance data are
digitally filtered with sharp-cutoff low-pass interpolation fil-
ters. These filters ensure that aliased subcarrier, chromi-
nance, and luminance frequencies are sufficiently suppressed
above the video base-band.
Figure 10. Chrominance and Luminance Interpolation
Filter – Full Spectrum Response
-70
-60
-50
-40
-30
-20
-10
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Frequency (MHz)
A
24490A
Wideband
Narrowband
-70
-60
-50
-40
-30
-20
-10
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13
Frequency (MHz)
A
24487A
Virtually all digital-to-analog converters have a response
with high frequency roll-off as a result of the zero-order hold
characteristic of classic D/A converters. This response is
commonly referred to as a sin(x)/x response. The sin(x)/x vs.
sampling frequency is shown in Figure 12.
Figure 11. Chrominance and Luminance Interpolation
Filter – Passband Detail
The TMC2490(1)A’s digital interpolation filters convert the
data stream to a sample rate of twice the pixel rate. This
results in much less high frequency sin(x)/x rolloff and the
output spectrum between f
S
/4 and 3 x f
S
/4 contains very lit-
tle energy. Since there is so little signal energy in this fre-
quency band, the demands placed on the output
reconstruction filter are greatly reduced. The output filter
needs to be flat to f
S
/4 and have good rejection at 3 x f
S
/4.
The relaxed requirements greatly simplify the design of a fil-
ter with good phase response and low group delay distortion.
A small amount of peaking may be added to compensate
residual sin(x)/x rolloff.
Figure 12. Sin(x)/x Response
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
0
1
2
Frequency (MHz)
3
4
5
6
A
24488A
-6
-5
-4
-3
-2
-1
0
0
1
2
3
4
5
6
7
8
Fs=13.5Msps
Fs=27.0Msps
(Oversampled)
Frequency (MHz)
A
24489A
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