bt865a Conexant Systems, Inc., bt865a Datasheet - Page 61

• Manufacturer Part Number: bt865a
• Description: Ycrcb To Ntsc/pal Digital Video Encoder
• Manufacturer: Conexant Systems, Inc.

Bt864A/Bt865A Data Sheet
3.5.4
Figure 3-4. Wiring for the Reset Input Pin
3.5.5
100138C
02/17/03
Reset Precautions
Filtering RF Modulator Connection
The user should make the length of the traces connected to the RESET* input pin as
short as possible. In addition, Conexant recommends that a 0.1
connected across the RESET* input pin and the digital ground pins (GND) for
decoupling purposes.
All of Bt864A/Bt865A’s programmable register bits can be reset through software
(i.e., setting register bit SRESET= 1). Furthermore, both the Bt864A/Bt865A’s
registers and timing can be reset by a low pulse of at least 0.05 µs (>1 complete period
of CLK) input directly to RESET*. If noise, having a pulse width close to 0.05 µs, is
inadvertently input to RESET*, it could cause the encoder to unintentionally reset the
subcarrier phase and/or the horizontal and vertical counters (see
type of timing error could cause faulty system operation (see
The Bt864A/Bt865A internal upsampling filter alleviates external filtering
requirements by moving significant sampling alias components above 19 MHz and
reducing the sinx/x aperture loss up to the filter’s passband cutoff of 5.75 MHz. While
typical chrominance subcarrier decoders can handle the Bt864A/Bt865A output
signals without analog filtering, the higher frequency alias products pose some EMI
concerns and may create troublesome images when introduced to an RF modulator.
When the video is presented to an RF modulator, it should be free of energy in the
region of the aural subcarrier (4.5 MHz for NTSC, 5.5–6.5 MHz for PAL), hence
some additional frequency traps may be necessary when the video signal contains
fundamental or harmonic energy (as from unfiltered character generators) in that
region. For example, a pixel rate of 13.5 msps is three times the NTSC-M aural carrier
of 4.5 MHz, hence significant harmonic energy can fall on the FM aural carrier for
character cell sizes which are multiples of three. Where better frequency response
flatness is required, some peaking in the analog filter is appropriate to compensate for
residual digital filter losses with sufficient margin to tolerate 10% reactive
components.
A three-pole elliptic filter (1 inductor, 3 capacitors) with a 6.75 MHz passband can
provide at least 45 dB attenuation (including sinx/x loss) of frequency components
above 20 MHz and provide some flexibility for mild peaking or special traps. An
inductor value with a self-resonant frequency above 80 MHz is chosen so that its
intrinsic capacitance contributes less than 10% of the total effective circuit value. The
RESET*
GND
Conexant
0.1 µF
Short as Possible
Figure
µ
100138_032
Figure
F capacitor be
PC Board Considerations
3-4).
1-9). This
3-7