參數資料
型號： TDA938X
廠商： NXP Semiconductors N.V.
英文描述： TV signal processor-Teletext decoder with embedded m-Controller
中文描述： 電視信號處理器與嵌入式米圖文電視解碼器控制器
文件頁數： 123/140頁
文件大?。?/td> 570K
代理商： TDA938X
1999 Sep 28
123
Philips Semiconductors
Preliminary Device Specification
TV signal processor-Teletext decoder with
embedded
μ
-Controller
TDA935X/6X/8X series
36. To obtain a good performance for both weak signal and VCR playback the time constant of the first control loop is
switched depending on the input signal condition and the condition of the POC, FOA, FOB and VID bits in
subaddress 24H. The circuit contains a noise detector and the time constant is switched to ‘slow’ when too much
noise is present in the signal. In the ‘fast’ mode during the vertical retrace time the phase detector current is increased
50% so that phase errors due to head-switching of the VCR are corrected as soon as possible. Switching of the time
constant can be automatically or can be set by means of the control bits.
The circuit contains a video identification circuit which is independent of the first loop. This identification circuit can
be used to close or open the first control loop when a video signal is present or not present on the input. This enables
a stable On Screen Display (OSD) when just noise is present at the input.
To prevent that the horizontal synchronisation is disturbed by anti copy signals like Macrovision the phase detector
is gated during the vertical retrace period so that pulses during scan have no effect on the output voltage. The width
of the gate pulse is about 22
μ
s. During weak signal conditions (noise detector active) the gating is active during the
complete scan period and the width of the gate pulse is reduced to 5.7
μ
s so that the effect of noise is reduced to a
minimum.
The output current of the phase detector in the various conditions are shown in Table 122.
37. The ICs have 2 protection inputs. The protection on the second phase detector pin is intended to be used as ‘flash’
protection. When this protection is activated the horizontal drive is switched-off immediately and then switched-on
again via the slow start procedure.
The protection on the EHT input is intended for overvoltage (X-ray) protection. When this protection is activated the
horizontal drive is directly switched-off (via the slow stop procedure).
The EHT protection input can also be used to switch-off the TV receiver in a correct way when it is switched off via
the mains power switch or when the power supply is interrupted by pulling the mains plug. This can be realised by
means of a detection circuit which monitors the main supply voltage of the receiver. When this voltage suddenly
decreases the EHT protection input must be pulled HIGH and then the horizontal drive is switched off via the slow
stop procedure. Whether the EHT capacitor is discharged in the overscan or not during the switch-off period depends
on the setting of the OSO bit (subaddress 25H, D4). See also note 56.
38. The control range indicates the maximum phase difference at the top and the bottom of the screen. Compared with
the phase position at the centre of the screen the maximum phase difference at the top and the bottom of the screen
is
±
0.5
μ
s.
39. During switch-on the horizontal drive starts-up in a soft-start mode. The horizontal drive starts with a very short T
ON
time of the horizontal output transistor, the ‘off time’ of the transistor is identical to the ‘off time’ in normal operation.
The starting frequency during switch-on is therefore about 2 times higher than the normal value. The ‘on time’ is
slowly increased to the nominal value. When the nominal frequency is reached the PLL is closed in such a way that
only very small phase corrections are necessary. This ensures a safe operation of the output stage. The switch-on
characteristic is given in Fig.54
During switch-off the soft-stop function is active. This is realised by decreasing the T
ON
of the output transistor
complimentary to the start-up behaviour. The switch-off time is about 43 ms. The soft-stop procedure is synchronised
to the start of the first new vertical field after the reception of the switch-off command. Furthermore the EHT capacitor
of the picture tube is discharged with a fixed beam current which is forced by the black current loop. The discharge
time is about 38 ms. During switch-off the vertical scan is stopped so that the discharge takes place in the overscan.
The horizontal output is gated with the flyback pulse so that the horizontal output transistor cannot be switched-on
during the flyback time.
40. The vertical blanking pulse in the RGB outputs has a width of 26 or 21 lines (50 or 60 Hz system). The vertical pulse
in the sandcastle pulse has a width of 14 lines. This to prevent a phase distortion on top of the picture due to a timing
modulation of the incoming flyback pulse.
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