OM8373 datasheet(86/108 Pages) PHILIPS

86 / 108 page

2004 Jan 15

Philips Semiconductors

Tentative Device Speciﬁcation

TV signal processor-Teletext decoder with

embedded

µ-Controller

OM8373; OM8378

51. This parameter indicates the bandwidth of the complete chrominance circuit including the chrominance bandpass

filter. The bandwidth of the low-pass filter of the demodulator is approximately 1 MHz.

52. The subcarrier output is combined with a 3-level switch output which can be used to switch external circuits like

sound traps etc. This output is controlled by the CMB1 and CMB0 bits in control byte 22H. The subcarrier signal is

available when CMB1/0 are set to 0/1.

53. Because of the 2-point black current stabilization circuit both the black level and the amplitude of the RGB output

signals depend on the drive characteristic of the picture tube. The system checks whether the returning measuring

currents meet the requirement and adapts the output level and gain of the circuit when necessary. Therefore the

typical value of the black level and amplitude at the output are just given as an indication for the design of the RGB

output stage.

The 2-point black level system adapts the drive voltage for each cathode in such a way that the 2 measuring currents

have the right value. This has the consequence that a change in the gain of the output stage will be compensated

by a gain change of the RGB control circuit. Because different picture tubes may require different drive voltage

amplitudes the ratio between the output signal amplitude and the inserted measuring pulses can be adapted via the

I2C-bus. This is indicated in the parameter ‘Adjustment range of the cathode drive level’.

Because of the dependence of the output signal amplitude on the application the soft clipping limiting has been

related to the input signal amplitude.

54. The alignment system for the Vg2 voltage of the picture tube can be activated by means of the AVG bit. In that

condition a certain black level is inserted at the RGB outputs during a few lines. The value of this level can be

adjusted by means of the brightness control DAC. An automatic adjustment of the Vg2 of the picture tube can be

realised by using the WBC and HBC bits in output byte 01. For a black level feedback current between 12 and 20

µA

the WBC = 1, for a higher or lower current WBC = 0. Whether the current is too high or too low can be found from

the HBC bit. The indication of these bits can be made visible on the screen via OSD so that this alignment procedure

can also be used for service purposes.

55. When the reproduction of 4 : 3 pictures on a 16 : 9 picture tube is realised by means of a reduction of the horizontal

scan amplitude the edges of the picture may slightly be disturbed. This effect can be prevented by adding an

additional blanking to the RGB signals. The blanking pulse is derived form the horizontal oscillator and is directly

related to the incoming video signal (independent of the flyback pulse). This blanking is activated with the HBL bit

(see Fig. 42).

56. This parameter is valid only when the CCC loop is active.

57. Signal-to-noise ratio (S/N) is specified as peak-to-peak signal with respect to RMS noise (bandwidth 5 MHz).

58. This is a current input. The timing of the measuring pulses and the vertical blanking for the 50/60 Hz standard are

given in Fig.40

The start-up procedure is as follows.

When the TV receiver is switched-on the RGB outputs are blanked and the black-current loop will try to adjust the

picture tube to the right bias levels. The RGB drive signals are switched-on as soon as the black current loop is

stabilised. This results in the shortest switch-on time.

When this switch-on system results in a visible disturbance of the picture it is possible to add a further switch-on delay

via a software routine. In that case the RGB outputs must be blanked by means of the RBL bit. As soon as the black

current loop is stabilised the BCF-bit is set to 0 (output byte 01). This information can then be used to switch-on the

RGB outputs with some additional delay.

59. The vertical guard function has been combined with the black current measuring input. For a reliable operation of the

protection system and to avoid that the black current stabilization system is disturbed the end of the protection pulse

during normal operation should not overlap the measuring pulses (see also Fig.40). Therefore this pulse must end

before line 14.

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