TDA9845 查看數據表（PDF） - Philips Electronics

零件编号

产品描述 (功能)

生产厂家

TDA9845

TV and VTR stereo/dual sound processor with digital identification

Philips Electronics 

Philips Semiconductors

TV and VTR stereo/dual sound processor

with digital identification

Preliminary specification

TDA9845

FUNCTIONAL DESCRIPTION

AF signal handling

The input AF signals, derived from the two sound carriers,

are processed in analog form using operational amplifiers.

Dematrixing uses the technique of two amplifiers

processing the AF signals. Finally, a source selector

provides the facility to route the mono signal through to the

outputs (‘forced mono’).

De-emphasis is performed by two RC low-pass filter

networks with internal resistors and external capacitors.

This provides a frequency response with the tolerances

given in Fig.4.

A source selector, controlled via the control input ports

allows selection of the different modes of operation in

accordance with the transmitted signal. The device was

designed for a nominal input signal (FM: 54% modulation

is equivalent to ∆f = ±27 kHz) of 250 mV RMS (Vi 1, Vi 2)

and for a nominal input signal (AM: m = 0.54) of 500 mV

RMS (Vi 1), respectively 250 mV RMS (Vi 3). A nominal

gain of 6 dB for Vi 1 and Vi 2 signals (0 dB for Vi 1 signal

(AM sound)) and 6 dB for Vi 3 signal is built-in. By using

rail-to-rail operational amplifiers, the clipping level

(THD ≤1.5%) is 1.60 V RMS for VP = 5 V and 2.65 V RMS

for VP = 8 V at outputs Vo 1, Vo 2. Care has been taken to

minimize switching plops. Also total harmonic distortion

and random noise are considerably reduced.

Identification

The pilot signal is fed via an external RC high-pass filter

and single tuned LC band-pass filter to the input of a gain

controlled amplifier. The external LC band-pass filter in

combination with the external RC high-pass filter should

have a loaded Q-factor of approximately 40 to 50 to

ensure the highest identification sensitivity. By using a

fixed coil (±5%) to save the alignment (see Fig.2), a

Q-factor of approximately 12 is proposed. This may cause

a loss in sensitivity of approximately 2 to 3 dB. A digital

PLL circuit generates a reference carrier, which is

synchronized with the pilot carrier. This reference carrier

and the gain controlled pilot signal are fed to the

AM-synchronous demodulator. The demodulator detects

the identification signal, which is fed through a low-pass

filter with external capacitor CLP (pin 4) to a Schmitt-trigger

for pulse shaping and suppression of low level spurious

signal components. This is a measure against

mis-identification.

The identification signal is amplified and fed through an

AGC low-pass filter with external capacitor CAGC (pin 3) to

obtain the AGC voltage for controlling the gain of the pilot

signal amplifier.

The identification stages consist of two digital PLL circuits

with digital synchronous demodulation and digital

integrators to generate the stereo or dual sound

identification bits which can be indicated via LEDs.

A 10 MHz crystal oscillator provides the reference clock

frequency. The corresponding detection bandwidth is

larger than ±50 Hz for the pilot carrier signal, so that

fp-variations from the transmitter can be tracked in the

event of missing synchronization with the horizontal

frequency fH. However the detection bandwidth for the

identification signal is made small (±1 Hz) to reduce

mis-identification.

Figure 2 shows an example of the alignment-free fp

band-pass filter. To achieve the required QL of around 12,

the Q0 at fp of the coil was chosen to be around 25

(effective Q0 including PCB influence). Using coils with

other Q0, the RC-network (RFP, CFP) has to be adapted

accordingly. It is assumed that the loss factor tanδ of the

resonance capacitor is ≤0.01 at fp.

Copper areas under the coil might influence the loaded Q

and have to be taken into account. Care has also to be

taken in environments with strong magnetic fields when

using coils without magnetic shielding.

Control input ports

The complete IC is controlled by the three control input

ports C1, C2 and C3 (TTL-level). With these ports the user

can select between different AF sources according to the

transmitter status (see Table 1). Finally Schmitt-triggers

are added in the input port interfaces to suppress spikes

from the control lines C1, C2 and C3.

After a power-on reset, the logic is reset (mute mode for

the AF channel). After some time (≤1 ms), when the

power-on reset is automatically deactivated, the switch

position of the Main channel is changed according to the

control input port levels C1, C2 and C3.

For standard L, the AM sound is fed via the AF input (Vi 1)

to the two AF outputs (Vo 1,Vo 2). This can also be

achieved by feeding at AF input Vi 3.

The logic level combination 111 of the control input ports

(C3, C2 and C1) is not allowed (see Table 1).

1995 Mar 20

6

Share Link: