MC44BS373CADR2 datasheet(25/52 Pages) FREESCALE

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MC44BS373CA Data Sheet

MC44BS373CA Modes of Operation

15.11 Sound Section

The oscillator is fully integrated and does not require any external components. An internal low-pass filter

and matched structure provide very low harmonics levels.

The sound modulator system consists of an FM modulator incorporating the sound subcarrier oscillator. The

audio input signal is AC-coupled into the amplifier, which then drives the modulator. For mono applications,

the audio pre-emphasis circuit is a high-pass filter with an external capacitor and an internal resistor

(106 K

Ω typical).

The recommended capacitor value for BG standard (with a time constant of 50

µS) is 470 pF.

The recommended capacitor value for M/N standard (with a time constant of 75

µS) is 750 pF.

The sound reference divider is a programmable divide-by-128 or divide-by-64, resulting in a reference

frequency of 31.25 kHz or 62.5 kHz with a 4.0-MHz crystal.

This reference frequency can be selected by bus (bit SREF), depending on the application. For instance,

when the audio bandwidth is required to be extended (for stereo application), it is possible to select the

higher value (62.5 kHz) to avoid any interference between the reference and the audio signals.

For stereo applications, the audio signal is already encoded in the baseband stereo encoder. It is not

necessary to use a pre-emphasis, and in this case the pre-emphasis capacitor must be removed.

Sometimes for the same application it is useful to increase the FM deviation. To increase FM deviation,

replace the original pre-emphasis capacitor with a fixed 100-nF capacitor. Adding this value boosts the

deviation and does not create a pre-emphasis.

Without pre-emphasis, the audio bandwidth extends to 60 kHz.

To increase the audio bandwidth at low frequencies it is recommended to change the sound PLL loop filter.

It is also recommended to increase the values of the loop filter capacitors. The recommended values are 1

µF

and 100 nF.

It is possible to create a peak for the low frequencies (from 30 to 80 Hz) by adjusting the resistor of the sound

loop filter. A flat response is given with a value of 5.6 K

Ω but this value can be set to 4.7 KΩ. This gives a

peak of about 1 dB at 40 Hz and extends the low-frequency cut-off to 28 Hz.

The following figure plots the resulting bandwidth for low frequencies and for the whole bandwidth (up to

60 kHz).

The first chart represents the audio bandwidth for low frequencies (20 to 150 Hz) for two sets of sound loop

filters. The first set with 1

µF + 100 nF + 5.6 KΩ gives a flat response. The second set with 1 µF + 100 nF

+ 4.7 K

Ω gives a peak of about 1 dB at 60 Hz. It is possible to adjust this peak by changing the resistor value.

MC44BS373CADR2 Datasheet(PDF) 25 Page - Freescale Semiconductor, Inc