Here’s a high quality stereo encoder using PIC18F1220 microcontroller. The sampling frequency used in this stereo encoder is 97 times higher than the pilot tone frequency. This makes very easy to reject all spectral residues around the sampling frequency without affecting the main signal characteristics.

Using of a microcontroller allows to build this stereo encoder with reduced part count and get excellent results in real operation. Here’s a stereo encoder block diagram:

This stereo encoder advisedly does not contain any preemphasis circuit. This configuration ensures loud sound without exceeding the maximum frequency deviation limit (75 kHz). The stereo encoder is designed to provide really good sound. This always needs to use the compressor/limiter/clipper device where the preemphasis is precisely assured.

Download Stereo Encoder Parts List

HEX file: stereo.zip (WDT: on, Osc.: HS PLL, MCLRE: RA5), version 2.0.
Suitable free PIC programmer is for example here: http://www.members.aon.at/electronics/pic/picpgm/. Actually Pira CZ does not provide the PIC programming, PCB nor complete kit sale for this device.

Adjusting elements description
Pilot level adjustment: The pilot level should be set to 9 % of the total deviation (75 kHz), measured in peak-to-peak values. For example, if audio input level is 2 V pp, set the pilot tone level to 0.2 V pp (the adjustment is linear with the max. of about 0.5 V pp).

Output level (gain) adjustment tip: If possible, it’s recommended to set high output level and adjust lower sensitivity on the transmitter/exciter rather than set low output level and adjust high sensitivity. This way gives getter S/N ratio. Is it clear?

Note: The output level adjustment has no effect on pilot-to-audio ratio.

Mode selection: Click and hold the button until the mode change is indicated by acoustic signal (number of beeps indicates the mode set). The device will remember the setting. Mode table:

Mode 1 is the default and recommended. Mode 2 switches to monaural operation. Modes 3 to 5 can compensate transient characteristics in some cases and improve channel separation, but have no reason for nonspecialists.

Stereo Encoder Printed Circuit Board (PCB)

Source: Pira CZ Stereo Encoder for FM broadcasting

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