BH1415 HI FI Stereo Encoder / Multiplexer with Limiter and Low Pass Filter

This is the latest BH1415 Stereo Encoder design from RHOM that includes a lot of great features in one small package. It comes with pre-emphasis, limiter so that the music can be transmitted at the same audio level, low pass filter that blocks any audio signals above 15KHz to prevent any RF interference and crystal based stereo encoder for stereo transmission.

BH1415 can be supplied with 6 - 15V voltage, consumes only around 25mA while providing very sound quality and improved 40dB channel separation. BH1415 is only available in SOP22 IC case and this may be an inconvenience for some folks. On the other hand, because the chip is smaller than regular DIP-based ICs it is possible to fit the entire stereo coder on a small PCB.

BH1415 requires 7.6MHz crystal oscillator which is pretty hard to find. The good news is that you can use 7.68 MHz crystal instead. In fact our BH1415 stereo encoder prototype uses 7.68 MHz crystal. This has absolutely no effect on stereo encoding process, we have tested it and stereo sound is crystal clear.

Main blocks of BH1415 chip are; pre-emphasis, (audio level) limiter, stereo encoder (MPX), PLL circuit, oscillator and RF buffer.

BH1415 PLL Transmitter

How to Connect BH1415 Stereo Encoder to FM Transmitter

1) First test FM Transmitter and make sure that an audio signal is properly transmitted on your desired frequency. Once that is done turn off transmitter and disconnect its audio inputs.

2) Connect left and right audio outputs from audio source to the inputs of stereo encoder.

3) Connect stereo encoder’s MPX output to transmitter’s input. Audio coax cable highly recommended.

4) Turn on transmitter, FM receiver and audio source (making sure its audio volume is not too loud), and apply 4-6 voltage to stereo encoder.

5) By now you should hear a stereo sound. Adjust the volume of your audio source making sure an audio signal is as clear as possible.

Frequently Asked Questions

1. Can I replace BH1415 with BA1404?
This is not possible because these two chips have different internal architecture and pin connections.

2. Can I use my own pre-emphasis circuit?
No, this is not possible because pre-emphasis is built into BH1415 chip.

3. Stereo Encoder works but the transmitted music comes out distorted. What can I do to eliminate that distortion?
Make sure you are not over-modulating BH1415 inputs with higher / louder audio signals than it can handle. Placing 10K stereo potentiometer on audio inputs can help eliminate that problem, although it is not necessary to achieve a good sound quality. Lowering the volume of incoming audio source should do it.

4. Stereo Encoder works but the transmitted music comes out with noise. What can be done to eliminate that noise?
Lack of proper filtering on your power supply may be a problem. If power supply is being used make sure that all the diodes on the rectifier are bypassed with a 10n capacitors and that your voltage supply is well filtered with at least 4700uF capacitor.

5. Can I use my transmitter's existing pre-emphasis?
Pre-emphasis can only be used before stereo encoder’s audio inputs not after its MPX output. Doing otherwise may produce undesired results as MPX signal contains crucial 19KHz PILOT signal needed for stereo decoding.

6. If I use a stereo encoder will any mono FM receiver be able to receive the audio signal?
Yes. MPX stream that is generated by BH1415 contains both stereo and mono audio signals.

7. Is it true that stereo transmitters require stronger power output comparing to mono FM transmitters?
Stereo transmitters may require 'slightly' stronger power output to achieve clear stereo sound on the receiving part because transmitter has to also carry out 19KHz PILOT signal without which stereo transmission is not possible. If stereo FM receiver does not receive a clear PILOT tone it may change from stereo to mono reception.

Where to get the Parts