NJM2035 - HI-FI Stereo Encoder / Multiplexer

How Does Stereo Encoder Work?

The stereo encoder consists of three main stages; pre-emphasis, digital encoder and mixer stages.

Pre-emphasis stage is achieved by using two 47K resistors and two 1nF capacitors. This helps to eliminate the noise that is produced during the FM transmission of your audio signal.

Second stage that is built around NJM2035 is a digital encoder. All of its internal blocks except for two audio amplifiers (pins 1 & 14) that act as separators are created using digital circuits. The first digital circuit is a 38KHz oscillator that is generated by using external 38KHz crystal (pin 7), 10pF capacitor (pin 6) and 100pF bypass capacitor (pin 5). Once 38KHz frequency is generated it is then buffered and divided into two 19KHz signals with a phase difference of 180 degrees. Once that is done these two frequencies are connected with two time division MPX digital alternating switchers ,one for each audio channel. Here audio channels are switched between each other with a total frequency of 38KHz. If you would be able to slow this frequency to 1Hz per second you would be able to hear that this all but a trick. During the first half of the second you would hear the left audio channel and during the second half of the second you would hear the right audio channel. Due to the fact that the channels are switched with a fast frequency of 38KHz per second our brain is unable to recognize that these channels are really switched and receives this as a continuous audio signal. At the same time another signal from the 38KHz oscillator is divided by half into 19KHz. This signal is called a PILOT tone because it will help a stereo decoder on the receiver’s part to slice the MULTIPLEX signal (mixed L and R audio channels) and separate them back into left and right audio channels.

The third stage is a mixer that consists of 33uF and 100nF capacitors and 82K and 10K resistors. The role of this circuit is to mix the multiplex subcarrier and pilot signals together. The multiplex subcarrier signal that is coming out from the pin 9 of the NJM2035 IC is the sum and difference of both left and right audio channels that are switched at 38Khz rate. The PILOT signal that is coming out of pin 8 is a 19KHz frequency that is used to distinguish what channel is currently being switched and without which stereo decoding would not be possible.

How to Connect a Stereo Encoder to your FM Transmitter

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

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

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

4) Turn on your transmitter, FM receiver and audio source (making sure its audio volume is not too loud), and apply voltage supply from a single 1.5 battery cell to a stereo encoder.

5) By now you should hear a stereo sound. Adjust 50K potentiometer making sure your L-R balance is set to the middle and adjust the volume of your audio source making sure an audio signal is as clear as possible.

Frequently Asked Questions

1. Can I replace NJM2035 with BA1404?
No. BA1404 chip has been manufactured by a different company and these two chips have different internal architecture and pin connections.

2. Can I use my own pre-emphasis circuit?
Yes that’s feasible. All you have to do is short circuit 47K resistor / 1nF capacitor and connect your pre-emphasis to audio inputs.

3. Stereo Encoder works but the transmitted music comes out distorted. What can I do to eliminate that distortion?
You have to remember not to over-modulate NJM2035 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. All you have to do is lower the volume of the incoming audio source.

4. Stereo Encoder works but the transmitted music comes out with noise. What can be done to eliminate that noise?
Stereo Encoder has to be grounded properly and cannot be too close to the transmitter’s oscillator as it might cause an oscillation. Another reason might be a lack of proper filtering on your power supply, if you are using one. If that’s the case 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 supply the encoder with a single 1.5V battery cell.
Yes. In fact due to the very little power consumption (>3mA) of the NJM2035 chip your encoder should work for quite a while when supplying from just one single 1.5V battery!

6. I want to connect the encoder to the supply voltage of my FM transmitter which is higher than 1.5 V, how can this be done?
You can easily build a simple power converter for the stereo encoder with a few simple components as shown below.

7. 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.

8. If I will use a stereo encoder will any mono FM receiver be able to receive the audio signal?
Yes. MPX stream that is generated by NJM2035 contains both stereo and mono audio signals.

9. 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 clear 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

If you are building the above Stereo Encoder and have trouble finding some of the components like NJM2035 or 38KHz crystals, we are distributing the following components in our Electronic Store and also the kit version in Electronic Kits section.