FM stereo broadcasting was introduced during the early 1960s. The scheme that was adopted was chosen to be compatible with the monaural FM radios that were in existence at the time. Essentially, the system performs the multiplexing of two signals and further combines them into a complex baseband signal that modulates the FM carrier. Figure 1 shows a block diagram of the typical analog-stereo generator used to drive an FM transmitter.
Figure 1: FM Stereo Multiplexing
A Left and Right source of audio are first pre-emphasized (HPF) and then fed to adder circuits. The output of one adder is the sum of the two signals, or the L + R signal (the monaural signal), and the output of the other adder is the difference of the two signals, or L – R. The L – R signal is applied to a balanced modulator along with a 38-kHz signal. The output of the balanced modulator is a DSBSC AM signal centered at 38 kHz. A portion of the 38-kHz signal is divided in frequency to become 19 kHz, and all three signals are applied to a summary/adder circuit at the output of the generator.
The resulting stereo-generator output-signal spectrum is shown in Figure 2.
Figure 2 FM Stereo Multiplexing Spectrum
The L + R signal, which contains baseband frequencies from near 0 Hz to 15 kHz, occupies that portion of the frequency spectrum. The 19-kHz tone is pilot sub-carrier signal, which will be used at the receiver to aid in the demodulation of the received signal. The L – R signal, which has been DSBSC amplitude modulated by a 38-kHz tone, occupies the frequency range from 23 to 53 kHz.
Pre-Emphasis and De-Emphasis
In FM, the noise increases with modulation frequency. To compensate for this effect, FM communication systems have incorporated a noise-combating system of pre-emphasis and de-emphasis. Pre-emphasis provides increased amplitude to the higher modulating frequencies prior to modulation under a well-defined pre-emphasis (high-pass filter) curve. This added amplitude will serve to make the higher frequencies more immune to noise by increasing their index of modulation. De-emphasis is just the opposite operation (using a low-pass filter) and it is done at the receiver.
GENERAL DESCRIPTION
HI-FI Stereo Multiplexer for FM transmitter with NJM2035 circuit
The NJM2035 is an integrated circuit used to generate a stereo composite signal and obtain switching out and 19Khz pilot signal due two input audio signal and 38Khz X-tal and few external CR.
This stereo encoder is the perfect solution for those looking for a high quality stereo sound transmission at a low cost. This stereo encoder produces an excellent crystal clear stereo sound and very good channel separation that can match with many more expensive stereo encoders that are available on the market. It is all possible thanks to a 38KHz quartz crystal that controls the 19kHz pilot tone, so you will never have to calibrate or re-adjust the circuit.
This stereo encoder is the perfect solution for those looking for a high quality stereo sound transmission at a low cost. This stereo encoder produces an excellent crystal clear stereo sound and very good channel separation that can match with many more expensive stereo encoders that are available on the market. It is all possible thanks to a 38KHz quartz crystal that controls the 19kHz pilot tone, so you will never have to calibrate or re-adjust the circuit.
NJM2035 offers superb quality and is manufactured by NJR CORPORATION (JRC), a subsidiary of New Japan Radio, a company that is known as the world's best manufacturer of high end professional audio semiconductors. This transmitter will work with any mono FM transmitter which are available on our website. While building your stereo encoder please take your time and always double check with the schematic to make sure that all the connections are done correctly. If you have any questions, comments or suggestion we will be glad to further assist you.
HI-FI Stereo Multiplexer for FM transmitter with NJM2035 PART'S
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 12v to a stereo encoder.
5. By now you should hear a stereo sound. Adjust P 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.
HI-FI Stereo Multiplexer for FM transmitter with NJM2035 PCB
Technical Specifications:
Voltage Supply: 4V - 14V MAX
Current Draw: >3mA
Channel Separation: < 25dB
Signal to Noise Ratio: 67 dB
Operation Temperature: -20 - 75°C
Frequency Range: 20Hz - 15KHz
Current Draw: >3mA
Channel Separation: < 25dB
Signal to Noise Ratio: 67 dB
Operation Temperature: -20 - 75°C
Frequency Range: 20Hz - 15KHz
PARTS LIST
R1 = 47k
R2 = 47k
R3 = 47k
R4 = 47k
R5 = 150
R6 = 100
C1 = 1nF
C2 = 1nF
C3 = 100nF
C4 = 100nF
C5 = 10mF
C6 = 100pF
C7 = 10pF
C8 = 10mF
D1,.. D4 = 1N4148
P1 = 50k Trim. Pot.
T1 = BC549
IC1 = NJM2035
X1 = 38Khz
J1, J2, J3 = Female Cinch connector for PCB (RCA Jack)
J4 = DC Power Jack 2.0mm with center pin for PCB
