- HI-FI Stereo Encoder / Multiplexer
1x 50K POT
3x 100nF (104)
1x 100pF (101)
1x 10pF (10)
|1x NJM2035 IC
1x 38 KHz Crystal
|Voltage Supply: 1.2V - 3.6V MAX
Current Draw: >3mA
Channel Separation: < 25dB
Signal to Noise Ratio: 67 dB
Operation Temperature: -20 - 75°C
Frequency Range: 20Hz - 15KHz
Stereo Encoder / Multiplexer
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 including
TX300 and TX500 which are available on our website.
The whole circuit may easily fit on a small 1”
x 1.5” printed circuit board allowing to fit in
places where space is limited. 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.
- Internal Block
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.
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
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
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.
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