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Old 3rd August 2012, 09:10 AM   #1
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Smile Hybrid FM Stereo Encoder for FM Transmitters

Hi Friends,
I am going to make a Hybrid 256x oversampled FM Stereo Encoder. I know only basic idea of FM stereo multiplexing (like 2x, 8x oversampled circuit) also I read in an article circuits like DAC, Sine LUT ROM etc can be used for higher sampled MPX system. But I don't know how to implement the circuit and where to begin.Please help me. I expect good design ideas and circuits. Sorry for my poor English. Have a nice day
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Old 3rd August 2012, 12:03 PM   #2
DF96 is offline DF96  England
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Quote:
I know only basic idea of FM stereo multiplexing (like 2x, 8x oversampled circuit)
I suspect not. The basic idea of FM stereo multiplexing involves no sampling, DACs, oversampling etc. It is just AM DSB, plus a pilot tone at half the subcarrier frequency.
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Old 4th August 2012, 06:08 AM   #3
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Quote:
Originally Posted by DF96 View Post
I suspect not. The basic idea of FM stereo multiplexing involves no sampling, DACs, oversampling etc. It is just AM DSB, plus a pilot tone at half the subcarrier frequency.
Yes, I know the analog design of fm stereo encoder and I implemented successfuly that by using MC1496 DSBSC ic. But now I would like to use the hybrid circuit part that is Time Division Multiplexing method rather than the Frequency Division Multiplexing.( I know complexity of TDM method is high compaired to FDM method but I would like to combine the advantages of digital method also for the circuit ). Thank you Click the image to open in full size.
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Old 5th August 2012, 09:07 PM   #4
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I am afraid the link does not work on my PC.
The idea of time division mpx is new to me. So could you please explain?
Is the signal you are generating compatible to FM stereo receivers as we know them?
I.e does the signal consist of the three frequency bands: baseband (L+R) and both sidebands of 38kHz (L-R) and (R-L). + the pilot of course. That would be frequency division mpx imho.
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Old 5th August 2012, 09:10 PM   #5
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It can be done!

R&SSR8000 Family of VHF FM Transmitters (Rohde & Schwarz Deutschland - Produkte - Fernseh- und Hrfunktechnik - Hrfunksender)
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Old 6th August 2012, 11:23 AM   #6
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In a simple TDM MPX (2x oversampled) method we can use an SPDT electronic switch like CD4053 (analog ES) ic. Imagine left channel is applied to N/C arm of ES and right channel is applied to N/O arm of ES the common arm is applied to a wide band signal summing amplifier(Op amp as an adder) here 19kHz is also applied to get a composite MPX signal at output of said signal summer. Later is applied to a wide band mono FM transmitter. A 38kHz square wave derived from 19kHz pilot (phase locked) is applied to the control input of ES. The out put of ES is an alternate series of left and right channel with duty cycle 50% and repeating period equal to 1/38kHz.
On the other hand in FDM MPX method , first we have to derive L+R and L-R by matrixing. The L+R and pilot is applied to a signal summing amplifier, the output of later is applied to a wide band mono FM transmitter. The L-R signal is DSBSC modulated(multiply) with 38kHz square (or sine) wave. The DSBSC wave an alternate series of L-R and R-L with 50% duty cycle and 1/38kHz period. This DSBSC wave is algebraically added to the L+R signal in the said summer where L-R overlap with L+R for +ve half cycles of 38kHz subcarrier results 2L and R-L overlap with L+R for the -ve half cycles of 38kHz subcarrier results 2R. Thus the output of summing amplifier is an alternate series of 2L and 2R with period 1/38kHz and 10% pilot is also present.
Thus the output of FDM MPX method is nearly equal to TDM MPX method except TDM MPX method generates lot of harmonics of switching frequency. This can be overcome by using oversampling methods and filtering.
One more thing is that when switching alternatively from one channel to other the average value of output contains sum (L+R) of input channel. the peak differences of inputs (L-R) re-creates troughs and crests of the suppressed switching wave (here 38kHz) or DSBSC modulated L-R signal. Again we can say that TDM MPX method is equivalent to FDM MPX method.
For more information please see this website RBS FM 92.60 MHz the Best Radio Station in Tulungagung - Digital Stereo Encoder Simulation
Thanking you
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Old 6th August 2012, 02:56 PM   #7
DF96 is offline DF96  England
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Most MPX decoders use TDM, and so have to add some crosstalk to compensate for the difference between the fundamental component amplitude of a sine wave and a square wave with the same peak value (pi/4 IIRC). I assume a TDM coder would have to make a similar correction.
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Old 7th August 2012, 11:35 AM   #8
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Here's an interesting link -

SIMPLE FM STEREO MULTIPLEX ENOCDER CIRCUIT
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Old 7th August 2012, 01:02 PM   #9
DF96 is offline DF96  England
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Yes, although the simpler circuits there omit the necessary crosstalk for a TDM encoder. The more complex ones which partially synthesise a 38kHz sine wave modulator don't need as much crosstalk. Full 38kHz synthesis would need no crosstalk.
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Old 8th August 2012, 03:44 PM   #10
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Plz see this linkPLL FM STEREO for DDS FM Stereo coder and different methods to generate 19kHz and 38kHz sine wave.
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