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Old 17th July 2005, 02:24 PM   #1
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Default Problem in an Encoder circuit design. Pls advice..

hii everyone..!!

here is a circuit degisn from BIASCOMMS which i have attached with the post.. Its a simplest stereo encoder..!

however, i do not understand the use and functioning of the first half of this ckt (shown in the attachment).. Its the design part in which Left and Right speaker inputs are fed to a TL072 dual opamp and the o/p again is Left and Right..!

cant i directly feed the orginal left, right stereo to two individual isolation opamps with preemphasied...??

if anyone knows please advice.. i have found similar type of design parts in many of the stereo encoder ckts. available on the net.. None of them could explain that part...

If anyone knows, please respond..!!

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Old 18th July 2005, 02:56 PM   #2
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You can build the Stereo Generator without the resistors and capacitors that I have indicated on the attached schematic.

They may have been added to give you "expanded" stereo or to overcome some problem in the encoding process.

I'd try it both ways. You may also want to make the 220k resistors adjustable so that you can trim the circuit for best stereo.
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Old 19th July 2005, 10:33 AM   #3
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Thnx a lot sir.. I desgined my stereo encoder with LF351 opamps. I used a preemphasis across the initial isolation stage, and fed them to the 4066 IC..

The seperation is quiet good.. But if i increase the amplitude of the input signal, the audio in the fm receiver gets distorted..

Especially, i experience distortions in songs having heavy beats or high treble, only when transmitting in stereo, and not in mono.. To elimate the problem, i used a high pass filter instead of the preemphasis... The problem seem to be solved, but the beats gets suppressed, a bit more than normal; and then distortions with high frequency signals takes place..

Can you provide me with an intermidiate solution to this problem.. I would love to post my ckt. design, but i havent completed it.. I just stated to play around with SPICE..

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Old 19th July 2005, 10:57 AM   #4
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"i experience distortions in songs having heavy beats or high treble, only when transmitting in stereo, and not in mono.."

The problem with distortion may be caused by high frequency audio that is interfering (heterodyning) with the stereo pilot.
Commercial FM stereo generators contain 15 kHz low pass filters with a very sharp high frequency cutoff to prevent this problem.

In the absence of these filters, your only option is to reduce the modulation.

If you have access to good audio editing software, you might record one your favorite pieces of music and then apply a very sharp 15 kHz low pass filter to see if this corrects your problem.
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Old 19th July 2005, 04:14 PM   #5
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Sir, in that case i will opt for reduced modulation... the only concern is that i havent used bandpass filters to block L-R harmonics (which are generated during the L R chopping).. If i use them, then phase variations occur due to which i lose my seperation to some extent..

I have connected this encoder to my 1 WATT mono fm transmitter.. So how much power may be wasted due to the presence of these harmonics..?? Will it affect its o/p considerably when i use a dipole antenna for transmission..??

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