Stereo Multiplexing

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jaguar98
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Posted by jaguar98: Sun. Oct 11, 2009 - 04:12 PM
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Hi guys,

I'm trying (for a long time) to build an stereo generator for using with my little PLL transmitter but I CAN'T set it to work right!

After a lot of searching in the net, I still have many doubts about how the multiplexing works....Not to mention the wrong diagrams out there..

Actually, is the third multiplexer that I have build so far, but the sin wave that I put into one channel of the multiplexer keep playing together among the 2 speakers...(yes, the pilot light on receiver is ON)

What I know(and what I actually see in the scope):
-Chopping channels at 38 kHz
-join them with a ~10% overral amplitude 19kHz tune
-filter the signals (~60kHz LPF)

What I don't know
-I need a phase shift between the pilot and the chopping? Or the shift has to be exatcly 0º?
-The wrong overral level would cause incorrect channel separation?
-Any critical advice to me?

Well, any help in places/books to find information is appreciated...I am almost giving up with this, cus I don't really see why the channel separation is almost zero even at a decoder that I built(with LM1800N)

Thank You guys!

Attachment(s): 

Image icon the chopped channels with the pilot tune

Tags:

Learning and Information, General Electronics
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ossi
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Posted by ossi: Sun. Oct 11, 2009 - 04:29 PM
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http://homepage.univie.ac.at/~do...

German, but perhaps the schematic is interesting.

I think by using a 4.332 MHz crystal and a R-2R DAC
with a TINY2313 it should be possible to
built a simple stereo-test-signal generator
generating 1kHz left and 2 kHz right or similar.

The following coder
http://www.mts.co.jp/service/SDC...
has an adjustable pilot phase from -12 to +12 deg.
Seems it will be adjusted that the overall
system has 0 deg. phase difference.

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jayjay1974
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Posted by jayjay1974: Sun. Oct 11, 2009 - 08:02 PM
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From Wikipedia:

Quote:
The (L+R) Main channel signal is transmitted as baseband audio in the range of 30 Hz to 15 kHz. The (L−R) Sub-channel signal is modulated onto a 38 kHz double-sideband suppressed carrier (DSBSC) signal occupying the baseband range of 23 to 53 kHz.

A 19 kHz pilot tone, at exactly half the 38 kHz sub-carrier frequency and with a precisely defined phase relationship to it, is also generated. This is transmitted at 8–10% of overall modulation level and used by the receiver to regenerate the 38 kHz sub-carrier with the correct phase.

As I understand this, you don't chop. You modulate a 38KHz subcarrier with the difference between L and R. The subcarrier itself is suppressed.

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ka7ehk
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Posted by ka7ehk: Sun. Oct 11, 2009 - 08:56 PM
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Agree with Jayjay -

You transmit L+R on the base-band as normal audio.

You add a 38KHz to the base-band audio and modulate that subcarrier with L-R.

The receiver detects the presence of the subcarrier to tell whether or not stereo is present. The receiver demodulates the primary signal, extracts the subcarrier, demodulates that, then recombines them to make the stereo audio.

Jim

 

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jaguar98
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Posted by jaguar98: Sun. Oct 11, 2009 - 09:44 PM
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ossi: Great page! There was the vital information that I was looking for. The phase is actually very important to correct separation!
As for the test circuit, i've built a DDS that is here on the project area. Very clean signal , with maximum of 65kHz.
The coder you sent looks awesome!

jayjay & Jim: Well, actually I have a limited knowledge in electronics, and I couldn't build a circuit that would do L+R, L-R in a 38kHz sub-carrier, etc.... Instead, I've build several setero MPX'ers just folowing the schematics, with little modifications that they may require. 90% of MPX out there use chopping technique to achieve stereo...Which isn't that good.....
What I know so far, is that using circuits with 4060, 4013 and 4066 IC's, is far better than BA1404...
------
I folowed the advices of the german page, and finally I could get separation! It is not a complete separation, but I can clearly notice it in the headphones of my stereo decoder!

Thanks to the help guys! I will stop the experiments for today, and try be a normal person who goes outdoor sometimes, lol!
(feel free to comment!! The more advices, the more I can improve the circuit)

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ossi
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Posted by ossi: Mon. Oct 12, 2009 - 06:26 AM
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Do you have an AVR procesor up and running that could
be used as test-generator ?

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jaguar98
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Posted by jaguar98: Wed. Oct 21, 2009 - 07:20 PM
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Sorry for the late.

Well, is there an AT90S2313, used to program a MC145170, but in my tests with Proteus simulator, it gave me a lot of offset between the pilot and the channels changing...

Also, I had many many problems with overall signal level, because in the scope the signal looks fine, but when putting it on the transmitter, it changed itself.....
Lot of room for experiments here ....

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