Does it (stereo encoder) really work???

[Willen]

Does it work? **broken link removed**
>Look its 'comments' too and suggest me too about its reliability. What about L? Is it general non critical choke?
>Can I connect its output from R11 to directly FM transmitter (without audio amplifier IC)? OR, can I use general purposes transistor instead of IC 741 to amplify audio?
>What is the friquency at pin 13 of IC3? Does R14 VR work for friquency adjust?

EDIT: Restarted thread for another simpler non-accurate encoder, see post #6

 

[rumpfy]

From memory, the FCC system for stereo broadcasting had a 38 kHz carrier which is suppressed in transmission. To regenerate this carrier, there is a 19kHz pilot tone.
The two signals A and B were added to give the baseband signal. The two signals A and B were also subtracted to give A-B. The A-B signal was used to amplitude modulate the 38kHz subcarrier. The carrier was not transmitted. To regenerate the carrier, a pilot tone at 19 kHz was transmitted. At the receiver, the 19 kHz tone was separated out from the recovered modulation and used to phase lock a 38 kHz signal to it. The A-B signal was demodulated and an adding circuit used to add the (A+B) to the (A-B) signal. ie. (A+B) + (A-B) = 2A. Similarly, the (A+B) - (A-B) = 2B . Thus the A and B signals were recovered.
In the circuit, IC1 is a 'Quad Bilateral Switch'. Two of the switch units are used and the other two are unused. Pin 1 is the input of the 'left' and its 'output' is pin 2. For the other switch, pin 4 is the input and pin 3 is the output.
IC2 is a flipflop which appears to switch 'left' and 'right' alternately.
Because of the way IC 1 operates, it is difficult to see how the A+B and A-B signals are generated. I have to say that it is unlikely that this circuit generates an FCC stereo signal. So, what does it do?
It appears to sample two audio signals at a fast rate and to simply combine them. To recover the two signals it is necessary to switch them at the same rate that they were generated. There is no information how this is done. One would have to have a synchronous switch.
The components L and C5 constitute a parallel resonant circuit. For the values given, C5 = 10nF and L=6.8 milli henry, the resonant frequency is about 19 kHz.
IC4 appears to function as a unity gain amp with frequency selective drive to each input. The inverting input has a rising inut current with frequency; the in-phase input has a falling input current with frequency.
Bit of a mystery.

 

[audioguru]

It is a horrible stereo encoder. Its 19kHz and 38kHz frequencies are not even crystal-controlled.
The FM transmitter must have a flat phase-free response to 53kHz which the antique 741 opamp cannot do in the encoder anyway.

 

[ronsimpson]

If you read the comments it looks like you should not build it.
"It has a filter L, C5 for frequencies higher than 15kHz" Say shat? Is it a high pass filter or a high blocking filter? With R9=47k it can't do much! If anything it is a band pass. I would have put a filter there to block 38khz and reduced R9. I believe there should be no 38khz on the output.

From experience audio above 15khz really trashes the stereo separation. The input on broadcast decoders have a low pass filter on the R and L channels. The filter is typically -1db at 15khz and way down at 19khz.

The 741 amplifier is a mystery! Just like the LC filter.

The pilot must be a crystal. (19/38khz)

 

[rumpfy]

I had another look at the design.
Firstly, the RC values in the oscillator(IC3) should be such as to produce a frequency of 76 kHz. The value of C should be therefore about 200 to 300 pF and NOT 10,000 pF.
The IC2 is a flipflop and it divides the input square wave from IC3 by 2 to give a 38 kHz signal out of pins 1 and 2 of IC2. This frequency is important because it is the 38 kHz subcarrier for the (L-R) signal. Because the RC values at IC3 are wrong, the subcarrier frequency is also wrong.
The signals on pin 1 and 2 of IC2 are of the opposite phase to each other. The IC1 is simply a 2 channel on-off switch and when pins 1 - 2 are ON; pins 3 - 4 are OFF. This is ok, but the input signal needs to be only the (L-R) signal and not the R and L signals. IC1 can function as a suppressed carrier modulator but the output signals wont be (L+R) and (L-R). They will be the R signal and the L signal upper and lower sidebands centred on 38kHz.
The circuit still looks suspicious to me.
As part of my efforts, I found a reference to an IC type BA1404.This is a ROHM IC which takes a L and a R signal and produces a MPX output signal. It needs a 38 kHz oscillator crystal to make it work. It looks like a simple solution to your problem.
I think ronsimpson and the guru are right on the ball with their comments.

 

[Willen]

Hi, the thread is 2 years old. I was trying to get its Xtal from 3 years. I was failed.

Now I got 4013, 4060, 4066 and 555 ICs but no crystal. So may be this non-accurate stereo encoder is only one alternative for me.

**broken link removed**

You all are familier with this old stereo encoder schematic . As a transmitter I have MOD 4 FM Tx by audioguru.

Hope I don't need pre-emphasis on the stereo encoder because Mod 4 has pre-emphasis.

I also want to remove few more filter opamps from here if they are not MORE critical here. If it is not more bad without filter, I will not use any filter, Because I am going to make 'non- accurate' stereo encoder so I want more simple, less components like Harry Lythall's 222 stereo coder. Please if it is possible, just delete few stage simply. If it will not work at any cost, I NEVER make ANY stereo encoder!

 

[audioguru]

My Mod4 FM transmitter was designed for mono, not stereo. Its frequency response attenuates the important stereo sidebands and changes their phase.
Pre-emphasis is done to EACH channel ahead of the stereo encoder but many stereo encoders do not have pre-emphasis so their sound is muffled like an AM radio.
The audio channels should each have a lowpass filter following pre-emphasis to attenuate 19kHz sounds and audio harmonics that will beat with the stereo pilot tone and beat with the stereo sidebands.

 

[dr pepper]

There is a project on the net that uses a pic micro and just a few comps to transmit fm stereo, it works well.
Dont expect studio quality though.

 

[Willen]

Hi Ag
The site has its own transmitter designed for stereo (Harley OSC). If the stereo could be little more small in size, I would make them both.

Hi Dr paper,
Finding advance components is impossible for me. So trying to make simple one from few years.

 

[dr pepper]

Have a look at this, it uses standard parts:

**broken link removed**

 

[Dick Cappels]

Same approach as Mr. Lythall's, different execution. No pretension of being hi-fi.

 

[dr pepper]

I was thinking of your design in post #8
I have made a couple of your projects mr Cappels, your site is excellent.

 

[Willen]

Have a look at this, it uses standard parts: **broken link removed**

Hi dr pepper,
Found the 2nd simplest stereo coder by Harry, linked by you. I found all components. I have 4060x2, 4013x2, 4066x4, lots of high speed opamps. One years ago with the help of audioguru, few stages (pre-emphasis, additive mixers) are also corrected. But eventually 4.864 crystal I couldn't get. I searched almost ALL stereo encoder found on internet but all has some limitation for me. But **broken link removed** by Harry looks really great! What can I do?

Same approach as Mr. Lythall's, different execution. No pretension of being hi-fi.
View attachment 84586

Wow Dick Cappels!!!
Feeling so nice to see you here on the forum! I am very familier with you site. I said 'the stereo coder by Harry is 2nd simplest' because 'The Simplest' your! I am visiting your site from few years. Few years ago I had talked with you via e-mail too about this. But again what can say....ectually... Here I can find LOTS and LOTS electronics shop but all of them sale parts which can be used on DVD players, TVs and radios. Some shops started to sale whole players, TVs and radio instead of the components because they have lots of benefit. If I bought an IC for FM Rx at $3, China sales entire FM radio at just $1. So components are disappearing. I searched ANY crystal which can be devide down to 38KHz but failed. Simplicity of your device makes me amazing!

 

[dr pepper]

I dont know how successful it would be however there is an xtal at 4.9152 mc's, I think used in computing applications, still might be difficult for you to get through.

Herer in the uk components for prototyping circuits esp microcontrollers are very easy to get, however consumer electronics spares are the opposite not being that easy to get, things nowadays last longer and when they fail are usually thrown away.

 

[Willen]

I dont know how successful it would be however there is an xtal at 4.9152 mc's, I think used in computing applications, still might be difficult for you to get through.

Herer in the uk components for prototyping circuits esp microcontrollers are very easy to get, however consumer electronics spares are the opposite not being that easy to get, things nowadays last longer and when they fail are usually thrown away.

Hi,
If I multiplied 38KHz with 129, it will be 4.902 instead of 4.915(Xtal). There is 13Hz mismatch. I don't know what it does in stereo encoder. But again I never seen the Xtal of this values before too. I can find 100s of 4MHz, 12MHz etc. I think I have to wait few years more.

How opposite there! Here nobody will throw their consumer electronics device because almost all can be repaired here. Thousands of repairing centre exist to do it. Spears are easily available for them. So nobody are selling parts for students projects. Few shop at capital city sales parts for students but they don't have the rare Xtal. In each project of mine, I feel to do gardening instead of electronics suggested by AG hehe.

 

[audioguru]

Hi Willen,
Many electronic products thrown away here ARE NOT DEFECTIVE, instead they simply do not look modern anymore. They still work fine!
Remember when we talked about incomes? We get paid 50 times more than your people get paid but products cost the same so our old (maybe only one year old) products are thrown away.
The city garbage pickup department now refuses to pickup the millions of televisions thrown away. Now people must pay somebody to take an old TV to the dump. Most of the TVs still work perfectly but have an ordinary CRT instead of a modern huge high definition LCD screen. Soon the high definition TVs will be thrown away when everybody replaces them with ultra-high definition or 3D TVs.

I have old or worn out telephones, telephone answering machines and laptop computers (some still work) that I kept and used the parts inside for some of my projects because it is too much trouble to for me drive for 10 minutes to buy inexpensive parts at a local electronics parts store. Today, most of the local stores are gone because we buy on the internet now then the products or parts are delivered to us. Even huge department stores are closing here but I get their online ads almost every day.

Cars here are also thrown away because they do not look "nice" anymore or have a simple fault that could be repaired. Repair labour costs more than the old car is worth.

 

[Dick Cappels]

FM stereo transmitters are analog and seeing very many home stereo encoder designs that use multivibrators instead of crystals, I tend to believe that the receivers PLL's that generate the demodulation carrier have some capture range and therefore while using a crystal has the great advantage that you don't have to "tune it in" and its drift is negligible, RC oscillators can be acceptable. If you can't find a suitable crystal, then you might want to try an RC osicllator.

For example, using Harry Lythall's circuit, the oscillator on the CD4060 can be connected as an RC oscillator, and the rest of the circuit behaves as before.

As an aside:
I have been living and experimenting in Thailand for more than 10 years and I see that with the exception of a few electronics distributors with limited product lines, the situation is the same; primary sources of parts are shops that sell replacement parts for consumer electronics. Yep, its a problem.

The reason I came back to this forum recently is because I had realized that Audioguru was no longer active on that other forum.

 

[audioguru]

Hi Dick,
Did I bring you back here? It is good to have you back.

I made a mono Eico FM tuner kit when I was beginning university in 1964. Eico made a stereo multiplex adapter for it the next year so I made one. They used vacuum tubes but sounded great.
The stereo demodulator did not use a crystal and it did not use an oscillator. It simply doubled the 19kHz pilot to exactly 38.0kHz and used it as the stereo sidebands carrier.

 

[Willen]

* same post removed!

 

[Willen]

Hi Dick,
Thailand and my country are almost same part of the world. So may be we are facing almost same problem. But here in my country, more poor accessibility exist than in Thailand due to poor economic and poor technical status of my country.

I also thought to use any external oscillator like 555 based or like you said RC OSC, but I have no more measurement device than a cheap ($ 5) multimeter. Without an oscilloscope I couldn't tune it to 38KHz exactly. I could tune it by hearing L and R output channel but may be such mechanism wouldn't make me fully satisfied. If I like any device I will keep them with me for whole life. I made a mod 4 simple FM tx by audioguru and I will keep it in my whole life as a symbol of my great project. First time I designed PCB on designer software (ExpressPCB). It looks like a professional because in such work I am so so much careful, so sometime it takes me lots of minutes to bend a normal lead of a component . Same way I want to make a better stereo encoder to keep it forever with me. So I wish to make very accurate or controlled by a crystal.

Your email made me so happy! Thank you lot.