Cheap Stereo Coder

[Albert H]

This is based on a (very old) Dutch design, and a slightly more basic version of this was the NRG Pro II Coder. I upgraded it to the Pro III (this circuit) that guaranteed which input was which (no random channel selection like certain cheap coders) and added stereo / mono switching. You can add pre-emphasis just by bypassing the input 47k resistors with 1n capacitors (or 1n in series with 4k7 to be even more accurate - though you won't hear the difference).

I've got a nifty stereo 15kHz lowpass filter that eliminates all the splashy noises (and blinking stereo indicators) and doesn't need any inductors. There's also a simpler version that does use inductors (but costs a lot more to build for slightly inferior results)

I'll also put up a cheap and simple stereo limiter and clipper that works well with this coder (and provides the pre-emphasis before the limiting, as it should be).

All the circuits can be built on Veroboard - a bit fiddly, but a quick way of building stuff if you don't have a PCB. I've also got PCB layouts for all this gear, and have just sent the artwork to a PCB manufacturer in the Far East, so I might make some PCBs or complete kits available.

I'll see whether it's worth selling the PCBs when they arrive, and let you all know.

We used to build a complete basic airchain in a nice 19" rack box. It had bargraph level and gain reduction indicators, silence detection, mono / stereo switching and optional RDS coder, programmed using USB. The complete unit went for around £650...

Here's the simple coder Vero:

Stereo Coder Vero Nouveau.png

I'll add the circuit diagram when I've cleaned it up and corrected it.

[Albert H]

Here's a slightly improved layout - I've just built another one!

Stereo Coder Vero Nouveau.png

The input leads (and the MPX output) should be screened leads. All the electrolytic capacitors should be rated at 16V or greater. The CMOS ICs - the 4060, 4013 and the 4066 - must all be the normal cheap CMOS type - NOT the "74HC" or "74HCT" types - since the logic all runs at the full supply voltage. The TL074 quad op-amp is a good choice because it's low noise, but you could use TL064 or 084 instead.

A few quick checks:

If you put your frequency counter on pin 9 of the 4060 you'll see 4.864MHz. If it's a bit high, put a small value ceramic capacitor in parallel with the 33p capacitor. If it's too low, change the 33p for 27p and then pad as necessary. You don't need to be really precise - if the 19 kHz coming out of pin 14 of the 4060 is within a couple of Hz, it'll be fine.

Pins 1, 7, 8, and 14 of the TL074 should all be at half the voltage measured on the TL074 pin 4. If any of the voltages are more than 100mV away from the mid-rail voltage, there's something wrong - check for shorts, incorrect components, or you overheated the TL074 when you soldered it in!

With no audio applied, you'll see a pretty good 19kHz sinewave coming from the output. Connect the coder to your exciter, and turn the mod up until the stereo pilot on your test receiver lights. Turn it up just a tad more to guarantee that there's enough 19kHz for all receivers. Apply audio, and you'll be pleased at the quality of the results. The stereo image will be pretty good.

One warning - remember that you need roughly four times as much RF power for the same coverage in stereo!

[Electronically]

Here's a slightly improved layout - I've just built another one!

Stereo Coder Vero Nouveau.png

The input leads (and the MPX output) should be screened leads. All the electrolytic capacitors should be rated at 16V or greater. The CMOS ICs - the 4060, 4013 and the 4066 - must all be the normal cheap CMOS type - NOT the "74HC" or "74HCT" types - since the logic all runs at the full supply voltage. The TL074 quad op-amp is a good choice because it's low noise, but you could use TL064 or 084 instead.

A few quick checks:

If you put your frequency counter on pin 9 of the 4060 you'll see 4.864MHz. If it's a bit high, put a small value ceramic capacitor in parallel with the 33p capacitor. If it's too low, change the 33p for 27p and then pad as necessary. You don't need to be really precise - if the 19 kHz coming out of pin 14 of the 4060 is within a couple of Hz, it'll be fine.

Pins 1, 7, 8, and 14 of the TL074 should all be at half the voltage measured on the TL074 pin 4. If any of the voltages are more than 100mV away from the mid-rail voltage, there's something wrong - check for shorts, incorrect components, or you overheated the TL074 when you soldered it in!

With no audio applied, you'll see a pretty good 19kHz sinewave coming from the output. Connect the coder to your exciter, and turn the mod up until the stereo pilot on your test receiver lights. Turn it up just a tad more to guarantee that there's enough 19kHz for all receivers. Apply audio, and you'll be pleased at the quality of the results. The stereo image will be pretty good.

One warning - remember that you need roughly four times as much RF power for the same coverage in stereo!

Could always use this one Albert

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[Albert H]

That will work pretty well, but requires an input op-amp to guarantee the source impedance (but you could use it for your pre-emphasis too), and you'd need an op-amp at the output to ensure that the filter sees a constant impedance.... Also, the inductors will be expensive and prone to picking up hum - even if you can find them these days. My 15kHz filter uses several dual op-amps, and sounds great.

I'm going to put up a cheap and simple stereo limiter here, and it's also (sometimes) worth having a slow-acting ALC before all the rest of the processing especially when you're using various input sources - it will even out the volume differences without being obvious in operation, and without destroying the dynamic range of the programme material.

Most of the audio stuff is quite simple, and if you're reasonably smart about laying it out, Veroboard is a great way of making one-offs. I have made complete radio studios on Vero! Most of my audio stuff gets prototyped and experimented-on on Veroboard. I work on the principle that if it's stable on Vero, it'll be stable on a proper PCB!

I've also got simple designs for "Aphex"-alike circuits and delay-line limiters. I've been at this since the 1960s, and have accumulated a huge portfolio of designs. I'm going to share a few of them for the benefit of the "hobby broadcasters" around here - especially those who're operating on a shoestring budget, and can operate a soldering iron. It might be worthwhile to put some kits together, and sell them on Ebay.

[Electronically]

That will work pretty well, but requires an input op-amp to guarantee the source impedance (but you could use it for your pre-emphasis too), and you'd need an op-amp at the output to ensure that the filter sees a constant impedance.... Also, the inductors will be expensive and prone to picking up hum - even if you can find them these days. My 15kHz filter uses several dual op-amps, and sounds great.

I'm going to put up a cheap and simple stereo limiter here, and it's also (sometimes) worth having a slow-acting ALC before all the rest of the processing especially when you're using various input sources - it will even out the volume differences without being obvious in operation, and without destroying the dynamic range of the programme material.

Most of the audio stuff is quite simple, and if you're reasonably smart about laying it out, Veroboard is a great way of making one-offs. I have made complete radio studios on Vero! Most of my audio stuff gets prototyped and experimented-on on Veroboard. I work on the principle that if it's stable on Vero, it'll be stable on a proper PCB!

I've also got simple designs for "Aphex"-alike circuits and delay-line limiters. I've been at this since the 1960s, and have accumulated a huge portfolio of designs. I'm going to share a few of them for the benefit of the "hobby broadcasters" around here - especially those who're operating on a shoestring budget, and can operate a soldering iron. It might be worthwhile to put some kits together, and sell them on Ebay.

I will be honest with you Albert I'm no a big fan of stereo coders I find they do sprog on the fm but there fine in mono. Stereo sounds great. But further they go out to a listener they tend to hiss at there end. Even the the licenced stations tend to hiss alot.

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[Albert H]

You need four times the power to get the same signal to noise ratio in stereo!

I've always found that stereo is necessary if you want to build a big audience. Mono just sounds and looks amateur. Getting higher RF power together has never been a problem for me. When the other stations in town were running 20 or 30 Watts mono, I'd be running 200 Watts stereo and getting all the listeners!

[mikroman]

Here is an interesting graph of RVR STL..

[Electronically]

You need four times the power to get the same signal to noise ratio in stereo!

I've always found that stereo is necessary if you want to build a big audience. Mono just sounds and looks amateur. Getting higher RF power together has never been a problem for me. When the other stations in town were running 20 or 30 Watts mono, I'd be running 200 Watts stereo and getting all the listeners!

Fair enough Albert. But what about all the big licenced stations that kilowatts of power with stereo. There stereo doesn't sound good at all they do hiss alot. But if I put hifi on to say mono there sounding perfect.

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[Electronically]

You need four times the power to get the same signal to noise ratio in stereo!

I've always found that stereo is necessary if you want to build a big audience. Mono just sounds and looks amateur. Getting higher RF power together has never been a problem for me. When the other stations in town were running 20 or 30 Watts mono, I'd be running 200 Watts stereo and getting all the listeners!

Fair enough Albert. But what about all the big licenced stations that kilowatts of power with stereo. There stereo doesn't sound good at all they do hiss alot. But if I put hifi on to say mono there sounding perfect.

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Fair enough Albert. But what about all the big licenced stations that have kilowatts of power with stereo. There stereo doesn't sound good at all they do hiss alot. But if I put hifi on to say mono there sounding perfect

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[mikroman]

Albert will certainly say more, but here is a brief review of the unwanted noise from me.
Kilowatts may be one of the conditions for good reception, but not the rule. From the emitter aspect, there are several critical points in the technology chain where unwanted noise can easily occur. At the beginning of the chain it can be caused by poor production of sound material, cheap sound cards, insensitive microphones. Another very common cause of unwanted noise lies in misconfigured broadcast processors. Overdriving AGC, Mband processor or clippers regularly ends with the introduction of audible and unpleasant noise. The next point is STL. Insufficient level of the receiving field on the frontend of the receiver. Anything lower than -50dBm for analog MPX transmission should be unacceptable. In practice, things are completely different, and very often this weak point is the cause of unwanted noise noticed by the listener. Speaking of listeners, it should be emphasized that there is no quality stereo reception on FM without optical visibility. Even when you have a good reception field, and you do not have optical visibility, multiple obstacles lead to multipath distortion. Its effects are nasty even when listening to mono.

[teckniqs]

Fair enough Albert. But what about all the big licenced stations that have kilowatts of power with stereo. There stereo doesn't sound good at all they do hiss alot. But if I put hifi on to say mono there sounding perfect

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I agree with you there, I reckon sometimes you could try running 10 times the power and still have Stereo hiss compared to running in Mono with no background hiss at all.

....I think it depends on how good the receiver is as well as they all vary in reception quality, also most car radios seem to work really well with Stereo and don't hiss whereas 99.999% of domestic home FM radio tuners will almost always hiss with FM stereo stations - even if they are strong signals.
(You may not notice it as much through the speakers with the stronger stations, but try plugging in some headphones and you'll soon hear that stereo noise/ hiss...)

[Electronically]

Fair enough Albert. But what about all the big licenced stations that have kilowatts of power with stereo. There stereo doesn't sound good at all they do hiss alot. But if I put hifi on to say mono there sounding perfect

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I agree with you there, I reckon sometimes you could try running 10 times the power and still have Stereo hiss compared to running in Mono with no background hiss at all.

....I think it depends on how good the receiver is as well as they all vary in reception quality, also most car radios seem to work really well with Stereo and don't hiss whereas 99.999% of domestic home FM radio tuners will almost always hiss with FM stereo stations - even if they are strong signals.
(You may not notice it as much through the speakers with the stronger stations, but try plugging in some headphones and you'll soon hear that stereo noise/ hiss...)

Good point there teckniqs. I checked on my car radio stereo sounded OK. That's baffling why hifi stereos hifis hiss and car stereo seems perfect.

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[LeeCavanagh]

We used to be hiss free when using 600w into a 3element beam, although we did have good height also, Radio 2 sounds hiss free to me on my hifi

[Electronically]

We used to be hiss free when using 600w into a 3element beam, although we did have good height also, Radio 2 sounds hiss free to me on my hifi

I have to say I had good height also. but I did notice the big stations hissed alot on a hifi with the stereo on. But only a few stood out perfect was radio 1. Rest hissed alot. But as I just not that long ago checked in my car the hiss is not there on any of them.

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[Albert H]

The hiss on stereo is simply due to the increased bandwidth that the receiver has to "see" to receive the whole of the stereo multiplexed signal. Mikroman is partially right when he points out that the quality of the link goes some of the way to determining the quality of a stereo signal. You need a fully noise-quietening link for good results. We always used UHF links, and getting a bigger signal into the receiver was usually just a question of using more elements on the transmitting aerial! We used TV aerials at both ends, so getting a 20+ element aerial was cheap enough, and just needed a Trip to B&Q!

The studio end also needs to be low noise - crappy compressor / limiters, cheap mixers and so on can all add to the noise. If you haven't got a good source, the output can't be any good!

[fmmpastouni]

Here's a slightly improved layout - I've just built another one!

Stereo Coder Vero Nouveau.png

The input leads (and the MPX output) should be screened leads. All the electrolytic capacitors should be rated at 16V or greater. The CMOS ICs - the 4060, 4013 and the 4066 - must all be the normal cheap CMOS type - NOT the "74HC" or "74HCT" types - since the logic all runs at the full supply voltage. The TL074 quad op-amp is a good choice because it's low noise, but you could use TL064 or 084 instead.

A few quick checks:

If you put your frequency counter on pin 9 of the 4060 you'll see 4.864MHz. If it's a bit high, put a small value ceramic capacitor in parallel with the 33p capacitor. If it's too low, change the 33p for 27p and then pad as necessary. You don't need to be really precise - if the 19 kHz coming out of pin 14 of the 4060 is within a couple of Hz, it'll be fine.

Pins 1, 7, 8, and 14 of the TL074 should all be at half the voltage measured on the TL074 pin 4. If any of the voltages are more than 100mV away from the mid-rail voltage, there's something wrong - check for shorts, incorrect components, or you overheated the TL074 when you soldered it in!

With no audio applied, you'll see a pretty good 19kHz sinewave coming from the output. Connect the coder to your exciter, and turn the mod up until the stereo pilot on your test receiver lights. Turn it up just a tad more to guarantee that there's enough 19kHz for all receivers. Apply audio, and you'll be pleased at the quality of the results. The stereo image will be pretty good.

One warning - remember that you need roughly four times as much RF power for the same coverage in stereo!

Hello.Can you post the schematic of the stereo encoder?

[Albert H]

Here you go!

stereocoder3A final.png

It works well on any supply from 12 - 15V, and gives pretty good results. You can easily add a mono / stereo switch (the original board had a jumper) and the LEDs are red for mono and green for stereo. Obviously if you're putting your pre-emphasis before your limiter, you can omit the pre-emphasis capacitors on this board, and replace the paralleled 100k resistors with single 47k on each channel.

Unlike the A****F stereo coder, this one knows its right from left! That's what the monostable made from the otherwise redundant half of the 4013 is for. You can use a 4066 instead of the 4016 without any change in performance - the 4066 may be easier to find these days. The diodes (other than the LEDs) are 1N4148. You could use a TL072 for the audio input op-amps and a TL071 for the output filter, if you don't like "wasting" op-amps. I have used the "spare" op-amp in the TL074 to provide the mid-rail bias (at low impedance), but this isn't necessary and doesn't improve anything (it's just a way to use the left-over op-amp).

[radionortheast]

I am looking to build this! the cd4066be came today, the cd4060 has no be on the end hope it will be ok, I will have to look at the tl074 when it comes, I got the crystal from engima, they seem to deliver fast it was abit expensive, the veroboard will take a few days to come, I intend to use ic sockets for the ics. I’m glad it has pre-emphasis and mono switch some encoders don’t have that, i’ve used pre-emphasis in stereo tool found it didn’t sound as good as the on board pre-emp.