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Anyone ever build a GOOD decoder for SCA?

wa2ise

wa2ise

Super Member
Oh, I've built the usual SCA decoder that uses a PLL chip, but they all suffered from crosstalk from the main program of the FM station. As the SCA subcarrier is about 20dB lower, that's not a surprise. So it would seem that a good band pass filter ahead of the decoder circuit with good rejection below 57KHz would be needed, and it did help. But maybe bad multipath reception of the FM station also cause crosstalk?
 
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I built this one many years ago and got very good results with careful adjustment of the 10k pot I put in instead of the specified 5k pot.
I did notice that strong stations did perform very cleanly. Weak ones with multipath weren't so great.
 

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Did some simulations of a passive band pass filter tuned to pass 57KHz and higher, and to stopband about 15dB down around 55KHz (high end of the stereo difference subcarrier). This to reduce the amount of main program channel audio hitting the SCA decoder. Used a passive filter as it won't have issues with overload or oscillations. This helped reduce crosstalk from the main channel to the decoded SCA signal (still present but faint). That remaining crosstalk might be blamed on multipath... :scratch2:
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The coils I salvaged from dead CRT computer monitors (these coils probably operated around 68KHz horizontal deflection rates you get with a computer's VGA card set to 1024x768@85Hz vertical refresh). So they'd be a natural choice for SCA subcarrier frequencies.
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Multipath does have a large effect on recover of the sub!

Also the stations tuning on the transmitter does too! The more symetrical the bandpass tuning is, the cleaner it will be!

The pro's in the business of providing retail background music found that they liked using yagi's turned at a 45 degree midpoint between vertical and horizontal to work best for clean reception.

Mark
 
Mark,

Seconded on all the above. Do remember the modulation on a SCA is a bit weaker and signal needs to be of good consistent quality. Also, the quality of and maintenance done on the station SCA equipment has a lot to do with the quality of SCA transmissions. If a transmitter is even a little bit assymmetrical the SCA audio gets fluttery!
 
evsentry3 said:
The more symmetrical the bandpass tuning is, the cleaner it will be!
Click to expand...

I tightened up and made it flatter the bandpass filter, and it helped lower the distortion on the SCA decoded audio. It's not hifi, but listenable...
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The 10K resistor dampens the ringing some, albeit at the loss of a couple dB of signal level (I normalized the plot above ignoring that).
 

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G'day all, I don't know a thing about SCA as it was never used here in Australia (as far as I know), but here is a scan from a very old (1970's) American electronics magazine that I have that might be of use to you. I don't know if the chip used is still available! Regards, Felix (vk4fuq) aka Catman.

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catman said:
G'day all, I don't know a thing about SCA as it was never used here in Australia (as far as I know), but here is a scan from a very old (1970's) American electronics magazine that I have that might be of use to you. I don't know if the chip used is still available! Regards, Felix (vk4fuq) aka catman.
Click to expand...

Looks like a copy of the one shown in post number 2 above.

The only decoder I actually heard working (and it sounded quite good)
was part of a telecommunications training system by Lab-Volt:

http://www.labvolt.com/products/tel...-communications-training-system-8080/overview

Regards.
 
G'day mate, yes the two circuits are identical! My magazine circuit is from 1977. Interesting. Regards, Felix aka catman.
 
Circuit I'm using is built around a CA3089 FM IF-detector chip meant for regular FM broadcast receiver work. It detects FM with a quad circuit. I also built that LM565 circuit, but if memory serves, the CA3089 worked better (though it's many years ago I did it). What I did this week is the better band pass filter.
 
sca decoder

I want to build a sca decoder. I am looking for a kit from Ramsey for $40, it has a case and looks pro. It needs to be connected to the fm detector of a fm tuner (no problem,I know where mine is). The output is mono that goes to a line preamp. I assume this is mono because only one output. Can I feed this signal back via a switch to the mpx section of the tuner and get stereo (I assume sca can broadcast stereo). I plan to connect it to the 12 volt supply in my preamp instead of using the internal 9V battery.

What is the quality of the sound? Specs of the decoder are 5uV and 20db crosstalk. Can specs be better from other decoders?

Can I find rock and roll, classic rock on the sca from the 60's 70's? What kind of music can I expect to find. Don't say elevator or country music!!!

:music:
 
wa2ise said:
I tightened up and made it flatter the bandpass filter, and it helped lower the distortion on the SCA decoded audio. It's not hifi, but listenable...
attachment.php
The 10K resistor dampens the ringing some, albeit at the loss of a couple dB of signal level (I normalized the plot above ignoring that).
Click to expand...

I like your design. The best thing about it is, it is passive. Hoorah. In another thread I am considering buying the Ramsey SCA decoder kit for $40 and I will probably use your filter if I need to inprove the crosstalk (probably will use your filter anyways, it cannot hurt).
 
Mitchell said:
I want to build a sca decoder. I am looking for a kit from Ramsey for $40, it has a case and looks pro. It needs to be connected to the fm detector of a fm tuner (no problem,I know where mine is). The output is mono that goes to a line preamp. I assume this is mono because only one output. Can I feed this signal back via a switch to the mpx section of the tuner and get stereo (I assume sca can broadcast stereo). I plan to connect it to the 12 volt supply in my preamp instead of using the internal 9V battery.

What is the quality of the sound? Specs of the decoder are 5uV and 20db crosstalk. Can specs be better from other decoders?

Can I find rock and roll, classic rock on the sca from the 60's 70's? What kind of music can I expect to find. Don't say elevator or country music!!!

:music:
Click to expand...
1. SCA decoded audio is mono only, and only goes up to around 5kHz. Think telephone line quality and you'll be very close.

2. I built mine inside a Technics SA-222 receiver, with the adjustable pot on the top panel. I fed the output into a spare tape input. I did notice that I had to move the receiver's volume control very slowly, otherwise it sounded like one that badly needed a De-Oxit'ing.

3. The music I found was mostly instrumental. Yes, some elevator music, but also some supermarket background music. Some had feeds of remote mikes, others had National Weather Service weather broadcasts, info for doctors, or radio reading services for the blind. Some of the "programming" was FSK datastreams, usually RDS info. There are a maximum of three possible subcarriers, but I never heard more than two in use on any one station.

http://www.blackcatsystems.com/radio/sca.html
 
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sca decoder frequency range

Well I did some looking around on the www. I found out the upper frequency range for sca is 7khz. I know from school that telephone voice quality is 300 to 3000hz. This looks good. Looks like I will get a kit from Ramsey after the holidays.:music:
 
sca audio quality

I posted that sca can go to 7khz but most music only goes to 5.5 khz or was it 5khz/
Anyways that makes the quality bad, so I will drop this idea.

Instead I will put up a small satellite dish (3 feet dia) and get DMX on c-band.
Quality is suspose to be good. The signal is digital in the clear, I need to verify this. Project for spring time.
 
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