SCA broadcast

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Darth_vader

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Well, let's see: SCA was originally used to broadcast Muzak programming in the particular markets they had a presence in, either relayed via telephone trunk to the host FM radio station (often an easy-listening station... imagine that) or played back from large reels of 1/2" tape on site at the station. This was how they did it largely in the era before satellite transmission took over (and for quite a number of years afterward, as a legacy service) but Muzak SCAs are, to my knowledge, extinct today. For the most part what you'lll hear these days are radio reading services for blind people and ethnic programming. Occasionally you may also encounter data broadcasts and backup STLs for local mediumwave stations.

Until about the late 1980s SCA broadcasts were considered a "subscription only" medium, meaning one (usually) had to give an organisation money to hear their broadcasts. SCA was largely deregulated by the FCC (even though the information on their web page doesn't seem to reflect it) and transmissions like radio reading services can usually be listened to freely.

SCA audio quality generally has a dynamic range of around 4 kHz; about on par with a typical mediumwave station. Broadcasts are transmitted, in North America, on narrowband subcarriers 66.5 (usually rounded to 67) and 92 kHz above the mono baseband. SCA frequencies are determined based on harmonics of the FM stereo pilot (19 kHz). Audio is modulated, at best, around 10% power of the main channel, thus it is far less tolerant of multipath and distortion than the main FM broadcast. Spillover from the main channel can sometimes be heard faintly in the background of SCA broadcasts, depending how wide the radio's frontend is.

It is sometimes possible to hear SCA audio by setting a police scanner in narrowband FM mode to the SCA frequency (e.g. 88.367 in Portland gets you Radio Svoboda via KBVM) but the best way is by using an SCA receiver or an FM broadcast receiver fitted with an SCA demodulator (see links below.) Most conventional police scanners lack the filtering needed to receive SCA audio, and it almost always gets obliterated by the main carrier. It is, however, also possible to fit them with SCA demodulators. Tine to dust off that old PRO-2006 you've had setting in the corner for the last 20 years and put it back to work!

Are you familiar with a publication called "FM Atlas"? The publisher/author (Dr. Bruce Elving) died a couple years ago, but he was a wealth of information about SCA and sold modded FM radios and kits to convert them to FM/SCA receivers. His book, the "FM Atlas", was published every three (?) years and included lists of SCA transmissions in major cities.

For the most part, analogue SCA isn't as popular today as it was 15-20 years ago. The fairly recent Ibiquity "HD Radio" system may ultimately be its successor. Still, if you live around a major urban area (I believe you once said you live in/near Chicago, correct?) you should at least be able to receive one or two, likely a radio reading service or a foreign-language service (or both).

http://metrosonix.com/ - one vendor of SCA radios
http://mistman.pdp10.org/pub/radio/sca.txt - article written by Dr. Elving explaining it
http://mistman.pdp10.org/pub/radio/sca_demodulator.png - schematic diagram to build an SCA demodulator circuit (I don't remember where I found it)

I also have an SCA radio and could record some audio so you can hear what it sounds like, if you want...
 
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mancow

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You can demod it with an SDR dongle hooked to the discriminator of a radio set to wide FM as well.
 

poltergeisty

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Didn't think of that. How would you feed baseband audio into the SDR?

Edit - Just looked for HF SDR's on eBay and there are none, just up convertors. You need a SDR that can tune 20KHz to 99KHz.
 
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poltergeisty

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I also have an SCA radio and could record some audio so you can hear what it sounds like, if you want...
Yeah, I would like to hear some stuff. BayFiles

I found a seller on eBay that sells SCA radios, but I'm not too sure what they are like. I like the AM/FM/SCA ones you linked to.

Can the MS-3330 Silver listen to AM SCA too?
 
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mancow

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Didn't think of that. How would you feed baseband audio into the SDR?

Edit - Just looked for HF SDR's on eBay and there are none, just up convertors. You need a SDR that can tune 20KHz to 99KHz.
If you can get access to the discriminator you feed it to the SDR directly. I was using an AOR SR2000A in FMW feeding a nooelecric HF upconverter hooked to an RTL dongle.
 

mancow

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So you are feeding the baseband audio into the SDR's antenna?
Not directly, it was going into the upconverter first then than fed the SDR's antenna jack.

Radio 455 kHz IF ---> Upconverter antenna input, Upconverter antenna output ---> SDR antenna input.

So the IF gets converted to 100 MHz + 455 kHz then the SDR tunes that frequency but with the software set to subtract the 100 MHz visually you get the equivalent perception of receiving the raw HF/MW signal directly when in reality you are receiving it with 100 MHz added.

Confused yet?
 

Darth_vader

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"I found a seller on E-Bay that sells SCA radios, but I'm not too sure what they are like."

Is it this one? http://ebay.com/itm/261271369674

I've had one in service for around 10 years now (acquired from E-Bay) and it works very well. The only thing I've had to replace on mine was the speaker, since it had been left out in the rain one afternoon, but is otherwise solid. The slide rule on mine looks slightly different, being an older model (picture here: http://imgs.tootoo.com/f4/7b/f47b2e1c1922d0b115211ecf9e619437.jpg) but they're probably all the same inside.

Manufacturer (?) is a somewhat obscure Hong Kong company called "Okano":
http://tootoo.com/d-rp3194412-sca-scmo-decoder-radio-subcarrier/
http://made-in-china.com/showroom/okanohk
http://radiosca.com/ (long-defunct E-Bay vendor)
 

n5ims

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The small market radio station I worked for used to get their ABC Radio Network feed via SCA from a station in the nearby large city. This was when the network was phone line based and it was cheaper (and often more reliable) to feed the small market stations via SCA from powerful FM stations than them leasing phone lines everywhere. Once they went satellite, the network made us all buy dishes and get their feed that way.

What you find on SCA channels vary largely with the market conditions. When they weren't very popular you had Muzak (perhaps several channels), some Remote Reading Service channels (think "reading for the blind" type of thing), and a few things for the station's use. This was somewhat fun at times. There were some IFB channels for the station and it's AM sister station (often after sunset, the low "nighttime" power levels made these AM signals hard to pick up on the gym floor for high school basketball games). Some used it as an STL for the remote AM tower feed. The most interesting one I knew of was a sort of slow-scan TV signal from the transmitter site so the engineer at the station could monitor the transmitter site visitors.

Once SCA distribution became popular (and a real revenue stream) it was pretty serious business and the fun stuff (that generated no income) was shut down. You had data streams of the financial markets, feeds to make sitting in waiting rooms (often Dr. office waiting rooms) more pleasant, informative, and often pushing certain meds. There were a few "distant learning" services that provided audio broadcasts (or would that be narrowcasts?) to train folks in various subjects like mechanics on how to work on the new (at the time) electronic controls in cars or other subjects.
 

N9NRA

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Been reading this with intrest, and got to wondering, i have an IC-92AD ham HT that`ll recieve the AM and FM broadcast bands as well as the usual stuff, just curious, but will my HT be able to recieve an SCA broadcast or not? Got to thinkin` that since the front end is a bit "tighter" than a scanner i might at least try it just for kicks. Thoughts? Thanx! N9NRA
 

Darth_vader

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You could try it, if your rig tunes 1 kHz increments. For example, if a station on 88.3 transmits a 67 kHz subcarrier, try "88.367" (or if you want to be super-precise, 88.3665). Can't guarantee you'll hear anything, since baseband and stereo difference signals are considerably more powerful than SCAs and will probably walk on them when using that sort of rig. If your rig has a de-emphasis or selectivity control (probably doesn't if it's an HT) try opening it up, then tighten it back down gradually until you can hear the SCA audio clearly.

By the way, radio data service (RDS) subcarrier = 57 KHz (19 * 3). If you hear a constant high-pitch "buzzing" noise when tuning past some stations without SCAs, particularly on 67, it's spillover from RDS. This is normal.
 
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poltergeisty

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Darth_vader

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I would strongly recommend it, though, especially if you travel and want to do any SCA DXing at your destination (or in transit). The mediumwave section is nothing special (doesn't get mediumwave SCA, either) but the FM section is very sensitive and was probably one of the best monophonic DX rigs I had before Sony introduced the XDR-F1HD.
 

Darth_vader

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Yeah, that one. It does receive mediumwave (AM) stations but only receives 67/92 SCA on FM (88-108). I don't know of any receiver today that does SCA on mediumwave, or if that method was ever advanced beyond experimental trials in the 1950s or '60s.

Words of advice: the mediumwave section, should you decide to use it, may need alignment. Out of the box, mine was so far off it was deaf to nearly everything between 530 and about ~860 kHz. Even after adjustment it's still a little "fadey" near the band's low end but not like it was. FM/SCA has always been solid and not needed adjustment (so far). (Individual results may vary.)
 
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