SCMO Steals Power from FM Main Signal?

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SCMO Steals Power from FM Main Signal?

Postby jon » Sun May 17, 2015 8:28 am

I just read a recent post elsewhere where the poster implied that FM stations that stopped providing SCMO services now have a stronger signal because the power to deliver the SCMO channel(s) was "counted" as part of their allowed power: Average and Maximum ERP values.

Anyone know the technical details of any of this?
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Re: SCMO Steals Power from FM Main Signal?

Postby Mike Cleaver » Mon May 18, 2015 12:31 pm

I never worked at an FM station where SCMO was permitted.
All of the senior engineers were opposed to it for various reasons.
The most common complaint was that it degraded the main signal but I never really got into the discussion as to how that occurred, being mainly an AM guy.
CHUM-FM refused attempts by CHUM owned Muzak in Canada to use it's subchannel to distribute Muzak.
The same was true with CJCA-FM later CIRK-FM in Edmonton, even though the Muzak franchise there was owned by the then Station Manager.
Luckily, that controversy went away when Muzak went to satellite transmission.
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54 years experience at some of Canada's Premier Broadcasting Stations
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Re: SCMO Steals Power from FM Main Signal?

Postby groundwave » Mon May 18, 2015 12:44 pm

jon wrote:I just read a recent post elsewhere where the poster implied that FM stations that stopped providing SCMO services now have a stronger signal because the power to deliver the SCMO channel(s) was "counted" as part of their allowed power: Average and Maximum ERP values.

Anyone know the technical details of any of this?


As the initials convey, SCMO is a modulated subcarrier and as such, represents the use of a small (but finite) amount of modulation room as "seen" from the perspective of the transmitter. Thus, an existing non-SCMO FM plant would theoretically require that it's composite (stereo) audio modulation be lowered by some degree (likely a few dB or so) to accommodate the "room" required for SCMO. Conversely, an existing SCMO transmission would be able to reclaim some modulation space upon discontinuing SCMO. Given the constant-amplitude (carrier) nature of FM, the presence or absence of SCMO should have absolutely zero impact on power - at least as measured at the transmitter.
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Re: SCMO Steals Power from FM Main Signal?

Postby jon » Mon May 18, 2015 3:40 pm

As a teenager in Vancouver, I hated SCMO. At that time, CKLG-FM provided the SCMO for Muzak. And CHQM-FM provided the SCMO for their own Q Music.

My Father bought an EICO FM Tuner kit in 1961 in Bellingham, shortly after CHQM-FM signed on. He did not buy the Multiplex unit, which was an expensive add-on. Both QM and LG had significant white noise on that receiver. LG-FM could be really bad because they did not run the audio compression that CHQM-FM did, so quiet passages of music got hit pretty badly. It only got worse when Tim Burge (and possibly others) cut back on the levels for spoken content, in reaction to listener complaints about the announcer's voice and commercials being so much louder than the music.

I should clarify: CHQM ran different audio compression specs on AM and FM. When I worked there in 1971 and 1972, you could easily see it as they had modulation percentage meters, which looked somewhat like VU meters, in the racks in or near the transmitter remote controls. The FM "attack" when a piece of music got suddenly quiet was fairly slow, and you could watch the meter as the compressor gradually raised the level. AM was pretty much sub-second response. Lest you think I should have been doing my job rather than watching the meter, I should mention that the FM meter was in the same racks as the automation machine (the ancient IGM Serial Number 0000005), which was right beside the AM transmitter remote controls and monitoring.

A couple of interesting points have come up since I originally posted this. One post I read says that a Toronto ethnic station on SCMO has always been careful to contract with a Mono FM station for SCMO because they claim it gives them a cleaner signal on the SCMO side, because there is no Stereo Multiplexer 19KHz "tone" being transmitted. Another post pointed me to a bunch of different no-name SCMO-capable portable radios that also receive standard AM and FM. That poster has seen them for $2 used at Goodwill, but the ones on eBay are $20-$35 U.S., though I think the cheapest one was just a circuit board. It might be interesting to see just what is being carried on SCMO these days. I haven't seen an SCMO license being granted or renewal on the CRTC site in a very long time, but that could be more to do with the new Admin Renewal "quick stream" that the CRTC uses now.
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