FM Modulators

[gordguide]

This shouldn't be a problem, but apparently it is. I'm looking for a wired FM modulator (not a wirless FM transmitter, which for some reason everyone thinks is the same thing) and apparently if you live in Canada and want one, you may as well be on Mars. In a city of 250,000 not one reseller I've checked out has one; most insist I "really want" a FM transmitter because [insert lame "tell 'em anything" reason here], which translates to "we have this transmitter in stock, and it's only $70". Well, I don't want a transmitter, thank you very much.

eBay is an option, but I would rather avoid paying $40+ to get a $20~30 item to my door.

Got any leads?

I would prefer one with the on/off switch (like the Clarion FM200) but I'm willing to look at anything. I don't understand why this should be so difficult; no wonder nobody shops at bricks-and-mortar stores anymore.

[Jonathan@TCS]

I have both Wired and Wireless FM mods in stock. The Wired ones sell for $69. They don't have a switch, but it wouldn't be hard to wire one in with it.

If you are in Calgary, come down to THE CAR SALON on 32 AVE.

[gordguide]

The switched units cut the regular FM antennae off to prevent interference, noise and in some models ground loops (others use a standard balun in the circuit to eliminate ground loops), which is a problem with my vehicle as it uses the crappy GM in-windshield antennae that is very difficult to get at in order to float the ground which I haven't been able to get to yet (you need to use a mirror to see the connector at the base of the windshield; it's actually much easier to remove the windshield to do it).

+12V is wired to the accessory bus so that the unit is un-powered when the vehicle is off, but that's not the function of the switch in the units that have them, and the switch shouldn't be mistaken for that function.

Some others are auto-sensing, cutting off the antennae via a relay when signal is present and restoring it when absent (eg the Sirius Modulator; like the Sirius Receivers they are manufactured by Directed Electronics # 14110 $US 24.95 MSRP).
http://www.directed.com/audio/sirius/carAccessories.asp#ca_fmMod

Perhaps you missed it in my parent post, but last time I checked Calgary was a bit bigger than 250,000 people. It's a bit of a drive from here, so don't expect to see me wandering in anytime soon, but thanks for the quick response.

[Jonathan@TCS]

I have never had a problem with the original Ant interfering with the MOD signal. Only the MOD interfering with FM reception, that is why I suggested the switch to turn it off when not in use.

[rage2]

We're in Calgary, there's a lot of good empty slots where interference with other channels is NOT a problem. We don't have that many radio stations here, compared to big cities in the states. They have a friggin' new station every 0.4 on the dial.

92.9 I find is the best slot with absolutely zero signal overlap, but I heard they're starting a new station there.

[gordguide]

High and Medium power transmitters in Calgary (some of these are "special" and won't be available on a regular FM radio; eg. the MUZAK or "elevator music" channels, but they still can affect channels in the regular band with interference) :
Class (C= high, B= medium)

C1 Calgary 47 88.10
C1 CJSI-FM 88.90
C1 CJSI-FM(1) 88.90
C CBCX-FM 89.70
B CJSW-FM 90.90
C CJAY-FM 92.10
B Calgary 42 92.90
C CKUA-FM-1 93.70
C1 CHKF-FM 94.70
C CHFM-FM 95.90
C1 CKIS-FM 96.90
C CIBK-FM 98.50
C CBR-FM 102.10
C CIQX-FM 103.10
C CBRF-FM 103.90
C CKRY-FM 105.10
B RCRFM-015 106.10
C CFGQ-FM 107.30

Add in the broadcast TV stations (the FM band is between channels 6 and 7 on the television band, channel 6 is directly next to FM at < 88 Mhz) and the low power transmitters (there are literally hundreds of active 10 Watt and Low Power licenses from Industry Canada in Calgary, all of which are much more powerful than the one sold for car, XM/Sirius or iPod use) and you can see that the whole idea of a non-interfereing channel is silly.

You must rely on the quality of your transmitter, your FM antennae and radio electronics, and basic luck (too bad if you live near an airport, for example).

Intermodulation is a problem, and affects all radio in the 30~300 Mhz VHF band (VHF-TV, FM radio, portable and mobile VHF-FM radios, a lot of private transmitters for aviation communication and control next to FM beginning at 108+, and the 720 Aircraft Communication frequencies).

Frequency A + Frequency B = intermod frequency
Frequency A - Frequency B = intermod frequency
You must avoid the intermodulated frequencies as well as the broadcast frequencies; they will have relatively high powered interference on that channel.

C1 Calgary 47 88.10 vs C1 CJSI-FM 88.90 = difference of 1.8 Mhz, therefore there will be interference at:
88.10 +/- 1.8 and 88.9 +/- 1.8, so that:

86.3 (outside FM band, so no problem); 87.1, 88.1, 89.9, and 90.7 will all have some interference being broadcast on them.

(If there is just one station, no problem. Two or more, and they interact with each other, producing interference at mathematically related frequencies).

You would need to calculate the intermods for all the station frequencies being used to determine the clear channels (there are second, third, etc. harmonic intermods but much lower in level, so we can ignore them, luckily).

FM radios have problems when there is interference 200 Mhz away and usually bigger problems when they are 100 Mhz away; ie if you are set to transmit at 88.1 then it's bad if there's a transmitter in your town at 88.0 or 88.2, and it's a bit better, but not good enough yet, if they are at 87.9 and 88.3).

They are actually much better if both your and the other transmitter are on the exact same frequency; a capture ratio of 1dB (1.5 or 1 dB on good home FM receivers is hardly rare, and although it's usually not that good on most car stereos, it's still only a few dB) means if your transmitter is only 1 dB louder than the local radio station, the receiver can reject the unwanted station perfectly.

Since so many people who use them complain about interference, it seems obvious the transmitters are not so hot to begin with. But, if you are using one, choose a channel either 300 Mhz away from regular broadcast channels (+/- xx.30) or if that's impossible, you should choose to sit right on top of a fairly weak station or intermodulation frequency, as your radio is better at rejecting that than it is at rejecting adjacent channels.

Personally, I've definitely given them the old college try, and I'm not happy with the results. Since every radio and every car installation is different, your mileage may well vary, and you might be happy with the results.

But, I'm looking for other options.

[theken]

visions mcleod put mine in, had a switch, give them a call

[Afrodeziak]

I just purchased one on ebay, installed it last week, no switch.

I am actually getting some interference. High pitched buzz when I have my ipod plugged in and playing through the frequency I have it set on. I will have to try the other stations and as well narrow the squeal down to the modulator, since i just installed the deck also (older deck with no Audio input)

[gordguide]

" ... am actually getting some interference. High pitched buzz ..."

The "high pitched buzz" is a classic ground loop, probably, although it's possible it can be HF (EMI) interference from the alternator's circuitry or the path of your wiring, or poor cables.

Most people wire up the +12V to a single point (usually, the factory accessory socket at the fuse box, but wherever) but then use chassis ground (where you either rely on metal-to-metal contact where you mount the gear, or run a very short wire to a point on the chassis, usually a bolt or screw nearby).

This works OK for very simple installations, sometimes, but will fail sooner or later if you start getting good gear or multiple pieces of gear.

At that point, at a minimum you should insure there are no ground loops. From your brief description, you have ground points at:
The deck you installed;
The antenna in your car;
The FM modulator;
The iPod's power adapter (if you use one in the car).

You can make two checks:
Run the iPod off the iPod's battery (unplug from any car power adapter) and see if the noise goes away.
Unplug the car antenna from your modulator (leave the modulator connected to your deck) and see if the noise goes away.

If it does, you have a ground loop, son. If not, you still might have a ground loop, but you will have to fix it as if you knew you did have one to find out for sure. It's a good idea anyway, actually.

Ideally, you would float the grounds on all gear and connect the -12V to a single point, much like you already do (or should be doing) for +12V.

You may find that's not enough, if the alternator is causing RFI interference in your power wiring (ie it's more than just a ground loop that's causing your problem).

In that case, connect directly from your +12V point to the car battery rather than the accessory connector on your car fuse box, and do the same for the -12V ground.

Note that I mean to continue to use "star wiring" technique; ie one +12V from the deck, one from the modulator, one from the power adapter, connected together at one single point, then run one wire to the battery. Same for GND; use "star wiring".

Fuse the +12V line as close to the battery as possible. Be sure to use wire of sufficient grade and gauge to avoid unpleasant things like, Oh, I dunno, a fire.

If you don't know what you're doing, get a pro to do it.

I use things like foam mounting tape, nylon bolts, washers, nuts, and screws, and high voltage rubber tape (3M) to isolate gear. Good old wood and fiberglass work fine as well, depending on what you need to do.

If you're using the iPod with a power adapter, you might want to consider buying an extra accessory/cigarette lighter socket and wiring that, leaving the factory one alone. You can get 2 and 3 socket ones too, if you want to add XM or Sirius later, or use one with a cellphone or GPS, and they can still be mounted out of sight if you want.

[gordguide]

Well, it was the most difficult purchase I've ever tried to make. Ever. I bet I spent 40 hours trying to hunt one down, including the better part of a day going to every Bricks-And-Mortar store I could think of, not one of which had one in stock, ever brought them in stock, or even offered to order one in; instead insisting I didn't really want one.

All the places in Canada were out of stock ** (when I say "all", I mean the very, very few I could find online in Canada that sold them); all the places in the 'states either didn't do Canada (fairly common with car stereo, it seems) or shipped via UPS, which is an automatic deal-killer for me due to their usurious brokerage fees.

So, after more than a month of looking, I have one on the way.

I found it at The Source's Online Store (you know, the old Radio Shack), and it happens to be a model that is highly recommended by those who've tried this route.

Online only; not sold in the stores, for $C 56 to your door, and while that is not a great price, it's fairly close to what it would cost to land one from New Jersey or California at a good price plus shipping, not forgetting $8 for Canada Post's brokerage fee.

** Why is it that retailers won't bring in an item that sells out online? The Source was sold out for weeks, and of the dozen they brought in this week, I ordered the same day and managed to snag the second-to-last one.

In fact, if it wasn't so hard to get my hands on one, I would have walked because, just like the Radio Shack of old, they still want to know your life story before they sell you anything.

Unfortunately, when you buy online you can't give them crap (there must be thousands of Radio Shack catalogs waiting for me at the Toronto Airport, whose postal code I memorized just for such occasions, it's what I used when they wouldn't buy "Mr Cash" and "I'm homeless").