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Old 13th February 2009, 07:33 PM   #1
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Default PC interferes with FM transmitter

(Moderator: This is a corrected version of a post I submitted minutes ago.)

I have a 500mW FM stereo transmitter that I acquired to transmit streaming audio (pandora.com, last.fm, etc.) and MP3 files from my laptop computer to the various FM radio receivers in my home, and out in the yard in the warm months. (Unit is: mobileblackbox.com, Eclipse-4000).

The transmitter has a BNC-connected 6.5" rubber antenna, and I output the sound from my computer's headphone jack with a 5' cable with 3.5mm stereo plugs into the transmitter's line-in jack.

The problem is that the sound coming from my FM radios is scratchy (mostly on the higher, treble sounds). It's not an overload sound (I have all of my volume settings carefully adjusted, and the XMTR has automatic gain control). It sounds like an FM radio does when it's slightly off frequency. This happens both when I'm streaming audio from the Internet or playing MP3 files with Windows Media Player. Somebody more audio-savvy than me said that what I'm hearing is "high-end clipping."

ALL other audio input sources that I've tried with my transmitter give an excellent FM signal and great range (two iPods, two other laptops, a desktop PC, and audio from my stereo TV).

And I've tried just about every possible way to eliminate outside interference (moving my PC and XMTR to a different room, unplugging my cordless phone, turning off my laptop's wireless capability and using an ethernet cable, etc). So, I'm pretty confident that the problem resides within my computer.

On another web site, somebody told me: What it sounds like based on what you're describing is a problem with high frequency sounds from your program material competing with the 19 KHZ pilot. An easy work-around is installing an equalizer between the source and the transmitter. Then you 'hard limit' anything above 15 KHZ. Carefully adjust your audio so you don't over-deviate (75 KHZ) and you're good to go.

So I've pretty much decided on buying a small, 9-band graphic equalizer: the Behringer FBQ800 <http://www.behringer.com/EN/Products/FBQ800.aspx>.

BUT, before I spend the $60 on the EQ and cables, I wanted to ask if anybody has any idea what might be going on with my computer that's apparently causing the problem with my FM transmission.

Thanks.
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Old 13th February 2009, 08:17 PM   #2
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Some of the 3.5mm audio cables are not well shielded, try a different brand cable also try a different length. Get a 2 piece clip-on ferrite choke from Radio Shack. Try it at different locations on the power cord and audio cable.
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Old 13th February 2009, 09:32 PM   #3
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Default Cable

Thanks, Kevin.

I'm using an expensive, well-shielded cable which came with a molded-on ferrite filter. I have tried other cables; all 6 ft and under. Maybe I should try a MUCH longer one.

Also, I should have mentioned in my original post that I believe my PC's sound card and headphone jack are OK because I get great sound from my powered speakers and from headphones.
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Old 14th February 2009, 12:21 AM   #4
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Hum,well,I was going to suggest some ferrite cores as well,but you say the cable already has them. Is there one at each end?

Did the antenna come with the transmitter? Hopefully it's somewhat tuned,and doesn't represent a high SWR/mismatch. (could cause issues.)

So,it's fine with other sources,but crummy with the one lappy..
Is it still noisy if you unplug the laptop from the charger/power supply,and just run it from battery power?

As for the EQ,if the transmitter already has an AGC,and some filtering,it should be fine as-is. It doesn't seem as though you've got the distortion/noise with any other sources,so I would guess that's it's OK...but for whatever reason,the laptop is just noisy.

Also,many times you can adjust the EQ a bit in software.When I was playing with my FM transmitter,I'd usually just set Winamp to 'shuffle' mode,and use it's built-in EQ to cut the treble,no other hardware needed.
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Old 14th February 2009, 03:21 AM   #5
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Hum,well,I was going to suggest some ferrite cores as well,but you say the cable already has them. Is there one at each end?

I have a core at one end of the audio cable. None on the AC cable. Could this be a big potential issue?

Did the antenna come with the transmitter? Hopefully it's somewhat tuned,and doesn't represent a high SWR/mismatch. (could cause issues.)

The antenna came with the XMTR. And, as I said, the XMTR and antenna work great with all of the other input sources that I've tried.

So,it's fine with other sources,but crummy with the one lappy..
Is it still noisy if you unplug the laptop from the charger/power supply,and just run it from battery power?

I have tried with battery power only; no help.

Also,many times you can adjust the EQ a bit in software.When I was playing with my FM transmitter,I'd usually just set Winamp to 'shuffle' mode,and use it's built-in EQ to cut the treble,no other hardware needed.

I haven't tried any software EQ. I'm using Windows Media Player which doesn't seem to have that option. And the streaming audio from pandora.com (that's about 75% of what we listen to) won't allow me to do any software EQ. I can try Winamp for my MP3s. Is there some way to set up software EQ for both MP3s AND the streaming sites?

Thanks for your reply.
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Old 14th February 2009, 04:13 AM   #6
dmills is offline dmills  United Kingdom
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FM stereo is low pass filtered with a "Brick wall" response at 15Khz to avoid trashing the 19Khz pilot tone that is used for both stereo switching and as a phase reference for the 38Khz DSB modulated S component.

A normal graphic eq will not really help as it can often only apply 12db of cut when you are really looking for at least a 6th order lowpass response.

Further, FM applies a hf boost to the audio to counteract the rising noise floor inherent in this form of transmission, so it is some 14db easier to hit the limiters at 14K then it is at 1Khz, combine these two effects and if that transmitter is missing the appropriate filtering and your sound card has badly behaved outputs, you can very quickly get into a real mess.

Try playing a lowish bit rate mp3 from that computer (these are normally lowpassed as part of the coding precess), and if that sounds as reasonable as low bitrate mp3 ever does, then it is a filtering problem, otherwise it is something else.

Regards, Dan.
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Old 14th February 2009, 04:15 PM   #7
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Thanks, Dan.

I tried a few low-to-very-low bit-rate MP3s on the laptop. But I still get the clipping in my FM output.

Somebody suggested that I might be experiencing an impedance mismatch between my headphones-out and line-in connections. Does that sound like a possibility? If so, does this inexpensive "hi/lo impedance adapter" look it it might do the job?: http://tinyurl.com/pyramidNS60
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Old 14th February 2009, 05:28 PM   #8
dmills is offline dmills  United Kingdom
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Unlikely.
You might just be driving the transmitter too hard, try turning the output of the computer down.

Regards, Dan.
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Old 14th February 2009, 07:32 PM   #9
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Turning the computer's output down was the first thing I tried. I carefully adjusted the system and Windows Media Player and pandora.com volumes.

Plus, the other input sources that I tested (which all worked well) needed no adjustment. I could max out the output on iPods and other PCs, and the transmitted FM audio was excellent.
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Old 14th February 2009, 07:40 PM   #10
dmills is offline dmills  United Kingdom
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Any way you can have a look at the spectrum of the signal on the computers output?

I am thinking that maybe there is a lot of ultrasonic hash up there (due to poor filtering of the DAC) that may be being mixed down into the audio band by the DSB modulator in the stereo multiplex generator.

Can you try a different soundcard?

Regards, Dan.
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