Recording FM stereo radio with Audition: ringing

[mikebo]

Hi - I'm new to this forum. I've used Cool Edit for many years but finally broke down and got Adobe Audition. Here's my problem.

I'm trying to record material from FM stereo broadcasts, but there is this super annoying ringing that seems to be triggered by high-end material such as snare and cymbal hits. With the spectrum analyzer in Audition I can see a constant spike at 19Khz which I suspect is the FM stereo multiplexor signal. But there is also a spike around 10Khz which is variable in amplitude depending on what's playing - I think this is my ringing noise. If I filter a wide enough chunk of bandwidth around 10Khz, it seems to eliminate the annoying ringing sound. Unfortunately, it also takes a big chunk of audio with it.

I'm piping one of the VCR outputs from my Pioneer AV receiver to my computer's built-in sound card input with a mini-stereo to stereo RCA plugs adapter cable. The sound chip in the computer is a C-Media CMI9739A. I can record other stuff through the receiver, like output from my MP3 player, without any ringing. It seems like only FM radio broadcasts are affected.

Any suggestions? I'm willing to buy a better sound card if necessary and go totally optical, but I don't think that's the problem here.

Regards, mikebo

 

[KBlair]

Intermod...

I would first suggest changing your FM reciever and see if those freq components are still there, that would be the easiest thing to try. How's your RCA cable? Use a shielded one if possible.

For years I had a Cadet PC Radio card from ADS Technologies. It was an ISA card, but had both FM and AM recievers in it. Ran it on an old 486 and it worked great. Nowadays most TV tuner cards will also do FM reception, like my Haupage USB 2.0 tuner box.

If changing recievers doesn't fix your problem, then you'll need to determine where that interferance is coming from. If it's internal to your PC, and it could be depending on your system, then I'd say ditch the onboard audio and get something better. 19kHz and 10kHz are so close that one might be a byproduct of the other, especially if the 10kHz is really 9.5kHz. What processor/chipset/memory/video are you running? Today I located a problem for a customer who was complaining about their 2way radios and pagers. Turns out their laptop computers were interfering with their radios on those freqs. The video chipset in the laptop ran at 166MHz and the memory was 333MHz, so those two components produced a pronounced spike around 166MHz when viewed on an analyzer.

 

[mikebo]

OK... I worked with a friend who is an EE. We discovered that the 19KHz FM stereo multiplexor signal is mixing with the source audio signal in the A/D converter in my on-board soundcard. This is creating a modulated ringing tone centered on 10Khz and is roughly 1KHz wide. Turning the receiver to mono eliminates the problem. The multiplexor signal used to cause a similar problem in analog tape decks, so they often had FM multiplexor filters which notched out 19Khz.

So, there are several ways to solve this problem. 1) install a notch filter to suppress the stereo multiplexor signal at 19KHz. 2) install a low pass filter and suppress anything above 15-16KHz (you won't get anything usefull above 15KHz on FM broadcast anyway). 3) buy a new sound card that uses different intermediate frequencies in its A/D converter. Ironically, I've been told that the more expensive the audio card, the more likely it will use frequencies that cause the ringing. I don't know if this is true, but I'm prepared to do trial and error comparison of several cards until I find one that works.

More to come...
- mikebo

 

[mikebo]

I installed an old SoundBlaster PCI512 and its A/D converter did NOT have the problem with intermodulation with the 19KHz FM multiplexor signal. I can record FM stereo just fine... But, of course, this card employs a fairly ancient chipset which is kind of noisy.

I bought a 24-bit SB Audigy Value for $30 from Best Buy, but I need to return it because it doesn't have seperate line-in and mic jacks. Looks like the SB Audigy 2 Value is what I need. But no stores in my area carry it and I need to be able to return it if I run into the 19KHz problem again. Dar...

Cheers!
- mikebo