I know you need to clip, compress, and limit the audio to 15khz before you send it to a transmitter for best results.
I'm getting conflicting advice on what to use to process audio. I see small "cards" to process audio for about 75 bucks that purport to do the job. I also see some sites that say it's money wasted to buy those cards when units like the Behringer MDX2600 or the SU9920 can do what is required and more.
How about some advice from someone who knows the audio side of things really well?
Towards the bottom I posted a link to MBL4 which is free audio processing software for both AM and FM using your soundcard. If you have nothing more than an equalizer you can just cut your audio right at 15khz. It's best to use a brick wall cut though and that requires either a darn good EQ or specialized equipment. The main reason to do this is to not screw up your pilot signal for stereo. Other than that there is a number of ways to compress and make your modulation sound better but most FM broadcasters run clean anyways rather than modify the sound like is so common with AMers.
I'd like to pass this schematic to you that features a hardware processor that looks intersting.
I'll try the software processing route for now, but I think I'm also going to build this hardware device and see what it will do.
I like building stuff, but I wish the fellow offered the circuit board for sale. I'll probably just build it on a perf board and wire it up the old-fashioned way.
I do understand having just clean audio with the 15khz wall though. It still is nice to have the audio "tailored" a bit so the listener doesn't have to "ride" his volume knob to compensate for different levels of audio on the programming being aired.
The link to MBL4 is dead and the software has been replaced with not so userfriendly Sonos.
Breakaway would be a good alternative if the developers didn't force you to use their god awful presets, which drastically cut down on mid range sounds and jack up the highs to levels that cause distortion.
I guess it appears that there's no viable software solution. I use a BSR equalizer and run flat, with the exception of cutting out highs above 15 khz. This seems to work the best.
I'm glad I didn't give up on breakaway broadcast. After playing with the tilt and PEQ settings, I was able to adjust the "curve" of the audio to get the flat sond I was looking for on the Cleveland preset. The meter bars on the BSR equalizer I have made for a good audio monitoring tool whie I played around. My transmitter now sounds identical to a professional radio station and i no longer have to ride the volume knob.
I'm using the demo version until it runs out and will probably end up purchasing it. However, there's an alternate option. I won't name the site, but it starts with a pirate and ends with a bay dot org.
By the way, I'd recommend 2 sound cards. One for the input and one for the output to the transmitter. Any sound cards should do. I'm using onboard realtek HD and an SB Audigy I got from Walmart. I had to lower the L/R Ref level to -19.5 db to avoid distortion.
The MBL4 softwave seems to only process WAV files, and only plays one at a time.
Am I missing something?
The idea is to use the live input for it so that you can use a dedicated computer and then run your audio into the machine and back out of it to your transmitter much like a hardware audio processor would be setup like.
The wav thing you describe is just for processing prerecorded audio off of your harddrive. You want to run the program in live mode. You also need two soundcards to use it that way, one for running your audio in, and another one for the audio out. Not too sure if a fullduplex soundcard makes a difference or not, but it's best to use two soundcards anyways for it.
As I mentioned, I'm an RF guy... audio processing is pretty new to me.
The two soundcard thing seems beyond the use of a simple "netbook" pc, which was going to be what I was going to use for processing.
I've been having some success using mp3gain to equalize volume on harddrive mp3's. I've also used Audacity to limit all the mp3's to 15khz by saving all the files at the 32000 setting. That makes a sharp cutoff at 15khz.
I then import the playlist into winamp. I also use the winamp equalizer to knock down everything above 15khz.
Thanks for the tip on MBL4. Now I understand how to make it work if go that route.