My softball team thought it would be fun to play intro music for each person as they went up to bat, like some major-league teams. I welded together a portable sound system complete with pneumatic wheels, second-hand shelf speakers, an AGM battery, and a 40W head unit.
I recently purchased a refurbished iPhone 3GS from an online retailer. I needed one for testing an application that I've been hired to build. Since I had the iPhone anyway, I figured the touch interface would work well for our team's sound box.
The first problem, I learned, is that the iPhone cannot play a single song, then stop. It always continues on to the next song unless it is the only song in a playlist. Luckily I wasn't the only person who found this extremely stupid, and before reaching for my compiler I checked the App Store and found Benjamin Pung's One track Mind app.
The next problem was that when I plugged in the cord from the headphone jack of the iPhone to the Line In jack of the head unit, there was an annoying beat, beat, beat sound.
For the benefit of those searching, it could also sound like: click, click click; thump, thump, thump; pop, pop, pop; beat, beat beat.
I considered loose connections, impediance mismatch, and EM interference, but I was able to rule those out. Oddly, rebooting the device would make it work normally until I fiddled with the jack (I still haven't figured this out).
It is noteworthy that this symptom is not present when listening with headphones (this is where, I believe, impediance plays a role).
Many internet searches informed me that this was a very common problem without a solution.
I notice that pushing the plug into the jack forcefully and holding made the noise stop. I also noticed that when I did so, there was just a little end-play; maybe 0.2mm. I had a hunch.
I grabbed a toothpick and went digging. I scraped the tip of the toothpick against the bottom of the jack repeatedly, and was able to pull out several small pieces of what I'll call lint â€“ probably a mixture of dust, fibres and oil.
I plugged it in to the stereo and ta da! No more noises. No more end-play.
Once again, the simplest solution (though not necessarily the most obvious one) usually is the correct one.
I'm posting this tip because many people have this problem, and most keywords like click, beat, and pop result in pages of results completely unrelated information (thanks Dr. Dre).