Digital Signal Shuts Down FM Transmitter.  How? - Page 1
Forum section: electronics / beginners

author: insanuschannel
author_rank: Newbie
timestamp: January 15, 2014, 02:00:13 am
content: I'm wondering why my FM transmitter turns off when a specific song on my iPod is played. (YouTube video linked at bottom)Over the past few years, I noticed a consistent error. At first, I assumed a faulty connection or iPod coming loose as I hit a bump or turn a sharp corner. I have since eliminated those as contributors by playing the song with the car parked / nothing touching it.What happens:Using my iPod in my car, when a specific song reaches a specific spot, the FM transmitter turns off. The iPod continues to play, the FM transmitter simply turns off. If I disconnect the iPod, and reconnect it, the FM transmitter restarts and continues normally.The song file was acquired by some means other than "legit purchase" years ago when Napster / Kazaa was all the rage, so who knows what is in the data stream.What I have done to troubleshoot:Purchased the song of iTunes and tried to replicate. Result: iTunes file plays normally, FM transmitter did not turn off.Used 3 different iPods. Result: FM Transmitter turned off for all iPods used.Plugged iPod into FM transmitter via Audio Jack, instead of proprietary connector, and held iPod ~a foot away from FM transmitter. Result: FM Transmitter turned off.Used FM Transmitter in another vehicle. Result: FM transmitter turned off.Used iPod on multiple other docks (non-FM-transmitting) Result: No issues, song played normally.For some reason that makes no sense to me, the streaming of a digital signal thru an electronic device causes it to turn off.Anyone able to explain this?Video here...

author: C
author_rank: Super Contributor
timestamp: January 15, 2014, 03:10:43 am
content: As just a guessthe bad song has some very high level low frequency sounds or some very high level very high frequency sounds. This causes Transmitter overload so the transmitter turns off to protect it self.You might look at the bad song on a scope to see or you could look at it with one of the pc programs that can display the signal on the screen like a scope to what it is.C

author: gman4925
author_rank: Regular Contributor
timestamp: January 15, 2014, 05:09:54 am
content: I had an iPod 4th generation unit with FM transmitter that plugged into the top.To configure the FM transmitter you had to play an mp3 track from the iPod and it would select the FM transmit frequency.The tracks produced different tones so even though the transmitter plugged into the serial port as well as the audio port I believe it was the tones that changed the transmitter frequency, not the file name being sent out the serial port.It sure was annoying if you put the iPod on shuffle and it selected one of the tracks while you were using it.What I'm getting at is perhaps there is something in the audio stream, possible higher than you can hear, possibly audible, that is telling the FM transmitter to either turn off or change frequency.

author: amyk
author_rank: Super Contributor
timestamp: January 15, 2014, 07:29:19 am
content: I hope we have not reached a stage where FM transmitters can sense copyrighted content and shut off...Have you tried a different FM transmitter?

author: insanuschannel
author_rank: Newbie
timestamp: January 15, 2014, 11:48:13 am
content: The high or low frequency thing makes sense, thanks. So I guess it is possible to embed a tone into a data signal that shuts down electronics.As far as mechanism for failure, is there a "watchdog" looking for frequency limits and shuts the thing down? My knowledge is too limited to understand how digital devices overload to the point of shutting down. I would have guessed a choppy / stuttered output signal rather than turning off.I have not deliberately tried a different transmitter...meaning this is the 3rd I've owned, had the file longer than I have used FM transmitters, and only recently did I "connect the dots" this song shuts down my transmitter. (Apologies for rambling, trying to provide completeness)The FM transmitter I use has the ability to select most any frequency in the FM band. I have also tried this on different frequencies...the failure always occurs. (Sorry for forgetting that in the original post)The O scope thing sounds great, I'll see if I can give that a go!

author: C
author_rank: Super Contributor
timestamp: January 15, 2014, 04:58:21 pm
content: you might the thinking a lot of fancy circuits for this to happen. Might be thinking fancy circuits to detect the power out of the FM transmitter as a form of protection to the FM transmitter to protect it.It could be very simple, A power supply with fold back current limiting. For a example and just using some easy random numbers, lets say that the FM transmitter has a max safe output of 1 watt. When transmitting at that 1 watt it needs 2 Amps of current. If you had a power supply with fold back current limiting with the limit set at 2 amps then when the FM transmitter tried to output > then 1 Watt it would exceed the 2 amp protection limit and the power supply folds back to a current of say 0.001 amps. Your FM transmitter output just died but you protected the circuit and the transmitter. This would match what you say is happening.Note: the actual numbers used here are garbage, just needed some for the explanation.look at Pre-emphasis and de-emphasis sectionhttp://en.wikipedia.org/wiki/FM_broadcastingThe "excessive deviation of the FM carrier" = more dc power used.As crazy as this sounds, shutting down the FM transmitter could also be saving the receiving radio speakers.C

author: ConKbot
author_rank: Super Contributor
timestamp: January 15, 2014, 05:42:24 pm
content: C's post sounds reasonable, and easy to diagnose. Turn down the ipod a bit, and see what happens. The new song you downloaded could have been at a lower volume than the original troublesome mp3. One could have been normalized to do horrible things to the dynamic range, and so the average amplitude could be higher. If it works fine with the volume reduced, then your ipod's output is too high.

author: Nerull
author_rank: Frequent Contributor
timestamp: January 15, 2014, 10:54:45 pm
content: A common anti-piracy tactic during the time kazaa and such were popular was to flood the system with corrupted mp3 that would randomly change from normal song to ear-splitting screeching sound partway through the song. You don't have one of those, by chance?

author: insanuschannel
author_rank: Newbie
timestamp: January 16, 2014, 12:04:01 am
content: Ha, no ear screeching files. I have a fair amount, probably a few dozen of the ~12k files I have that were obtained that way. No known issues with any other files.I shared the file with a EE friend, he was interested in investigating the signal. The file is AAC format, he mentioned about it being pre-2009 and DMR'ed or something.I'll try turning down the iPod volume all the way and see if that has an impact. At least I think that's what I'm supposed to try. C kinda blew my mind up with the "protecting my speakers" bit...

author: C
author_rank: Super Contributor
timestamp: January 16, 2014, 01:47:22 am
content: The speaker bitSay for example you have a nice fancy base speaker. It can move a long way as you can see with some songs. At a 100hz you may be able to have a huge signal going to the speaker as you are pushing against air that does not want to move at 100hz. As you lower the frequency it will get easer to move the air so the speaker cone can move further. At some point the speaker will be banging against it limits going in and could move to far out if you keep the high power that was safe at a higher frequency going to it.Going the other direction as you increase the frequency the air and the mass of the speaker cone will limit the amount of movement possible from the speaker cone. You are producing less sound and more heat in the voice coil of the speaker. Now a good speaker system will have a network that shifts the energy from the base speaker to the better built to handle the frequency range speaker so the base speaker is prevented from trying to move at frequencies it can not handle. So in simple terms a speaker has limits, a max power that can change with frequency.Now think of what exists in the system when going too high or low in frequency. Did they use band pass networks for all the speakers or are they counting on the amp to limit the range. Now throw in a few more things, A high frequency square wave which would have harmonics or hitting the limits of the amp to amplify a signal which will cause some clipping and again harmonics. in both of these cases you might not hear this happening but harmonic content will still cause heating of the high freq speaker if it gets to the speaker. And if you put too much energy to a speaker you will burn it out or damage it.Just to add to this nasty bit of thought all the different speakers in a speaker system probably have different power limits.As insanuschannel said a low volume level could prevent it from happening.As you know the spot in the song you could get one of the programs that has a audio spectrum analyzer capability and look at the frequencies of the song where this happens.C

author: insanuschannel
author_rank: Newbie
timestamp: January 17, 2014, 01:11:28 am
content: With the iPod volume all the way down, same thing happens.Any recommendations for an audio spectrum analyzer program?

author: C
author_rank: Super Contributor
timestamp: January 17, 2014, 01:24:45 am
content: A program by the name of Audacity is shipped with some audio equipment.You might like it for it other capabilities.it has what they call a Spectrograms that could do the trick as this shows frequency over time.http://audacity.sourceforge.net/Some examples of Spectrograms are on youtubeC

author: alxnik
author_rank: Regular Contributor
timestamp: January 20, 2014, 04:10:47 pm
content: Is it on a specific second of the track or "after some time of playing"?One thing I thought about is DC offset which might be biasing the electronics in strange ways (and generating heat?).Best bet is to check the file through audacity or something similar as C said.

author: richard.cs
author_rank: Super Contributor
timestamp: January 20, 2014, 05:29:57 pm
content: Alxnik's suggestion of a d.c. offset sounds plausible although I'd be surprised if both (either in fact) the ipod and the FM transmitter were d.c. coupled. I'm inclined to believe there's some clever programming mode that is entered by audio tones. If you look at it in Audacity do a FFT of the section where you think it happens, maybe some specific tones stand out.

author: EvilGeniusSkis
author_rank: Regular Contributor
timestamp: February 18, 2014, 01:29:52 am
content: Quote from: C on January 17, 2014, 01:24:45 amYou can also download Audacity for free, just google it

author: katzohki
author_rank: Frequent Contributor
timestamp: February 18, 2014, 04:20:51 am
content: You can try this audio file if you'd like. It's in .au format so you might use audacity to convert it to .mp3 or something.https://sites.google.com/site/katzohki/downloads/Warble_250_1000.au

author: true
author_rank: Frequent Contributor
timestamp: February 18, 2014, 05:27:16 am
content: So complicated...Is it a controllable FM transmitter? If so, it may be some sort of in-band signalling.