Sound static when playing content off of laptop to fm transmitter

[Jake Sinn]

hi all

i am playing content from my mac and transmitting through a fm transmitter. i have tested on multiple cars with high quality sound systems and it is static.

however, when i change the source from laptop to dvd player, the sound sounds great and there is no static at all.

any suggestions?

thanks

[Mark Gulbrandsen]

First off, get a real laptop. Second, be sure you are not overloading the input to the transmitter.

[Jake Sinn]

Why is a mac not a real computer?

How would i be sure im not overloading?

[Frank Cox]

Turn the output volume down on your mac. If the static stops, it was overloaded.

[Jake Sinn]

Im using VLC.

So would it be the output volume in there?

Since I have and HDMI plugged into my laptop, it does not let you adjust the volume for the computer

[Sam Chavez]

Try running the Mac without the charger. The static or noise sometimes starts there. Just a guess from past experience.

[Carsten Kurz]

VLC has it's own volume control, but besides that, even over HDMI, the audio volume can be adjusted. Since this FM transmitter quite probably has no HDMI audio input - how do you actually feed the audio from the laptop to the FM transmitter? You could use either an HDMI->analog audio converter, or use the Mac Books analog audio port. What about the audio connection when you use the DVD player - how is that connection made?

Can you describe more precisely what you mean with 'static'?


+1 to Sam - simultaneous video and audio to separate equipment fed from a mains-powered computer will often cause audio noise. There are different ways to get rid of this, but if a laptop is used, disconnecting the power adapter is an easy check.

- Carsten

[John Thomas]

Please describe the full signal path here. What devices are between the Mac and the fm transmitter?



To clarify, is it only static that you're hearing, or are you hearing the intended audio but with static behind it?



MacOS won't adjust volume when HDMI is the selected audio output, but like Carsten said, VLC does have its own volume adjustment.

On the bright side, that means you probably have the correct output selected on the Mac.



I wonder if you're using an audio disembedder and have plugged a spdif coax output into an analog input on the fm transmitter. I bet that would sound pretty gnarly.

[Jake Sinn]

The setup is:

Mac laptop connected via HDMI to projector.

Aux cord from audio out on projector to FM transmitter.

I said earlier that when testing, my laptop was NOT plugged in.


I am hearing the intended audio with static behind it.

[John Thomas]

Gotcha. Using the projector's aux output is a common source for a ground loop hum, and the unbalanced audio doesn't help. A chassis-to-ground jumper wire may be in your future, or an isolator. But let's make sure we're on the right track first.


Would the static be better described as a hum, or is it more of a hiss, or even a crackle? Does it come through constantly, or is it more like only when something audible happens the sound crackles?


Is the Mac in close enough proximity to the fm transmitter that you could plug the aux cord into the headphone jack of the mac rather than the projector's aux out? You'll definitely want to turn the Mac's volume down to about 1/3 if you do this.

[Gordon McLeod]

Is this a Dcinema projector as none I know of have aux audio outputs.

[Marcel Birgelen]

A ground loop doesn't sound like static, but like a 50 or 60 Hz hum/buzz.

Most MacBook power adapters I know are double isolated and ungrounded and therefore shouldn't cause a ground loop.

That being said, any non-amateur-hour setup should split the audio as close to the source as possible, before feeding it into the transmitter. I hope the transmitter does some compression of the input signal before broadcasting it over the airwaves.

[Carsten Kurz]

I would first try to switch the Mac's audio to it's line/headphone output and feed that to the FM transmitter. Using the projector aux out is an unnecessary complication.

You can switch between different audio output devices immediately from VLCs audio menu. Then play with the volume.

VLCs offers audio compression, however, I doubt we'll get Jack into configuring it properly.

[John Thomas]

Plenty of non-technicians and audience members would describe a ground loop hum as static. Considering the setup in question: ground loops, gain-staging issues, and interference are all possible here.

[Harold Hallikainen]

Any possibility it's a lack of decoding of DTS or Dolby encoding?