Video Interferes with Audio

In case its RF getting into the audio IF try looping the audio feed lead a few turns in/around a ferrite ring

It's a common problem probably caused by bad grounding on the main board. I've fixed similar problems by plugging everything into a common power strip and grounding everything to a common point. It never really went away completely but the grounding was a big help.

I tried an external USB dongle, but unless I ran the dongle on external power, which required cable butchery, I still had noise. Incidentally, different ports had different noise levels, which added another layer of entertainment to this exercise.

Finally, I decided that USB was out, and to try Firewire. Firewire is differential audio, and has power supplied locally with no common ground. Unfortunately, the device I borrowed for the ocassion was rather expensive ($300):

I suspect there are cheaper Firewire sound cards:

that may be more appropriate.

Google for "Macbook DVI audio noise".

etc...

Sometimes dual audio isolation transformers on the line out from the computer will resolve this issue. Chuck

On Mon, 12 Sep 2011 09:51:08 -0400, chuck wrote (in article ):

Hmm. Was not familiar with them. When you say "dual", I assume you mean one for each channel, left and right. I see they run about $40.

The hiss sounds like a higher frequency than the typical 60 cycle hum you get in speakers.

On Sat, 10 Sep 2011 10:15:10 -0400, N_Cook wrote (in article ):

Thanks. Not having a ferrite ring readily available, I tried a steel washer :-). Didn't seem to have much effect. I'm going to try using the core of the transformer out of a blown power adapter next.

On Sat, 10 Sep 2011 11:14:36 -0400, Jeff Liebermann wrote (in article ):

Thanks for the ideas and pointers, Jeff. I believe the Mac ground is floating with respect to the house since it runs off a battery. Everything is already plugged into a common power strip. I'll take a look at the Firewire cards, but I am really hesitant to invest any money on speculation :-)

As long as its the modern compact switched mode type, not conventional iron cored transformer, it should be much the same ferrite and splittable so you can get more audio cable turns through it

Yes I meant one for each channel. Chuck

It's not the house ground that's the problem. All the internal power supply voltages are referenced to a common ground, which is shared with the internal metal frame, shields, and the common ground for the audio. It's very difficult to get a good solid ground in a portable device, so there will naturally be conduction paths for high currents going through the frame and grounds. If any of these grounds has even a nominal resistance, a noise voltage will be impressed across it. That's where the noise is coming from.

The noise might be power supply switching noise at 60-100KHz, but methinks it's more likely high current processor noise. At 1.3v CPU voltage and 50 watts average dissipation, that's about 40 Amps of current going through the processor and coming from the power supply. It doesn't take much of that to create noise. You can sorta tell if it's coming from the CPU or the PS by just listening to the noise. If it changes when the CPU becomes busy, it's the CPU. If it's fairly constant, it might be the power supply or video. Either way, there's not much you can do about that doesn't involve redesign or major modifications. So, your best option is to move at least the analog portion of the sound system to outside the box where it doesn't share a common ground with the rest of the laptop. Incidentally, the same problem appears on many PC laptops.

Oh well.

You have my best guess(tm). What more could you want? Search eBay for "Firewire Audio Interface". $70 and up.

If you want to experiment with cheap, try a USB external sound dongle.

That's worked for me on some PC's but I've never tried one on a Mac for hum and noise control. For $3.00, it's worth a try.