Video Interferes with Audio

[Nelson]

I have a MacBook Pro that I use as an Entertainment Center. The video
out is connected to an Analog TV by a mini-dvi -> S-Video adapter. The
audio is connected from the Mac's earphone output to the sound RCA
inputs on the Analog TV. I know this is old technology, I shouldn't
use a Mac, etc, etc.

Everything works fine except that there is a constant low hiss and
changes in the MacBook Pro screen, eg opening a file, are reflected in
the a corresponding perturbation in the hissing. It also doesn't
matter if I use the S-Video or composite video inputs to the TV.

I'm guessing it is a grounding issue but not sure what I can do about
it or how to troubleshoot it.

Any advice would be appreciated.

 

[N_Cook]

I have a MacBook Pro that I use as an Entertainment Center. The video
out is connected to an Analog TV by a mini-dvi -> S-Video adapter. The
audio is connected from the Mac's earphone output to the sound RCA
inputs on the Analog TV. I know this is old technology, I shouldn't
use a Mac, etc, etc.

Everything works fine except that there is a constant low hiss and
changes in the MacBook Pro screen, eg opening a file, are reflected in
the a corresponding perturbation in the hissing. It also doesn't
matter if I use the S-Video or composite video inputs to the TV.

I'm guessing it is a grounding issue but not sure what I can do about
it or how to troubleshoot it.

Any advice would be appreciated.

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

 

[chuck]

I have a MacBook Pro that I use as an Entertainment Center. The video
out is connected to an Analog TV by a mini-dvi -> S-Video adapter. The
audio is connected from the Mac's earphone output to the sound RCA
inputs on the Analog TV. I know this is old technology, I shouldn't
use a Mac, etc, etc.

Everything works fine except that there is a constant low hiss and
changes in the MacBook Pro screen, eg opening a file, are reflected in
the a corresponding perturbation in the hissing. It also doesn't
matter if I use the S-Video or composite video inputs to the TV.

I'm guessing it is a grounding issue but not sure what I can do about
it or how to troubleshoot it.

Any advice would be appreciated.

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

 

[Nelson]

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

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.

 

[Nelson]

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

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.

 

[Nelson]

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):
<http://m-audio.com/products/en_us/FireWire1814.html>
I suspect there are cheaper Firewire sound cards:
<http://guides.macrumors.com/DJing_on_the_Mac#External_sound_cards>
that may be more appropriate.

Google for "Macbook DVI audio noise".
<http://reviews.cnet.com/8301-13727_7-10331142-263.html>
<https://discussions.apple.com/thread/1348342?start=0&tstart=0>
etc...

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

 

[N_Cook]

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.

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

 

[chuck]

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.

Yes I meant one for each channel. Chuck