Video Interferes with Audio

N

Nelson

Guest
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.

--
Nelson
 
Nelson <nelson@nowhere.com> wrote in message
news:0001HW.CA90DCB40055BC60B03139BF@news.astraweb.com...
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.

--
Nelson
Click to expand...
In case its RF getting into the audio IF try looping the audio feed lead a
few turns in/around a ferrite ring
 
On Sat, 10 Sep 2011 09:21:56 -0400, Nelson <nelson@nowhere.com> wrote:

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.
Click to expand...
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...

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
On Sat, 10 Sep 2011 09:21:56 -0400, Nelson <nelson@nowhere.com> wrote:

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.
Click to expand...

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 <bf3s67hkjd53pqikf7ut4nivai4mmukmkt@4ax.com>):

On Sat, 10 Sep 2011 09:21:56 -0400, Nelson <nelson@nowhere.com> wrote:

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
Click to expand...
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
 
On Sat, 10 Sep 2011 10:15:10 -0400, N_Cook wrote
(in article <j4frdm$bqc$1@dont-email.me>):

Nelson <nelson@nowhere.com> wrote in message
news:0001HW.CA90DCB40055BC60B03139BF@news.astraweb.com...
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.

--
Nelson


In case its RF getting into the audio IF try looping the audio feed lead a
few turns in/around a ferrite ring
Click to expand...
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
 
On Sat, 10 Sep 2011 11:14:36 -0400, Jeff Liebermann wrote
(in article <qmum67h9vdn7ie87lc6bvbv4ancatvbh4c@4ax.com>):

On Sat, 10 Sep 2011 09:21:56 -0400, Nelson <nelson@nowhere.com> wrote:

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.

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...
Click to expand...
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 :)

--
Nelson
 
Nelson <nelson@nowhere.com> wrote in message
news:0001HW.CA949447011A745EB02919BF@news.astraweb.com...
On Sat, 10 Sep 2011 10:15:10 -0400, N_Cook wrote
(in article <j4frdm$bqc$1@dont-email.me>):

Nelson <nelson@nowhere.com> wrote in message
news:0001HW.CA90DCB40055BC60B03139BF@news.astraweb.com...
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.

--
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
Click to expand...
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
 
On Tue, 13 Sep 2011 04:58:29 -0400, Nelson <nelson@nowhere.com> wrote:

On Mon, 12 Sep 2011 09:51:08 -0400, chuck wrote
(in article <bf3s67hkjd53pqikf7ut4nivai4mmukmkt@4ax.com>):

On Sat, 10 Sep 2011 09:21:56 -0400, Nelson <nelson@nowhere.com> wrote:

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

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.
Click to expand...

Yes I meant one for each channel. Chuck
 
On Tue, 13 Sep 2011 05:09:41 -0400, Nelson <nelson@nowhere.com> wrote:

I believe the Mac ground is
floating with respect to the house since it runs off a battery.
Click to expand...
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.

Everything is already plugged into a common power strip.
Click to expand...
Oh well.

I'll take a
look at the Firewire cards, but I am really hesitant to invest any
money on speculation :)
Click to expand...
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.
<http://www.ebay.com/itm/220685482076>
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.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
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