What Are the Cylyndrical Objects You Often See on Audio, USB

[Nelson]

What are those cylindrical objects you often see surrounding the cable
on USB, Audio, Firewire, etc cables? I am assuming they are some kind
of passive RF interference filter. How do they work? How effective are
they?

I am having trouble with interference with an audio cable connected
from a computer to a TV and was wondering if a cable with one of these
doo-dads would be worth the $.

--
Nelson

 

[petrus bitbyter]

"Nelson" <nelson@nowhere.com> schreef in bericht
news:0001HW.CAE0073A00514F8EB01029BF@news.astraweb.com...

What are those cylindrical objects you often see surrounding the cable
on USB, Audio, Firewire, etc cables? I am assuming they are some kind
of passive RF interference filter. How do they work? How effective are
they?

I am having trouble with interference with an audio cable connected
from a computer to a TV and was wondering if a cable with one of these
doo-dads would be worth the $.

--
Nelson

They are ferrites and used for RF suppression. You can buy types that can be
clamped on existing cables and they usually are effective. Nor that
expensive too so it's worthwhile to give it a try.

petrus bitbyter

 

[Jeffrey Angus]

On 11/9/2011 9:32 AM, Nelson wrote:

What are those cylindrical objects you often see surrounding the cable
on USB, Audio, Firewire, etc cables? I am assuming they are some kind
of passive RF interference filter. How do they work? How effective are
they?

I am having trouble with interference with an audio cable connected
from a computer to a TV and was wondering if a cable with one of these
doo-dads would be worth the $.

Ferrite cores

Jeff


--
"Everything from Crackers to Coffins"

 

[Jeff Liebermann]

On Wed, 9 Nov 2011 10:32:26 -0500, Nelson <nelson@nowhere.com> wrote:

What are those cylindrical objects you often see surrounding the cable
on USB, Audio, Firewire, etc cables?

Regulatory repellents. They're attached by the manufacturer to keep
the FCC off their back by limiting the amount of RFI/EMI sprayed by
their equipment.

I am assuming they are some kind
of passive RF interference filter.

Yep. Under the nearly impossible to remove plastic cover lies a heart
of powdered oxidized iron.

How do they work?

Quite well. If an FCC Enforcement Burro inspector approached, all one
needs to do is wave the lumpy cable at the inspector, and he will
vanish into a smog of legalese.

How effective are they?

100% successful. I haven't seen an FCC inspector for many years.

I am having trouble with interference with an audio cable connected
from a computer to a TV and was wondering if a cable with one of these
doo-dads would be worth the $.

Oh well. You finally decided to disclose what you're trying to
accomplish, so I guess I'll have to provide a reasonable answer.

I assume the computah generated interference is trashing the picture
on the TV or is being heard on the TV audio. What channel is the TV
watching? If it's channel 3/4 from some kind of set top box, you
might find it more useful to simply avoid the RF problem and rewire
your TV setup to use a non-RF input. HDMI, DVI, component video,
S-video, and component video inputs should all be present on the back
of your unspecified model TV.

If you're only using the TV for computer audio, you could also
eliminate the problem by purchasing a set of "computah speakers" for
about $30. The speakers inside most TV's are fairly disgusting.

If none of these alternatives seem useful, you can purchase clamp on
ferrite filters.
<http://www.delevan.com/web/PDF/Suppressors/Cable/BFseries/Page%20118_120_BF.pdf>
Radio Shock carries some:
<http://www.radioshack.com/product/index.jsp?productId=3012599>
<http://www.radioshack.com/product/index.jsp?productId=2103222>
or you can just cannibalize the ferrite beads off an old cable. For
audio, just about anything will work. Bigger is better and running
multiple turns through the core is even better:
<http://www.stevelarkins.freeuk.com/computer_interference.htm>

Gotta run... good luck.

--
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

 

[hrhofmann@att.net]

On Nov 9, 11:10 am, Jeff Liebermann <je...@cruzio.com> wrote:

On Wed, 9 Nov 2011 10:32:26 -0500, Nelson <nel...@nowhere.com> wrote:
What are those cylindrical objects you often see surrounding the cable
on USB, Audio, Firewire, etc cables?

Regulatory repellents.  They're attached by the manufacturer to keep
the FCC off their back by limiting the amount of RFI/EMI sprayed by
their equipment.

I am assuming they are some kind
of passive RF interference filter.

Yep.  Under the nearly impossible to remove plastic cover lies a heart
of powdered oxidized iron.

How do they work?

Quite well.  If an FCC Enforcement Burro inspector approached, all one
needs to do is wave the lumpy cable at the inspector, and he will
vanish into a smog of legalese.

How effective are they?

100% successful.  I haven't seen an FCC inspector for many years.

I am having trouble with interference with an audio cable connected
from a computer to a TV and was wondering if a cable with one of these
doo-dads would be worth the $.

Oh well.  You finally decided to disclose what you're trying to
accomplish, so I guess I'll have to provide a reasonable answer.

I assume the computah generated interference is trashing the picture
on the TV or is being heard on the TV audio.  What channel is the TV
watching?  If it's channel 3/4 from some kind of set top box, you
might find it more useful to simply avoid the RF problem and rewire
your TV setup to use a non-RF input.  HDMI, DVI, component video,
S-video, and component video inputs should all be present on the back
of your unspecified model TV.

If you're only using the TV for computer audio, you could also
eliminate the problem by purchasing a set of "computah speakers" for
about $30.  The speakers inside most TV's are fairly disgusting.

If none of these alternatives seem useful, you can purchase clamp on
ferrite filters.
http://www.delevan.com/web/PDF/Suppressors/Cable/BFseries/Page%20118_...
Radio Shock carries some:
http://www.radioshack.com/product/index.jsp?productId=3012599
http://www.radioshack.com/product/index.jsp?productId=2103222
or you can just cannibalize the ferrite beads off an old cable.  For
audio, just about anything will work.  Bigger is better and running
multiple turns through the core is even better:
http://www.stevelarkins.freeuk.com/computer_interference.htm

Gotta run... good luck.

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

Do what Jeff says. As past predident of the IEEE EMC Society, I have
used many ferrite beads to reduce interference levels to meet FCC
requirements. There are cylidrical beads for round cables and flat
beads for ribbon cables. If you scrap almost any piece of electronic
equipment, you can scrounge a ferrite bead or several.

But, there are different compositions of ferrite to cover the whole
frequency band, so the first bead you try might not be the one you
need. If you told us a little bit more about what your setup is and
what the interference is and where and what you turn off to clear the
problem, we might be able to give you a lot more help

 

[Nelson]

On Wed, 9 Nov 2011 17:02:18 -0500, hrhofmann@att.net wrote
(in article
<7fb37120-0e12-4560-8ad8-d18a7631cf02@k10g2000yqn.googlegroups.com&gt:

On Nov 9, 11:10 am, Jeff Liebermann <je...@cruzio.com> wrote:
On Wed, 9 Nov 2011 10:32:26 -0500, Nelson <nel...@nowhere.com> wrote:
What are those cylindrical objects you often see surrounding the cable
on USB, Audio, Firewire, etc cables?

[snip]

I assume the computah generated interference is trashing the picture
on the TV or is being heard on the TV audio.  What channel is the TV
watching?  If it's channel 3/4 from some kind of set top box, you
might find it more useful to simply avoid the RF problem and rewire
your TV setup to use a non-RF input.  HDMI, DVI, component video,
S-video, and component video inputs should all be present on the back
of your unspecified model TV.

If you're only using the TV for computer audio, you could also
eliminate the problem by purchasing a set of "computah speakers" for
about $30.  The speakers inside most TV's are fairly disgusting.

If none of these alternatives seem useful, you can purchase clamp on
ferrite filters.
http://www.delevan.com/web/PDF/Suppressors/Cable/BFseries/Page%20118_...
Radio Shock carries some:
http://www.radioshack.com/product/index.jsp?productId=3012599
http://www.radioshack.com/product/index.jsp?productId=2103222
or you can just cannibalize the ferrite beads off an old cable.  For
audio, just about anything will work.  Bigger is better and running
multiple turns through the core is even better:
http://www.stevelarkins.freeuk.com/computer_interference.htm

Gotta run... good luck.

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

Do what Jeff says. As past predident of the IEEE EMC Society, I have
used many ferrite beads to reduce interference levels to meet FCC
requirements. There are cylidrical beads for round cables and flat
beads for ribbon cables. If you scrap almost any piece of electronic
equipment, you can scrounge a ferrite bead or several.

That reminds me... my IEEE Life Insurance premium is due. It was such
a bargain when I was younger

But, there are different compositions of ferrite to cover the whole
frequency band, so the first bead you try might not be the one you
need. If you told us a little bit more about what your setup is and
what the interference is and where and what you turn off to clear the
problem, we might be able to give you a lot more help

I have a MacBook Pro set up as an entertainment center feeding an
analog TV. Video is out of the mini-display port through an S-Video
adapter. Audio is out of the headphone port into the RCA audio-in on
the TV. There is a constant hiss which modulates with screen changes.
Since I don't hear the hiss if I just connect earphones to the port, my
assumption is that it is RF pickup from the S-Video cable or the TV
being fed back through the audio cable. If any of my classes ever
covered RF interference, I must have slept through them

I posted this problem here a while back. One suggestion was to buy a
usb or firewire sound adapter... but my insurance premium is due
And, besides, there is no way to test it in advance to see if it solves
the problem. Another was to try to wrap the audio cable around a
ferrite core. I pulled one that looks like a washer off an old piece
of equipment and took a few turns but nothing changed. I also
considered grounding. But the Mac runs off the battery so it's not
clear how I could force a common ground. And I'm a little leery of
that anyway having fried one or two things in my younger days trying to
establish a common ground to an oscilloscope

I have an audio cable with a sub-mini-jack at one end and two RCA jacks
at the other which has one of these ferrite things at the mini-jack
end. Unfortunately, it is the wrong size for the earphone port, but it
got me to thinking. On my list of "to do's" is to go down to Radio
Shack and buy an sub-mini to mini adapter so I can try out the cable
but I'd like to figure out what the actual problem is before throwing
money at trial and error solutions.

--
Nelson

 

[Jeff Liebermann]

On Fri, 11 Nov 2011 05:01:44 -0500, Nelson <nelson@nowhere.com> wrote:

I have a MacBook Pro set up as an entertainment center feeding an
analog TV. Video is out of the mini-display port through an S-Video
adapter. Audio is out of the headphone port into the RCA audio-in on
the TV. There is a constant hiss which modulates with screen changes.

<http://www.htmblog.com/2010/01/macbook-pro-audio-hissing-in-sound-card.html>

Since I don't hear the hiss if I just connect earphones to the port, my
assumption is that it is RF pickup from the S-Video cable or the TV
being fed back through the audio cable. If any of my classes ever
covered RF interference, I must have slept through them

Fix your earphones. I've heard the hiss on several Macbooks.

I posted this problem here a while back. One suggestion was to buy a
usb or firewire sound adapter... but my insurance premium is due

I remember. Same problem, same suggestion. The MacBook audio output
is noisy. Ferrites are not going to fix it.

As for the cost, is $1.30 too much for you?
<http://www.ebay.com/itm/360385369541>

--
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

 

[Jeff Liebermann]

On Fri, 11 Nov 2011 08:37:37 -0800, Jeff Liebermann <jeffl@cruzio.com>
wrote:

http://www.htmblog.com/2010/01/macbook-pro-audio-hissing-in-sound-card.html

More on the hiss problem.
<http://forums.macrumors.com/showthread.php?p=13200418>
My guess(tm) is some manner of internal grounding problem. The his is
probably coming from processor switching noise, some of which is going
through the audio circuitry. Ferrites are not going to fix that.
Spend the $1.30 and get an external USB or firewire sound thing.
--
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

 

[Nelson]

On Fri, 11 Nov 2011 11:46:45 -0500, Jeff Liebermann wrote
(in article <45kqb79tqo3k69oucoipohaftd09ctoj9i@4ax.com&gt:

On Fri, 11 Nov 2011 08:37:37 -0800, Jeff Liebermann <jeffl@cruzio.com
wrote:

http://www.htmblog.com/2010/01/macbook-pro-audio-hissing-in-sound-card.html


More on the hiss problem.
http://forums.macrumors.com/showthread.php?p=13200418
My guess(tm) is some manner of internal grounding problem. The his is
probably coming from processor switching noise, some of which is going
through the audio circuitry. Ferrites are not going to fix that.
Spend the $1.30 and get an external USB or firewire sound thing.

OK, OK, I spent the $ 1.30 I'll let you know when the slow boat
from China gets it here. But I thought you said earlier that you tried
USB and it didn't work. Do you have any $1.30 firewire solutions up
your sleeve?

--
Nelson

 

[Nelson]

On Fri, 11 Nov 2011 11:37:37 -0500, Jeff Liebermann wrote
(in article <hgjqb71k2369dmu6plokcv1kubmkmkgnbq@4ax.com&gt:

[snip]

Fix your earphones. I've heard the hiss on several Macbooks.

Not to be argumentative but I just hooked up a set external speakers to
the audio output jack and cranked the volume up to Pain Level 1 and
could not detect the hiss. I tried with with the speakers powered by 6
AA batteries and with an AC adapter. No detectible difference.

Unfortunately this is not a solution because then my wife can't control
the volume with the TV remote.

--
Nelson

 

[Jeff Liebermann]

On Fri, 11 Nov 2011 14:09:36 -0500, Nelson <nelson@nowhere.com> wrote:

OK, OK, I spent the $ 1.30 I'll let you know when the slow boat
from China gets it here.

There are others available from domestic sources on eBay. I happened
to find the absolute cheapest.

But I thought you said earlier that you tried
USB and it didn't work.

Correct. Thanks for reading what I previously posted. However, you
missed or forgot an important point. We were trying to use the
MacBook for recording a music festival. The hiss was also present
with USB on record as well as playback. For playback, I doubt if you
could hear hiss that was more then -30dB down from the peak audio
level. That's 1/1000th of the peak volume. However, the recording
engineer wanted absolutely no buzz or hiss at any level. I couldn't
do that with the USB dongle. I got close with fancy USB external
sound card. However, when someone air dropped (literally) a firewire
sound card, the noise was almost totally gone on the spectrum
analyzer.

Note that in a previous URL, various users offered different
grounding, ungrounding, configuration, and equipment changes to fix
the hiss problem. Reading further, most came back indicating that the
fix was not permanent.

Do you have any $1.30 firewire solutions up
your sleeve?

No. I'm wearing short sleeves.

Do your own shopping:
<http://search.reviews.ebay.com/Firewire_Sound-Cards_W0QQftsZ2QQsubmitZGoQQucatZ5781QQufmZ1QQugatZ2QQuqtZrQQusopZ1003>

These look like the cheapest and can probably be found used somewhere:
<http://www.atpm.com/10.02/ifire.shtml>

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

 

[Nelson]

On Fri, 11 Nov 2011 23:44:50 -0500, Jeff Liebermann wrote
(in article <altrb75q38tlutamlbq66vlkta74un988d@4ax.com&gt:

[snip]

Correct. Thanks for reading what I previously posted.

I always read what you post I am appreciative of anyone who takes
the time to help me and I have always found this group to be
exceptionally knowledgeable and willing to help (as opposed to some
other groups I belong to who offer only snarky comments).

However, you
missed or forgot an important point. We were trying to use the
MacBook for recording a music festival. The hiss was also present
with USB on record as well as playback.

Indeed, I _did_ miss that.

For playback, I doubt if you
could hear hiss that was more then -30dB down from the peak audio
level. That's 1/1000th of the peak volume. However, the recording
engineer wanted absolutely no buzz or hiss at any level. I couldn't
do that with the USB dongle. I got close with fancy USB external
sound card. However, when someone air dropped (literally) a firewire
sound card, the noise was almost totally gone on the spectrum
analyzer.

Note that in a previous URL, various users offered different
grounding, ungrounding, configuration, and equipment changes to fix
the hiss problem. Reading further, most came back indicating that the
fix was not permanent.

Perhaps all instances of the symptom don't have the same cause.

Do you have any $1.30 firewire solutions up
your sleeve?

No. I'm wearing short sleeves.

Do your own shopping:

http://search.reviews.ebay.com/Firewire_Sound-

Cards_W0QQftsZ2QQsubmitZGoQQuca

tZ5781QQufmZ1QQugatZ2QQuqtZrQQusopZ1003

These look like the cheapest and can probably be found used somewhere:
http://www.atpm.com/10.02/ifire.shtml

I think I'll hold off until I see how the Chinese USB dongle works,
assuming the company actually exists It's hard for me to imagine
how they can even afford to ship it at that price. No wonder they are
killing us.

I am still struggling to explain why the hiss is very loud when the
audio output is connected to the TV and inaudible when connected to
earphones or the external speakers. I'm thinking that maybe the
S-Video ground and the audio ground are at different levels. But then
you had the problem with no video output involved, right?


--
Nelson

 

[Jeff Liebermann]

On Sat, 12 Nov 2011 04:37:38 -0500, Nelson <nelson@nowhere.com> wrote:

On Fri, 11 Nov 2011 23:44:50 -0500, Jeff Liebermann wrote
(in article <altrb75q38tlutamlbq66vlkta74un988d@4ax.com&gt:

I think I'll hold off until I see how the Chinese USB dongle works,
assuming the company actually exists

As I vaguely recall, the USB dongle didn't have much noise when the
laptop was running off battery, but was slightly noisy with the AC
adapter attached. I had 2 or 3 different USB audio dongles, which
were about the same.

You might try ordering from a USA vendor for faster delivery:
<http://www.ebay.com/sch/i.html?LH_PrefLoc=1&rt=nc&LH_BIN=1&_nkw=usb+sound&_sc=1&_sop=15&_sticky=1&_trksid=p3286.c0.m301>

It's hard for me to imagine
how they can even afford to ship it at that price. No wonder they are
killing us.

Agreed. If you purchase such a dongle retail in the USA, my guess is
that it would cost about $15. As long as we have enough money to
purchase such underpriced products, we will benefit from the low
prices. When we run out of money, China will own us.

I am still struggling to explain why the hiss is very loud when the
audio output is connected to the TV and inaudible when connected to
earphones or the external speakers. I'm thinking that maybe the
S-Video ground and the audio ground are at different levels. But then
you had the problem with no video output involved, right?

It doesn't require video to produce the hiss. My guess(tm) is that
it's coming from the processor, not the video. Changes in operating
configuration and wiring produce a different path to ground for the
processor. It it happens to go through or near the audio circuitry,
you get hiss. If I knew which ground path, it might be possible to do
something to fix it. However, the easiest fix is to insert something
that isolates the audio ground from the laptop ground. I don't know
if the USB dongle or the other devices do this, but it would make
sense to prevent ground loops.

--
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

 

[Nelson]

On Sat, 12 Nov 2011 14:05:21 -0500, Jeff Liebermann wrote
(in article <n1gtb7tuhua2u4tmu498o0fc5tla33fl8t@4ax.com&gt:

On Sat, 12 Nov 2011 04:37:38 -0500, Nelson <nelson@nowhere.com> wrote:

On Fri, 11 Nov 2011 23:44:50 -0500, Jeff Liebermann wrote
(in article <altrb75q38tlutamlbq66vlkta74un988d@4ax.com&gt:

I think I'll hold off until I see how the Chinese USB dongle works,
assuming the company actually exists

As I vaguely recall, the USB dongle didn't have much noise when the
laptop was running off battery, but was slightly noisy with the AC
adapter attached. I had 2 or 3 different USB audio dongles, which
were about the same.

You might try ordering from a USA vendor for faster delivery:

http://www.ebay.com/sch/i.html?LH_PrefLoc=1&rt=nc&LH_BIN=1&_nkw=usb+sou

nd&_sc

=1&_sop=15&_sticky=1&_trksid=p3286.c0.m301

It's hard for me to imagine
how they can even afford to ship it at that price. No wonder they are
killing us.

Agreed. If you purchase such a dongle retail in the USA, my guess is
that it would cost about $15. As long as we have enough money to
purchase such underpriced products, we will benefit from the low
prices. When we run out of money, China will own us.

I am still struggling to explain why the hiss is very loud when the
audio output is connected to the TV and inaudible when connected to
earphones or the external speakers. I'm thinking that maybe the
S-Video ground and the audio ground are at different levels. But then
you had the problem with no video output involved, right?

It doesn't require video to produce the hiss. My guess(tm) is that
it's coming from the processor, not the video. Changes in operating
configuration and wiring produce a different path to ground for the
processor. It it happens to go through or near the audio circuitry,
you get hiss. If I knew which ground path, it might be possible to do
something to fix it. However, the easiest fix is to insert something
that isolates the audio ground from the laptop ground. I don't know
if the USB dongle or the other devices do this, but it would make
sense to prevent ground loops.

Well, the Chinese Dongle arrived and seems to work fine. But it has
zero effect on the hiss. At least it has a nice green flashing LED to
let you know when a signal is being transmitted

Guess I'll just have to live with it.

--
Nelson

 

[Nelson]

On Sat, 12 Nov 2011 14:05:21 -0500, Jeff Liebermann wrote
(in article <n1gtb7tuhua2u4tmu498o0fc5tla33fl8t@4ax.com&gt:

On Sat, 12 Nov 2011 04:37:38 -0500, Nelson <nelson@nowhere.com> wrote:

On Fri, 11 Nov 2011 23:44:50 -0500, Jeff Liebermann wrote
(in article <altrb75q38tlutamlbq66vlkta74un988d@4ax.com&gt:

I think I'll hold off until I see how the Chinese USB dongle works,
assuming the company actually exists

As I vaguely recall, the USB dongle didn't have much noise when the
laptop was running off battery, but was slightly noisy with the AC
adapter attached. I had 2 or 3 different USB audio dongles, which
were about the same.

You might try ordering from a USA vendor for faster delivery:

http://www.ebay.com/sch/i.html?LH_PrefLoc=1&rt=nc&LH_BIN=1&_nkw=usb+sou

nd&_sc

=1&_sop=15&_sticky=1&_trksid=p3286.c0.m301

It's hard for me to imagine
how they can even afford to ship it at that price. No wonder they are
killing us.

Agreed. If you purchase such a dongle retail in the USA, my guess is
that it would cost about $15. As long as we have enough money to
purchase such underpriced products, we will benefit from the low
prices. When we run out of money, China will own us.

I am still struggling to explain why the hiss is very loud when the
audio output is connected to the TV and inaudible when connected to
earphones or the external speakers. I'm thinking that maybe the
S-Video ground and the audio ground are at different levels. But then
you had the problem with no video output involved, right?

It doesn't require video to produce the hiss. My guess(tm) is that
it's coming from the processor, not the video. Changes in operating
configuration and wiring produce a different path to ground for the
processor. It it happens to go through or near the audio circuitry,
you get hiss. If I knew which ground path, it might be possible to do
something to fix it. However, the easiest fix is to insert something
that isolates the audio ground from the laptop ground. I don't know
if the USB dongle or the other devices do this, but it would make
sense to prevent ground loops.



Well, the Chinese Dongle arrived and seems to work fine. But it has zero
effect on the hiss. At least it has a nice green flashing LED to let you
know when a signal is being transmitted

Guess I'll just have to live with it.

Update: So I got one of those new-fangled Digital TVs with an HDMI
input. Unfortunately the Mac only outputs video on the HDMI interface
so I had to use the Audio Out RCA jacks for the sound. Fortunately the
TV accomodates this. Result: No Hiss.

Summarizing my observations:

S-Video + RCA Sound to Analog TV - Hiss
S-Video + USB Dongle to Analog TV - Hiss
S-Video + External Speakers - No Hiss
S-Video + Earphones - No Hiss
HDMI + RCA Sound to Digital TV - No Hiss

The conclusion I draw from this is that the problem is not the MAC but
RF interference picked up by the analog TV.

--
Nelson