[EE] Is Optoisolated Audio Practical?

[Martin McCormick]

Are there any optoisolators that are suitable to carry
analog audio over the normal frequency range of 20 to 20000 HZ?

What I am trying to do is feed audio from a radio
receiver across the room to an audio mixer. There are enough
computers and switching power supplies in the room that noise is
picked up by the shield of the audio line and ends up being
induced in to the radio via the chassis ground.

If optoisolated, the noise would still be on the cable
but the path for it would break completely at the output side of
the isolator.

I am not holding my breath but I figured I can ask
whether any such device exists or maybe a conventional
optoisolator will work if one conditions the input to the LED to
negate the distortion one would get by feeding audio biased to
light the LED halfway during silence and then go to full
brightness and almost fully off at maximum volume.

One would hope for linear response.

Thanks for any good ideas.

Martin McCormick WB5AGZ

[James Cameron]

Use an optoisolator with two photodiodes. One of them to provide
feedback to the driver. Linearity will increase.

[James Cameron]

Vishay IL300 for instance;

https://datasheet.octopart.com/IL300-Vishay-datasheet-16918.pdf

figure 18 on the datasheet has an amusing schematic where the Greek
ohm character is accidentally expressed in the english character set,
giving resistances of "30 KW".

[RussellMc]

Post by James Cameron
Vishay IL300 for instance;
https://datasheet.octopart.com/IL300-Vishay-datasheet-16918.pdf

Before you try mega-overkill DO look at fig1 and fig18 on the IL300 data
sheet - as mentioned by James


http://www.learnabout-electronics.org/Downloads/IL300-photodiode-optocoupler.pdf
Memory says that HP started this game long ago with a similarly but not
identically named part.

Vishay application note 50 covers design but all Vishay links don't like me
at present.

*HERE IT IS - Hooray for Cornell!*

https://people.ece.cornell.edu/land/courses/ece5030/labs/s2014/appn50vishay.pdf

IL300 - in stock, Digikey


https://www.digikey.com/product-detail/en/vishay-semiconductor-opto-division/IL300/751-1292-5-ND/1731525

Googlabet offers a zillion variations on the theme


https://www.google.co.nz/search?q=linear+optocoupler&num=40&hl=en-NZ&authuser=0&tbm=isch&tbo=u&source=univ&sa=X&sqi=2&ved=2ahUKEwjG14fmz97eAhXFyLwKHZK2CMUQsAR6BAgEEAE&biw=1866&bih=1008

NIST version - bipolar input at 3500 VDC offset :-)

https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=620473


Russell

[James Cameron]

Post by RussellMc

Post by James Cameron
Vishay IL300 for instance;
https://datasheet.octopart.com/IL300-Vishay-datasheet-16918.pdf

Before you try mega-overkill DO look at fig1 and fig18 on the IL300
data sheet - as mentioned by James

Thanks. I had thought my mail had not got through to most people, as
I hadn't seen any replies that cited me. Problems with laptop.org
mail reputation perhaps. My job to fix that, but no clue yet.

Post by RussellMc
Vishay application note 50 covers design but all Vishay links don't
like me at present.

https://www.vishay.com/docs/83708/appnote50.pdf was the best for me,
and yes, it is very comprehensive, giving bandwidth expected for each
design.

[RussellMc]

On Mon, 19 Nov 2018 at 16:26, James Cameron <***@laptop.org> wrote:
https://www.vishay.com/docs/83708/appnote50.pdf was the best for me,
and yes, it is very comprehensive, giving bandwidth expected for each
design.

For interest:
Bottom of last page:

The zero biased photovoltaic amplifier offers a 50 kHz to 60 kHz usable
bandwidth. When the detector is reverse biased to -15 V, the typical
isolation amplifier response increases to 100 kHz to 150 kHz. The phase and
frequency response for the IL300 is presented in Fig. 41. When maximum
system bandwidth is desired, the reverse biased photoconductive amplifier
configuration should be considered.


Russell McMahon

[Sean Breheny]

Haha, hooray for my Alma Mater, but especially for my friend Dr. Bruce Land
(a neuroscientist by training but an EE at heart - who has taught this
microcontroller applications course for more than 25 years now)

Post by RussellMc
*HERE IT IS - Hooray for Cornell!*
https://people.ece.cornell.edu/land/courses/ece5030/labs/s2014/appn50vishay.pdf

[Dwayne Reid]

Hi there, Martin.

The standard approach is to use an audio isolation transformer. I
have had very good success with issues such as what you are describing.

Shameless plug: my company sells little isolation transformer modules
for not much money. These use a semi-custom transformer from
Datatronics and really do work very well. I think that cost is
around Can $25 in singles (soon to increase due to supplier cost
increases) plus shipping.

<https://trinity-electronics.com/products/audio-products/pbc-l475-transformer-isolated-mono-sum-card/>

Let me know if you want to try this out. I'll arrange to send one to
you and you can either pay for it or return it as you see fit.

dwayne

Post by Martin McCormick
Are there any optoisolators that are suitable to carry
analog audio over the normal frequency range of 20 to 20000 HZ?
What I am trying to do is feed audio from a radio
receiver across the room to an audio mixer. There are enough
computers and switching power supplies in the room that noise is
picked up by the shield of the audio line and ends up being
induced in to the radio via the chassis ground.
If optoisolated, the noise would still be on the cable
but the path for it would break completely at the output side of
the isolator.
I am not holding my breath but I figured I can ask
whether any such device exists or maybe a conventional
optoisolator will work if one conditions the input to the LED to
negate the distortion one would get by feeding audio biased to
light the LED halfway during silence and then go to full
brightness and almost fully off at maximum volume.
One would hope for linear response.
Thanks for any good ideas.
Martin McCormick WB5AGZ

--
Dwayne Reid <***@planet.eon.net>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
780-489-3199 voice 780-487-6397 fax [phone] Toll Free
www.trinity-electronics.com
Custom Electronics Design and Manufacturing

[Harold Hallikainen]

Post by Dwayne Reid
Hi there, Martin.
The standard approach is to use an audio isolation transformer. I
have had very good success with issues such as what you are describing.
Shameless plug: my company sells little isolation transformer modules
for not much money. These use a semi-custom transformer from
Datatronics and really do work very well. I think that cost is
around Can $25 in singles (soon to increase due to supplier cost
increases) plus shipping.
<https://trinity-electronics.com/products/audio-products/pbc-l475-transformer-isolated-mono-sum-card/>
Let me know if you want to try this out. I'll arrange to send one to
you and you can either pay for it or return it as you see fit.
dwayne

I agree that the simplest way to do this would be with an audio
transformer. The next simplest way would be with a differential amplifier.
I'd use two conductor (twisted pair) shielded cable. At the unbalanced
output, the - conductor is connected to chassis. The + conductor is
connected to the unbalanced "hot" line. At the differential amplifier (or
transformer), the + and - leads go to the + and - inputs of the
differential amplifier (or to the transformer primary). I'd still tie the
grounds on each piece of equipment together (shield tied to chassis at
each end) to minimize the common mode voltage on the differential pair.
There will be some shield current, but it should not induce anything into
the differential pair. If the grounds are not tied together, there can be
substantial common mode voltage that is difficult for the differential
amplifier or transformer to reject.

Harold
(I still have my Western Electric 111C audio transformer)

[Brent Brown]

Post by Martin McCormick
Are there any optoisolators that are suitable to carry
analog audio over the normal frequency range of 20 to 20000 HZ?
What I am trying to do is feed audio from a radio
receiver across the room to an audio mixer. There are enough
computers and switching power supplies in the room that noise is
picked up by the shield of the audio line and ends up being
induced in to the radio via the chassis ground.

Sounds like a case for differential balanced audio. The noise is induced equally
(more or less) in both conductors, noise mostly cancels as it's the same magnitude
but opposite polarity on each conductor, any remaining common mode noise
reduced by the CMRR of the receiving amplifier.

Post by Martin McCormick
If optoisolated, the noise would still be on the cable
but the path for it would break completely at the output side of
the isolator.

If the noise is induced in the cable, would it not also modulate the current in the LED
and then appear on the output side of the optocoupler? Sorry, not trying to pick
holes.

Post by Martin McCormick
I am not holding my breath but I figured I can ask
whether any such device exists or maybe a conventional
optoisolator will work if one conditions the input to the LED to
negate the distortion one would get by feeding audio biased to
light the LED halfway during silence and then go to full
brightness and almost fully off at maximum volume.

A proper optical fibre link should perform better than copper + optocouplers. Will for
sure be something commercially available... probably at commercial prices :-(

[Martin McCormick]

Post by Brent Brown
A proper optical fibre link should perform better than copper + optocouplers. Will for
sure be something commercially available... probably at commercial prices :-(

Thank you to everybody who replied.

Post by Brent Brown
The standard approach is to use an audio isolation transformer. I
have had very good success with issues such as what you are describing.
Shameless plug: my company sells little isolation transformer modules
for not much money. These use a semi-custom transformer from
Datatronics and really do work very well. I think that cost is
around Can $25 in singles (soon to increase due to supplier cost
increases) plus shipping.
Let me know if you want to try this out. I'll arrange to send one to
you and you can either pay for it or return it as you see fit.

That is quite reasonable and thanks for the information.

You got me to thinking, though, and I have a large spool
of CAT 3 cable which would provide 4 twisted pairs of telephone
wire. I do have some little 1-to-1 isolation audio transformers
so the only problem would be if the windings of the transformer
capacitively coupled the low-level EMI smog in to the radio. The
common mode signal in this case is the low-level RF noise while
the differential signal is the desired audio. The noise
shouldn't make it across the transformer unless it does so via
capacitive coupling. I'll just have to try it and see.

The fiber optic approach that Brent Brown describes would
be a great solution as there would be nothing conductive between
the ends of the cable. I imagine the best fiber optic link would
be either a digital carrier made by pulses of light at some high
sampling rate such as 10 Mb/S or so with a decoder back to audio
at the other end but then you get the possibility of your digital
link generating more noise to have to deal with which totally
defeats the purpose.

Before I retired in 2015, I worked at Oklahoma State
University in our Network Operations Group which managed our
Ethernet and telephone networks on campus and the links to our
auxiliary campuses and the Internet.

The thing that impressed me about fiber optics is that
the fiber isn't that terribly expensive but terminating it is.

You've got to polish the cut end of the fiber until it is
optically flat and free of deformations or one must use an
electric arc to melt two ends of fiber such that they fuse
together creating one continuous fiber. Equipment that will do
that reliably is quite expensive. Some of the consumer HiFi gear
that uses fiber-optic audio cables may use plastic fibers for all
I know but still terminating them is not trivial.

Basically, if the fiber optic connection is meant to be
easily connected and disconnected, you polish and lose 2 or 3 DB
per interface or you fuse if you are permanently joining a fiber
to something that you don't plan to remove under normal
circumstances. Those fusion splices don't have any loss since
there is no boundary.

In this case, thanks to all. I'll temporarily connect
the audio to a short jumper and the isolation transformer and
then the long line from the secondary of the transformer to the
audio mixer and see if the smog survives all that. If it does,
it's back to the drawing board for something different.

Martin McCormick WB5AGZ

[Denny Esterline]

Well, your subject line asks if it's practical. Toslink exists, so.. yeah,
it's practical.
I'm also running USB over optical isolators. I'm guessing that routing your
audio over two sound cards and another computer just to gain isolation is
probably not what your looking for, but ...

Post by Brent Brown

Post by Brent Brown
A proper optical fibre link should perform better than copper + optocouplers. Will for
sure be something commercially available... probably at commercial

prices

Post by Brent Brown
:-(

Thank you to everybody who replied.

Post by Brent Brown
The standard approach is to use an audio isolation transformer. I
have had very good success with issues such as what you are describing.
Shameless plug: my company sells little isolation transformer modules
for not much money. These use a semi-custom transformer from
Datatronics and really do work very well. I think that cost is
around Can $25 in singles (soon to increase due to supplier cost
increases) plus shipping.
Let me know if you want to try this out. I'll arrange to send one to
you and you can either pay for it or return it as you see fit.

That is quite reasonable and thanks for the information.
You got me to thinking, though, and I have a large spool
of CAT 3 cable which would provide 4 twisted pairs of telephone
wire. I do have some little 1-to-1 isolation audio transformers
so the only problem would be if the windings of the transformer
capacitively coupled the low-level EMI smog in to the radio. The
common mode signal in this case is the low-level RF noise while
the differential signal is the desired audio. The noise
shouldn't make it across the transformer unless it does so via
capacitive coupling. I'll just have to try it and see.
The fiber optic approach that Brent Brown describes would
be a great solution as there would be nothing conductive between
the ends of the cable. I imagine the best fiber optic link would
be either a digital carrier made by pulses of light at some high
sampling rate such as 10 Mb/S or so with a decoder back to audio
at the other end but then you get the possibility of your digital
link generating more noise to have to deal with which totally
defeats the purpose.
Before I retired in 2015, I worked at Oklahoma State
University in our Network Operations Group which managed our
Ethernet and telephone networks on campus and the links to our
auxiliary campuses and the Internet.
The thing that impressed me about fiber optics is that
the fiber isn't that terribly expensive but terminating it is.
You've got to polish the cut end of the fiber until it is
optically flat and free of deformations or one must use an
electric arc to melt two ends of fiber such that they fuse
together creating one continuous fiber. Equipment that will do
that reliably is quite expensive. Some of the consumer HiFi gear
that uses fiber-optic audio cables may use plastic fibers for all
I know but still terminating them is not trivial.
Basically, if the fiber optic connection is meant to be
easily connected and disconnected, you polish and lose 2 or 3 DB
per interface or you fuse if you are permanently joining a fiber
to something that you don't plan to remove under normal
circumstances. Those fusion splices don't have any loss since
there is no boundary.
In this case, thanks to all. I'll temporarily connect
the audio to a short jumper and the isolation transformer and
then the long line from the secondary of the transformer to the
audio mixer and see if the smog survives all that. If it does,
it's back to the drawing board for something different.
Martin McCormick WB5AGZ
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[Harold Hallikainen]

Post by Martin McCormick
In this case, thanks to all. I'll temporarily connect
the audio to a short jumper and the isolation transformer and
then the long line from the secondary of the transformer to the
audio mixer and see if the smog survives all that. If it does,
it's back to the drawing board for something different.
Martin McCormick WB5AGZ

I'd put the transformer at the receive end of the line. Either end breaks
the ground loop, but putting it at the receive end also removes common
mode voltage.

Harold
W6IWI

[Martin McCormick]

Post by Harold Hallikainen
I'd put the transformer at the receive end of the line. Either end breaks
the ground loop, but putting it at the receive end also removes common
mode voltage.
Harold
W6IWI

Thanks. Interestingly enough, that was what I had planned to do.
EMI can be very sneaky and it sometimes seems to outwit every
measure to stop it. It seemed to me that a short pigtail to the
audio output on the radio would not pickup the racket like that
15 or 20-foot run of cable to the mixer amplifier. The hash is
probably present on the transformer's frame but as long as it
does not touch the chassis of the radio, the receiver should be
quiet. If there is still any trouble, it may take a second
transformer and short pigtail at the other end but I'll try the
simplest approach first.

One thing for sure. The hash adds about 10 DB of racket
to the noise coming from the radio so I'll know I have it when it
is about the same with or without the connection.

Martin WB5AGZ

[AK]

Martin, I built what you are looking for back about 12 years ago using
optical methods. Using an analog to TOSLINK converter (I have extras if you
are interested), I modulated a laser instead of the LED in the TOSLINK
connector, and sent the audio over free space to a receiver. The sound
quality was perfect (no loss of highs I have experienced from transformer
isolators) and fun. You would just need to find a piece of equipment with
TOSLINK input.

Post by Brent Brown

Post by Brent Brown
A proper optical fibre link should perform better than copper + optocouplers. Will for
sure be something commercially available... probably at commercial

prices

Post by Brent Brown
:-(

Thank you to everybody who replied.

Post by Brent Brown
The standard approach is to use an audio isolation transformer. I
have had very good success with issues such as what you are describing.
Shameless plug: my company sells little isolation transformer modules
for not much money. These use a semi-custom transformer from
Datatronics and really do work very well. I think that cost is
around Can $25 in singles (soon to increase due to supplier cost
increases) plus shipping.
Let me know if you want to try this out. I'll arrange to send one to
you and you can either pay for it or return it as you see fit.

That is quite reasonable and thanks for the information.
You got me to thinking, though, and I have a large spool
of CAT 3 cable which would provide 4 twisted pairs of telephone
wire. I do have some little 1-to-1 isolation audio transformers
so the only problem would be if the windings of the transformer
capacitively coupled the low-level EMI smog in to the radio. The
common mode signal in this case is the low-level RF noise while
the differential signal is the desired audio. The noise
shouldn't make it across the transformer unless it does so via
capacitive coupling. I'll just have to try it and see.
The fiber optic approach that Brent Brown describes would
be a great solution as there would be nothing conductive between
the ends of the cable. I imagine the best fiber optic link would
be either a digital carrier made by pulses of light at some high
sampling rate such as 10 Mb/S or so with a decoder back to audio
at the other end but then you get the possibility of your digital
link generating more noise to have to deal with which totally
defeats the purpose.
Before I retired in 2015, I worked at Oklahoma State
University in our Network Operations Group which managed our
Ethernet and telephone networks on campus and the links to our
auxiliary campuses and the Internet.
The thing that impressed me about fiber optics is that
the fiber isn't that terribly expensive but terminating it is.
You've got to polish the cut end of the fiber until it is
optically flat and free of deformations or one must use an
electric arc to melt two ends of fiber such that they fuse
together creating one continuous fiber. Equipment that will do
that reliably is quite expensive. Some of the consumer HiFi gear
that uses fiber-optic audio cables may use plastic fibers for all
I know but still terminating them is not trivial.
Basically, if the fiber optic connection is meant to be
easily connected and disconnected, you polish and lose 2 or 3 DB
per interface or you fuse if you are permanently joining a fiber
to something that you don't plan to remove under normal
circumstances. Those fusion splices don't have any loss since
there is no boundary.
In this case, thanks to all. I'll temporarily connect
the audio to a short jumper and the isolation transformer and
then the long line from the secondary of the transformer to the
audio mixer and see if the smog survives all that. If it does,
it's back to the drawing board for something different.
Martin McCormick WB5AGZ
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[Richard Prosser]

Hi Martin,
I once looked at the Vishay IL300 (or it may have been an earlier version).
My application was isolating a current shunt but we couldn't get the gain
or offset sufficiently stable over the temperature range (-20C to +50 ?).
IIRC we were looking for <<1%.
This shouldn't be an issue with you so it may be worth a look. The package
has a transmitter LED and two receive sensors. One sensor is used to
correct non-linearities. Your main issue could be frequency response - I'm
not sure about that but the data sheet should be available, along with
suggested schematics.

RP

RP

Post by AK
Martin, I built what you are looking for back about 12 years ago using
optical methods. Using an analog to TOSLINK converter (I have extras if you
are interested), I modulated a laser instead of the LED in the TOSLINK
connector, and sent the audio over free space to a receiver. The sound
quality was perfect (no loss of highs I have experienced from transformer
isolators) and fun. You would just need to find a piece of equipment with
TOSLINK input.

Post by Brent Brown

Post by Brent Brown
A proper optical fibre link should perform better than copper +
optocouplers. Will for
sure be something commercially available... probably at commercial

prices

Post by Brent Brown
:-(

Thank you to everybody who replied.

Post by Brent Brown
The standard approach is to use an audio isolation transformer. I
have had very good success with issues such as what you are describing.
Shameless plug: my company sells little isolation transformer modules
for not much money. These use a semi-custom transformer from
Datatronics and really do work very well. I think that cost is
around Can $25 in singles (soon to increase due to supplier cost
increases) plus shipping.
Let me know if you want to try this out. I'll arrange to send one to
you and you can either pay for it or return it as you see fit.

That is quite reasonable and thanks for the information.
You got me to thinking, though, and I have a large spool
of CAT 3 cable which would provide 4 twisted pairs of telephone
wire. I do have some little 1-to-1 isolation audio transformers
so the only problem would be if the windings of the transformer
capacitively coupled the low-level EMI smog in to the radio. The
common mode signal in this case is the low-level RF noise while
the differential signal is the desired audio. The noise
shouldn't make it across the transformer unless it does so via
capacitive coupling. I'll just have to try it and see.
The fiber optic approach that Brent Brown describes would
be a great solution as there would be nothing conductive between
the ends of the cable. I imagine the best fiber optic link would
be either a digital carrier made by pulses of light at some high
sampling rate such as 10 Mb/S or so with a decoder back to audio
at the other end but then you get the possibility of your digital
link generating more noise to have to deal with which totally
defeats the purpose.
Before I retired in 2015, I worked at Oklahoma State
University in our Network Operations Group which managed our
Ethernet and telephone networks on campus and the links to our
auxiliary campuses and the Internet.
The thing that impressed me about fiber optics is that
the fiber isn't that terribly expensive but terminating it is.
You've got to polish the cut end of the fiber until it is
optically flat and free of deformations or one must use an
electric arc to melt two ends of fiber such that they fuse
together creating one continuous fiber. Equipment that will do
that reliably is quite expensive. Some of the consumer HiFi gear
that uses fiber-optic audio cables may use plastic fibers for all
I know but still terminating them is not trivial.
Basically, if the fiber optic connection is meant to be
easily connected and disconnected, you polish and lose 2 or 3 DB
per interface or you fuse if you are permanently joining a fiber
to something that you don't plan to remove under normal
circumstances. Those fusion splices don't have any loss since
there is no boundary.
In this case, thanks to all. I'll temporarily connect
the audio to a short jumper and the isolation transformer and
then the long line from the secondary of the transformer to the
audio mixer and see if the smog survives all that. If it does,
it's back to the drawing board for something different.
Martin McCormick WB5AGZ
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[AB Pearce - UKRI STFC]

My application was isolating a current shunt but we couldn't get the gain or offset sufficiently stable
over the temperature range (-20C to +50 ?). IIRC we were looking for <<1%.

HP (probably now Keysight, or whoever handles the semiconductors) make an opto for exactly this use.

Don't have the part number handy but can dig it out if wanted.

[s***@agilent.com]

Was HP then Avago now Broadcom soon to be...

"Avago" always brought a wry smile to me here in AU since it is the vernacular contraction for "have a go". Often followed by "ya mug" if directed towards a poorly performing sportsman (particularly a cricketer). I wish I had an Avago promotional cup ;o)

An acquaintance had this as a custom number plate on his car, hence his nickname of "Avago Bob". Despite his best intention, it was somewhat ironic.

Probably OT by now.

Stephen

-----Original Message-----
From: piclist-***@mit.edu <piclist-***@mit.edu> On Behalf Of AB Pearce - UKRI STFC
Sent: Friday, 23 November 2018 12:47 AM
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: RE: [EE] Is Optoisolated Audio Practical?

Post by Richard Prosser
My application was isolating a current shunt but we couldn't get the
gain or offset sufficiently stable over the temperature range (-20C to +50 ?). IIRC we were looking for <<1%.

HP (probably now Keysight, or whoever handles the semiconductors) make an opto for exactly this use.

Don't have the part number handy but can dig it out if wanted.



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[AB Pearce - UKRI STFC]

Post by s***@agilent.com
Was HP then Avago now Broadcom soon to be...
"Avago" always brought a wry smile to me here in AU since it is the vernacular contraction for "have a go".

Oh yeah, that's right, the semiconductor bit got split from the instrument group, didn't it.

I always had the same wry reaction to the Avago name as well, probably because I'm a kiwi ...

[Martin McCormick]

Post by AK
Martin, I built what you are looking for back about 12 years ago using
optical methods. Using an analog to TOSLINK converter (I have extras if you
are interested), I modulated a laser instead of the LED in the TOSLINK
connector, and sent the audio over free space to a receiver. The sound
quality was perfect (no loss of highs I have experienced from transformer
isolators) and fun. You would just need to find a piece of equipment with
TOSLINK input.

This is exactly why I asked this question as I had no
idea what might come up.

I actually have some devices that have Toslink outputs
and inputs. One is a Behringer usb sound card which is aimed at
musicians. It has no mic input but 2 line-level RCA phono jacks,
a headphone jack and a Toslink optical output that speaks SPDIF.
I also bought a rebuilt CD audio recorder back in the late
nineties that sports both a Toslink input and an output.

I didn't know much about Toslink up to this very moment so
I looked it up.

It is absolute overkill for the audio of that general
coverage short wave receiver, but all I really need do is to buy
a Toslink cable long enough to reach where the CAT 3 cable is
now and see what happens. The Toslink cables appear
to come in a connector style that looks somewhat like a 3.7 MM
headphone jack and some use a square socket and there may be
adaptors to hold one of the round plugs in that square opening.

That would give two good-quality audio channels and all I
don't have is the cable so the project has almost assembled itself.

If I find out that the encoder generates yet more RF
noise, I'll have to solve that problem but the Toslink system
seems very promising.

Thanks very much.

Oh yes. That usb sound card is aimed at musicians but I never
got good enough to be called one. The sound card works great for
recording audio from scanner radios and any other unbalanced line-level
source.

Martin McCormick WB5AGZ

[Sean Breheny]

Hi Martin,

I think that the difficulties you mention with fiber are mostly with high
grade singlemode fiber used for long distance links or very high data
rates. I interned with a (now defunct) company which packaged and sold
various optical assemblies for telecoms via fiber. We bought the bare die
parts (e.g., laser diodes, photodiodes, etc.) and put them into
hermetically-sealed packages with a pre-aligned singlemode fiber pigtail
with a connector on the end. I worked on the part of the system which did
the internal alignment of the fiber to laser diodes but I saw the benches
where people would terminate fiber in connectors and perform fusion
splicing in other instances.

Anyway, I think for short-range digital audio links you can use cheap
plastic fiber and hand tools which cleave the ends. You then use enough
transmit power that the reflections at each interface, along with the fiber
attenuation, don't reduce the signal too low at the receiver end.

One interesting tidbit that I learned at this company was that you could
make a quick-and-dirty optical attenuator by just winding fiber around an
object which had a radius tight enough that total internal reflection can
no longer happen inside the fiber and light is lost at a constant rate per
length (which depends on the internal indices of refraction of the core and
cladding, as well as the bend radius). You could then vary the attenuation
by changing the number of turns.

Sean

Post by Martin McCormick
The thing that impressed me about fiber optics is that
the fiber isn't that terribly expensive but terminating it is.
You've got to polish the cut end of the fiber until it is
optically flat and free of deformations or one must use an
electric arc to melt two ends of fiber such that they fuse
together creating one continuous fiber. Equipment that will do
that reliably is quite expensive. Some of the consumer HiFi gear
that uses fiber-optic audio cables may use plastic fibers for all
I know but still terminating them is not trivial.
Basically, if the fiber optic connection is meant to be
easily connected and disconnected, you polish and lose 2 or 3 DB
per interface or you fuse if you are permanently joining a fiber
to something that you don't plan to remove under normal
circumstances. Those fusion splices don't have any loss since
there is no boundary.