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S/PDIF help

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Anonymous
February 10, 2005 6:39:09 AM

Archived from groups: rec.audio.pro (More info?)

Anyone know the insides of S/PDIF? There's a few versions of the spec
out there but I am interested in the one that carries Dolby information
- IEC61937.

Some CD/DVD players have built-in Dolby decoders and they output the
data over S/PDIF. Is the data truly decoded into all the various
channels? Is it still compressed? Is it simply PCM at this point, with
channel information associated with it?

My understanding is that IEC60958 carries IEC61937 data, but sets some
flags to distinguish the data from normal PCM. That's about as much as
I know and am having trouble getting more details than that.

Oh, one other question - how does IEC950 differ from IEC60958?

I'd appreciate contact with anyone who can provide me with more insight
into the interface.

thanks

gene

More about : pdif

Anonymous
February 10, 2005 9:22:45 AM

Archived from groups: rec.audio.pro (More info?)

There are many CD/DVD/SACD manufacturers advertising DOLBY decoding
built-in. What is it, then, that they mean? If the S/PDIF interface
is NOT carrying channelized digital pcm stream, and the data is simply
dolby digital or DTS or whatever, why advertise built-in decoding?
They must have some reason for advertising as such.

I have not looked into this, but is it possible that they decode Dolby
and send out on HDMI, for example? Just a thought.
Anonymous
February 10, 2005 10:36:44 AM

Archived from groups: rec.audio.pro (More info?)

<carzrgr8@optonline.net> wrote in message
news:1108035549.434361.249220@o13g2000cwo.googlegroups.com

> Anyone know the insides of S/PDIF? There's a few versions of the spec
> out there but I am interested in the one that carries Dolby
> information

> - IEC61937.
>
> Some CD/DVD players have built-in Dolby decoders and they output the
> data over S/PDIF.

The SP/DIF output of a DVD player is not obtained by means of decoding or
encoding inside the player. The SP/DIF stream is raw data from the disc
being played, somewhat reformatted. The encoding process was part of the
production of the optical disc.

>Is the data truly decoded into all the various channels?

The SP/DIF data is decoded in the equipment that receives the SP/DIF data
stream from the optical player.

> Is it still compressed?

Yes. The data is similar in format to a 2-channel 48 KHz PCM data stream,
except that the actual data words are not PCM audio. If interpreted as PCM
audio (which can be done with the right equipment) the Dolby Digital data
stream sounds like white noise. The complex decoding process involves
separation of the six audio channels, and uncompression of each channel into
audio signals that represent music, voice and EFX.

> Is it simply PCM at this point, with channel information associated with
> it?

It is a complex data stream.


> My understanding is that IEC60958 carries IEC61937 data, but sets some
> flags to distinguish the data from normal PCM. That's about as much
> as I know and am having trouble getting more details than that.

For most practical purposes, that's all you probably really need to know.
Dolby decoders are separate components - you can be very successful if you
think about them as black boxes. A data stream goes in, and a number of
channels of audio come out.

> Oh, one other question - how does IEC950 differ from IEC60958?

I'm not a standards maven.

> I'd appreciate contact with anyone who can provide me with more
> insight into the interface.

You don't have to understand the standards to know enough about the
interfaces for most practical purposes.

Now that you know that you had just about everything wrong in your first cut
at the problem, why not reread the information at your disposal in the light
of what you now know.
Related resources
Anonymous
February 10, 2005 2:18:28 PM

Archived from groups: rec.audio.pro (More info?)

<carzrgr8@optonline.net> wrote in message
news:1108045365.856977.152150@g14g2000cwa.googlegroups.com

> There are many CD/DVD/SACD manufacturers advertising DOLBY decoding
> built-in.

OK.

> What is it, then, that they mean?

It means that they have analog outputs for multichannel. More specifically,
it means that they have at least 6 analog output jacks, one for each of the
5.1 channels in Doilby digital.

>If the S/PDIF interface is NOT carrying channelized digital pcm stream,

Every optical drive I've ever seen with multichannel analog outputs also had
a SP/DIF output, but there may be some exceptions. In essence, the SP/DIF
output is related to the the INPUT to the onboard Dolby decoder.

> and the data is simply dolby digital or DTS or whatever, why advertise
> built-in decoding?

The built-in decoding can have practical uses besides what you've
hypothesising.

> They must have some reason for advertising as such.

Built in Dolby decoding is useful to people who are using systems that lack
Dolby digital decoders but have multichannel analog inputs.

Your typical consumer is going to hook this digital player up to a receiver
that already has a dolby digital decoder built into it. Given the simplicity
and ease of hooking up just a single piece of coax or fiber, most people are
going to to that. Many multimedia receivers only accept digital multichannel
inputs anyway. BTW, this makes the recievers cheaper to build and it's OK
because it usually works fine.

OTOH, how many times have you seen features advertised that had little or no
real-world significance to most people?
Anonymous
February 10, 2005 2:28:00 PM

Archived from groups: rec.audio.pro (More info?)

<carzrgr8@optonline.net> wrote:
>Anyone know the insides of S/PDIF? There's a few versions of the spec
>out there but I am interested in the one that carries Dolby information
>- IEC61937.

That is not the S-PDIF spec. The S-PDIF spec (which is now replaced
with IEC-958) is just the interface that allows raw data to be sent.

The IEC61937 standard is actually a standard which allows various kinds of
compressed 5.1 data (including Dolby AC-3) to be sent over the existing
(older) S-PDIF hardware as if it were raw PCM data. It includes some flags
about the content of the data.

>Some CD/DVD players have built-in Dolby decoders and they output the
>data over S/PDIF. Is the data truly decoded into all the various
>channels? Is it still compressed? Is it simply PCM at this point, with
>channel information associated with it?

No, it's an AC-3 bitstream. These players do NOT have built-in Dolby
decoders. They take the AC-3 data off the disc as if it were standard
PCM 2-channel audio data, and they send it out of the S-PDIF port.

In this case, the thing with the Dolby decoder is whatever plugs into
the S-PDIF jack and recovers the data stream, uncompresses it, and splits
it into PCM data for conversion to analogue.

The players that have Dolby decoders use them not for the S-PDIF output,
but so they can have discrete-channel analogue outputs in the rear.

>My understanding is that IEC60958 carries IEC61937 data, but sets some
>flags to distinguish the data from normal PCM. That's about as much as
>I know and am having trouble getting more details than that.

Right. There are some subcode bits that are now defined for handling
such data. I think the IEC-60958 is a followon to the older IEC-958
standard and may supersede it.

>Oh, one other question - how does IEC950 differ from IEC60958?

IEC 950 is an electrical safety standard, I think having to do with
chassis leakage specs. It is not related to digital audio.

>
>I'd appreciate contact with anyone who can provide me with more insight
>into the interface.

Look at Rane applications node 149. Probably on the Rane web site.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
February 10, 2005 5:20:18 PM

Archived from groups: rec.audio.pro (More info?)

Thanks for the Rane tip - took a look and there's some usefull info
there.

I'm disappointed that SPDIF doesn't have already decoded audio.
Although it's nice to know that the CD/DVD stuff has decoded analog out
- it's not quite what I want to use.

So suppose I want to build a device with dolby decoder - I already know
that Dolby requires $10K just to get a license to run a decoder (+ per
unit royalty, but am not concerned about that). Does anyone know of
any Dolby decoders publicly available that can be used (i.e. OEM) in
another product? For example, if a decoder chip were available, that
would be great. So far, I've only found hardware + software solutions
(e.g. DSP + firmware) where you cannot get the vendors to begin a
dialog without a signed license from Dolby labs. What would be ideal
for me, is a chip that has the decoder built-in, no software to deal
with, and already includes the royalty!

I'm not saying this is the best solution to my problem, but there are
PC sound cards built that have the decoders (maybe a partial software
solution too), include S/PDIF, and ready to rock. How low-class is it
to include a sound blaster in another product, eh?
Anonymous
February 11, 2005 1:44:03 AM

Archived from groups: rec.audio.pro (More info?)

<carzrgr8@optonline.net> wrote in message
news:1108035549.434361.249220@o13g2000cwo.googlegroups.com...
> Anyone know the insides of S/PDIF? There's a few versions of the spec
> out there but I am interested in the one that carries Dolby information
> - IEC61937.
>
> Some CD/DVD players have built-in Dolby decoders and they output the
> data over S/PDIF. Is the data truly decoded into all the various
> channels? Is it still compressed? Is it simply PCM at this point, with
> channel information associated with it?
>
> My understanding is that IEC60958 carries IEC61937 data, but sets some
> flags to distinguish the data from normal PCM. That's about as much as
> I know and am having trouble getting more details than that.
>
> Oh, one other question - how does IEC950 differ from IEC60958?
>
> I'd appreciate contact with anyone who can provide me with more insight
> into the interface.
>
> thanks
>
> gene

See if the following helps...

http://www.epanorama.net/documents/audio/spdif.html

Craig
Anonymous
February 28, 2005 6:09:00 PM

Archived from groups: rec.audio.pro (More info?)

carzrgr8@optonline.net <carzrgr8@optonline.net> wrote:
>
>So suppose I want to build a device with dolby decoder - I already know
>that Dolby requires $10K just to get a license to run a decoder (+ per
>unit royalty, but am not concerned about that). Does anyone know of
>any Dolby decoders publicly available that can be used (i.e. OEM) in
>another product? For example, if a decoder chip were available, that
>would be great. So far, I've only found hardware + software solutions
>(e.g. DSP + firmware) where you cannot get the vendors to begin a
>dialog without a signed license from Dolby labs. What would be ideal
>for me, is a chip that has the decoder built-in, no software to deal
>with, and already includes the royalty!

Do you want to build a single unit, or a whole lot of them? If you only
want a single unit, it should be no problem to get your hands on a cheap
home receiver with an AC-3 decoder chip in it out of the junk bin at
your local TV repair, buy a Sam's Photofacts for it, and extract the
decoder chip. Getting it to play nicely with converters and the like
may turn out to be a lot of work, but you won't know until you try.

>I'm not saying this is the best solution to my problem, but there are
>PC sound cards built that have the decoders (maybe a partial software
>solution too), include S/PDIF, and ready to rock. How low-class is it
>to include a sound blaster in another product, eh?

You may not even need to include the whole sound blaster. You may just
be able to buy the card and remove the chip from it, although sadly you
will find that while complete documentation is available for most home
theatre receivers, there is very little technical documentation available
for soundcards and other low end computer gear.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
February 28, 2005 6:15:41 PM

Archived from groups: rec.audio.pro (More info?)

"Scott Dorsey" <kludge@panix.com> wrote in message
news:cvvtos$bb1$1@panix2.panix.com
> carzrgr8@optonline.net <carzrgr8@optonline.net> wrote:
>>
>> So suppose I want to build a device with dolby decoder - I already
>> know that Dolby requires $10K just to get a license to run a decoder
>> (+ per unit royalty, but am not concerned about that). Does anyone
>> know of any Dolby decoders publicly available that can be used (i.e.
>> OEM) in another product? For example, if a decoder chip were
>> available, that would be great. So far, I've only found hardware +
>> software solutions (e.g. DSP + firmware) where you cannot get the
>> vendors to begin a dialog without a signed license from Dolby labs.
>> What would be ideal for me, is a chip that has the decoder built-in,
>> no software to deal with, and already includes the royalty!
>
> Do you want to build a single unit, or a whole lot of them? If you
> only want a single unit, it should be no problem to get your hands on
> a cheap home receiver with an AC-3 decoder chip in it out of the junk
> bin at your local TV repair, buy a Sam's Photofacts for it, and
> extract the decoder chip. Getting it to play nicely with converters
> and the like may turn out to be a lot of work, but you won't know
> until you try.

Another similar approach would be to pick up one of the Technics SHAC-300 or
SHAC-500 stand-alone decoders. They show up occasional on eBay. They were
fine 2-channel boxes as well.

>> I'm not saying this is the best solution to my problem, but there are
>> PC sound cards built that have the decoders (maybe a partial software
>> solution too), include S/PDIF, and ready to rock. How low-class is
>> it to include a sound blaster in another product, eh?

> You may not even need to include the whole sound blaster. You may
> just be able to buy the card and remove the chip from it, although
> sadly you will find that while complete documentation is available
> for most home theatre receivers, there is very little technical
> documentation available for soundcards and other low end computer
> gear.

Typically, the sound card implementations of Dolby decoding are heavily
software-assisted. There may be no licensed Dolby hardware on them at all.
For extra fun, some NVidia sound sections included Dolby encoders, AFAIK
still heavily software-based.
!