Have your say & ask the experts

I'm still very much confused about the fact that
people say that digital source from one device is different that
digital source from another device which "in theory" doesn't make
sense. In mean binary output should be the same regardless of the
device and why it isn't? or is it really different?

I mean the
receiver side needs to receive full signal because I'm sure just like
in any network device there are checks that make sure that is the case.
In a situation that something is missing the sender is being asked to
resend the data. Also on the receiver side there is usually a buffer so
there shouldn't be a situation that you don't have any data at some
point.

madeinstein:

I mean the receiver side needs to receive full signal because I'm sure just like in any network device there are checks that make sure that is the case.

JohnDuncan:

No, it doesn't

Ok so that is the main problem with digital out.. It's simply unreliable!

So now I understand why Linn is using ethernet port in it's DS devices and in my opinion that is the way forward. You need to ensure that your DAC receives the full signal.

Continuing on the same subject..

That also explains why in Stereophile's test Logitech Transporter was better than Squeezebox + separate DAC. Even if DAC in Transporter is not as good as the separate DAC (maybe it is I don't know) Transporter has the edge of receiving better signal than DAC via digital out from Squeezebox.

Are you sure? The OP is right as far as I know. All WiFi systems have error checking built in as part of the transmission std AFAIK. So wireless transmission of digitised information should be the same irrespective of source given the transmitter power is sufficient (i.e you are not out of range).

Now converting that digital signal to an analogue signal is a different matter, here as the OP mentions, the DAC will play a role.

JohnDuncan:

Wifi does (it's file transfer), but the digital stream produced by a player (and received by a digital input) does not, AFAIK.

Taking CD as the obvious example there is error correction built into the CD track using Cross Interleaved Reed-Solomon coding.

The standard is laid out in the Red Book. but to paraphrase:

The smallest entity in a CD is called a frame. A frame consists of 33 bytes and contains six complete 16-bit stereo samples (2 bytes × 2 channels × six samples: equals 24 bytes). The other nine bytes consist of eight Cross Interleaved Reed-Solomon coding error correction bytes and one subcode byte, used for control and display. Each byte is translated into a 14-bit word using eight-to-fourteen modulation, which alternates with 3-bit merging words. In total there are 33 × (14 + 3) = 561 bits. A 27-bit unique synchronization word is added, so that the number of bits in a frame totals 588 (of which only 192 bits are music). Working backwards you can also see that a frame contains 1/75th of a second of music.

So any decent player will have a fault free digital output. All you need to do is get this into your DAC without corrupting it.

welshboy the question is what kind of signal is being send via digital out? not how it's stored on CD/HD etc..

When you're using ethernet port you have error correction build in into the protocol you're sending a data with (TCP, UDP) but as far as I understood from what JohnDuncan said earlier digital out has just raw data (bits of information) without any begining/end. 

Read a bit more on this subject so yes the frames are being send rather than raw data.

Is that correct?

Found more info..

I think the problem with digital out is that the player is sending frames but only in one direction. So if the DAC misses the frame or gets invalid frame it will be skipped as it has no means to ask for this data to be resend. This is the big advantage of ethernet connection.

madeinstein:

welshboy the question is what kind of signal is being send via digital out? not how it's stored on CD/HD etc..

When you're using ethernet port you have error correction build in into the protocol you're sending a data with (TCP, UDP) but as far as I understood from what JohnDuncan said earlier digital out has just raw data (bits of information) without any begining/end. 

Read a bit more on this subject so yes the frames are being send rather than raw data.

Is that correct?

The OP mentioned digital souce in his original post. Strictly speaking the CD or DVD etc is the digital source. Everything else in the "player" is a reading mechanism and DAC. 

I wasn't really talking about CD but about the error correction. As you know CD stores PCM (Pulse code modulated) audio data. My point was that a lot of people think you can lose data from a CD during the read process - in simple terms you can't because of the fact that the CD has inbuilt error correction so the data reciever can tell if it has the correct data. Now we come to the choices. The device recovering the data has freedom within the standard to either ask for the data again which is usual or if several contiguous frames are corrupted it can "estimate" what is missed to give what is usually refered to as an inaudible correction.

The benefit (as some people see it) of digital transmission is that signal reproduction does not happen in real time as with an analogue signal. (that is to say there is normally buffering) There is therefore time (in relative terms) to correct errors from the reading of the CD transmitting of data etc. Since the time codes are also transmitted in the frames there is also time to correct the sequencing. It's not like the reproduction of an analogue signal in that regard.

You mention  the S/PDIF standard (optical or coaxial digital) and you have no doubt read that this is a development of a high quality proffesional transmission standard. As such there is error correction available in time slot 31 of the sub frame as it happens. So again it's about how the equipment is designed and if error correction is enabled. Note though that it is possible to degrade the digital signal if the environment is electrically noisy. Same is of course true for any digital or analogue transmission cable. At some point with both transission systems the signal is unrecoverable if the noise is too bad. That said analogue tends to hang on longer even if what you end up hearing is unpleasant wheras digital tends to sound OK then fall of a cliff so to speak.

BTW the same factors will impact on ethernet as well - ultimately in an electrically noisy environment it can fail.

Jitter (more correctly seek jitter) tends to be caused by re-sampling errors as the Red book standard does not actually require block accurate addressing. Therefore if the extraction process re-starts a sample of a frame it may get that frame out of sequence. Most good CD players will correct this in software. Normally this is done by performing overlapping reads and sort of sliding the data blocks about electronically to find the overlaps at the edges. Think of putting 2 sheets of paper with the same thing written on them on top of each other with the light behind then and moving them about to get all the writing to line up. The differences (if any) stand out sharply. Astronomers do something similar with telescope images to see the relative movement of celestial objects.

welshboy:

You mention  the S/PDIF standard (optical or coaxial digital) and you have no doubt read that this is a development of a high quality proffesional transmission standard. As such there is error correction available in time slot 31 of the sub frame as it happens.

[...]

BTW the same factors will impact on ethernet as well - ultimately in an electrically noisy environment it can fail.

welshboy I do not agree with your that S/PDIF is the same as ethernet. Please correct me if I'm wrong.

I agree that with S/PDIF you have information in the frame to check if the frame is correct.. But the question is what if it's not correct? The frame is being skipped.. and the next one is received just like JohnDuncan said initially.

With ethernet if the packet is incorrect receiver asks the sender to resend the data and nothing is skipped. That means that skipping is much less likely. I mean you would have to either loose transmission completly.. or several send/receive signals would have to fail which doesn't happen often even in long distance networks.

Of course I agree with you that if S/PDIF sender + cable + DAC is good quality than it should be the same as ethernet.

Ethernet protocols are much more complex and are designed specificly not to loose any data.

madeinstein:

welshboy:

You mention  the S/PDIF standard (optical or coaxial digital) and you have no doubt read that this is a development of a high quality proffesional transmission standard. As such there is error correction available in time slot 31 of the sub frame as it happens.

[...]

BTW the same factors will impact on ethernet as well - ultimately in an electrically noisy environment it can fail.

welshboy I do not agree with your that S/PDIF is the same as ethernet. Please correct me if I'm wrong.

I agree that with S/PDIF you have information in the frame to check if the frame is correct.. But the question is what if it's not correct? The frame is being skipped.. and the next one is received just like JohnDuncan said initially.

With ethernet if the packet is incorrect receiver asks the sender to resend the data and nothing is skipped. That means that skipping is much less likely. I mean you would have to either loose transmission completly.. or several send/receive signals would have to fail which doesn't happen often even in long distance networks.

Of course I agree with you that if S/PDIF sender + cable + DAC is good quality than it should be the same as ethernet.

Ethernet protocols are much more complex and are designed specificly not to loose any data.

Of course you are correct ultimatly theS/PDIF standard is design for close to real time transmission of audio data over very short distances. Ethernet is designed for buffered transmission of any sort of digital data over longer distances on a non-dedicated transport structure. My point, from which I confess I think I wandered a bit, was that it's prefectly possible to get reliable transmission of a digitised audio stream using S/PDIF standards.

My other point was related to digital sources i.e. the thing that carries the digital information be that CD or DVD or DAB broadcast. These are the sources of the digital information IMHO.

I also agree that ethernet is specifically designed not to loose data. However that does not make it bullet proof. It is subject to all of the same problems as any other sort of tranmission system.

I'm going to have a go at this, but I don't proclaim to be 100% correct...

The way I've been reading this thread, I think that the original question has been a little misunderstood.  I understand OP to be asking something tantamount to 'If you have a £50 CD player with SPDIF out and a £500 CD player with SPDIF out, surely there should be no difference in what comes out of the SPDIF socket'

This is something that I once thought many moons ago.   As JohnDuncan said, it's about the ability of the player to read the data correctly in the first place more than anything else - you would expect the more expensive CD player to employ techniques that enable it to read as much data correctly as possible.  You would expect the cheaper one to not read the data as well in the first place, but to use error correction to pick up the pieces.  Therefore, playing the same disc in both machines would produce two different data streams and hence they will sound different (to some people, at least).

I understand what OP is thinking about with regard to Ethernet and error checking but as has been said, SPDIF is one way traffic; no error checking in place. However, even if CD players transmitted the data over ethernet, it wouldn't make any difference as the data coming out of the player could have errors in the first place so if the checking is done against that, then there would be no difference.  It would only make a difference if the player then went back to the CD to check for errors against the original but again, there could be different read errors due to the nature of reading a spinning optical disc and you could end up in some sort of error checking loop!  It of course begs the question of whether CD's are an appropriate medium to store other sorts of data!

This is why some people believe that Hard disks or Solid state memory is a better medium for reading audio data from - you can have all the error checking and the source medium isn't as likely to have the read errors associated with optical media (or is it?).

Going to lie down now...