Help me understand S/PDIF please

A

AndrewClaycomb

New member
I have a Firepod and I would like to add 2 more channels through S/PDIF. I was looking at the Art DPSII which has S/PDIF in and out.

If I bought the DPSII could I just plug the preamp into my interface using an RCA cable for both the input and the output and be done? Will the two match each other on sample rates? Would I have to buy an external word clock to make the two work together without pops and clicks?

I thought I had a firm grasp on this until I was reading some reviews and a guy mentioned word clock issues between the preamp and his interface.

Thanks.
 
This is from your manual:

S/PDIF IN and OUT. The S/PDIF I/O allows the FIREPOD to receive and transmit audio from other digital
audio devices. The S/PDIF standard allows for two channels of audio to be transmitted at up to rates of
24bit/96Khz. The S/PDIF input also allows the FIREPOD to receive and transmit word clock. Word clock is
the synchronizing signal that indicates the sampling frequency or rate of sample words over a digital audio
interface. Note: When using SPDIF In, you must select SPDIF in your FIREPOD Hardware Control Panel
(located in your system tray – Windows XP only).

All you need is a S/PDIF cable to connect this to your other device.
 
Honestly....
Word Clock and SPIDF are not the same thing the way I understand it. Spidf does indeed have clocking info embedded in the digital stream which will in fact allow the the interface and the the pre to synchronize. But a dedicated word clock source is different from what I understand.
The pre would send the master synch signal and the interface would be set to be the slave.
 
tmix said:
Honestly....
Word Clock and SPIDF are not the same thing the way I understand it. Spidf does indeed have clocking info embedded in the digital stream which will in fact allow the the interface and the the pre to synchronize.
Click to expand...

I wouldn't go so far as to use the word "info" to describe the clock, but yes. Word clock is a series of pulses, one per sample (IIRC), that keep two units sample-accurate with respect to one another.

By contrast, the very act of sending digital data in a serial format (one bit followed by the next) necessitates some sort of bit clock in order for the receiver to figure out where one bit ends and one bit ends. There are various techniques used for this, some of which send separate clocks on a separate line, the rest of which encode the clock and data together in various ways as S/PDIF does.

It's pretty neat. IIRC, the way S/PDIF does it is this: it doubles the bit clock rate. At the start of a bit (whether zero or one), the level changes from zero to one or one to zero (depending on the value of the previous bit). Halfway through the bit, if the value is one, the state changes back to the previous state. If the value is zero, it does not change. By doing this, you can watch the stream of bits and even if you don't know the data rate, it takes at most a single one bit for you to figure out the width of a bit. After that, it takes at most a single zero bit to determine which boundary is the half-bit boundary and which one is the full boundary. Once you know that, the state changes from logic low to high and vice versa give you not only the clock, but also the data.

The sad thing is that I wrote this off the top of my head and then checked Wikipedia because I couldn't remember the name of the encoding.... :) It's Biphase Mark Code. *sigh*

The upshot of this is that there is a data boundary every so often to indicate where each sample or group of samples begins (no, I don't know the frame size or the framing details up at that level of the stack; it is really just data at that point). From this, because it knows the number of bits per sample, the hardware can divide that bit clock down into something resembling a sample clock....
 
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Thanks everybody, so just to be clear.

All I need is a single S/PDIF cable going from the out on the tube preamp to the input on my Firepod? I don't need a 2nd cable going from the output on my Firepod into the In on the tube preamp? The Firepod doesn't need to transmit anything back to the Art preamp?

Sorry if I'm dense on this.
 
AndrewClaycomb said:
Thanks everybody, so just to be clear.

All I need is a single S/PDIF cable going from the out on the tube preamp to the input on my Firepod? I don't need a 2nd cable going from the output on my Firepod into the In on the tube preamp? The Firepod doesn't need to transmit anything back to the Art preamp?

Sorry if I'm dense on this.
Click to expand...
When I've used SPDIF, two cables were needed. You have to set the Firepod as master and the tube amp as slave (Or visa versa). In order to get the clocking information to the tube pre, don't you need a cable from the firepod SPDIF out to the tube amp SPDIF in as well? Correct me if I'm wrong.
 
guitar zero said:
When I've used SPDIF, two cables were needed. You have to set the Firepod as master and the tube amp as slave (Or visa versa). In order to get the clocking information to the tube pre, don't you need a cable from the firepod SPDIF out to the tube amp SPDIF in as well? Correct me if I'm wrong.
Click to expand...

If your tube pre is capable of syncing off of S/PDIF, you might get better quality using the FIREPOD's clock than using the clock in the tube pre. I have no idea which clock would be more accurate, so I can't answer that question either way. Most pres can only sync off of word clock inputs, though, and since the FIREPOD doesn't have a WC out, your only choice is to use the tube pre as the master clock and let the FIREPOD slave via S/PDIF.

BTW, if you can help it, try not to record other tracks while the FIREPOD is slaved via S/PDIF; my FIREPODs had less-than-stellar S/PDIF locking abilities, if memory serves, so you might have some quality loss and/or dropouts on tracks recorded from the FIREPOD's inputs while it is being driven by an external clock. (You might not have this problem; it may have been an issue specific to my setup.)
 
I regularly use a S/PDIF connection with two other pieces of pro-sumer equipment (an ART Digital MPA to a MOTU Traveler interface) - both devices support external word clock and both coax and optical S/PDIF. Each device also supports 44.1/48/88.2/96/.../192khz sample rates. I agree that the S/PDIF standard includes clock information, and so by specification, you should need only the digital cable (I'm sure the spec is like this for the optical cable, and I'm pretty sure it is for the coax, too).

But in practice, getting the devices to work together without hiss/click/pop problems was a nightmare. I tried many different combinations of cables, optical or coax, word clocked one way or the other, different sample rates, etc., and finally arrived at an external word clock at 96khz to each unit (having to terminate the connection on the ART with a BNC terminator) and coax S/PDIF -- this combination, for me, works great every time. My suspicion is that my solution is unique to my gear, and that changing any variable would require me to go back to the trial-and-error process that got me good results this time.
 
antichef said:
I regularly use a S/PDIF connection with two other pieces of pro-sumer equipment
Click to expand...
And that is the critical difference... I too would highly reccomend clocking an SPDIF audio connection from a seperate word clock connection when connecting with multiple devices... the original poster was looking to connect just two devices by spdif... no real benefit to a second clock path in this set-up
 
MOFO Pro said:
And that is the critical difference... I too would highly reccomend clocking an SPDIF audio connection from a seperate word clock connection when connecting with multiple devices... the original poster was looking to connect just two devices by spdif... no real benefit to a second clock path in this set-up
Click to expand...
Ah - I was being less-than-clear. I meant two devices other than the ones that OP mentioned, but still just two. Although recently I've added third (Echo Audiofire 12 connected via firewire daisy-chained with the Traveler) and it's now in the same external clock family and it all works nice :)

I definitely agree that it's worth trying the native S/PDIF sync first - much easier and cheaper if it works!
 
OK, so the answer is I might be able to use all 10 channels (Firepod-8, Art-2) at once without an external word clock... but if I get hiss/dropouts I should add an external word clock.
 
guitar zero said:
Got a 58 and an audix i5, but NO 57, so apparently they are not as ubiquitous as opinions these days.
Click to expand...
Well... not everyone has an opinion either... (but you won't find them here)

And if you've got an early 58 unscrew the windscreen... there's your 57
 
AndrewClaycomb said:
OK, so the answer is I might be able to use all 10 channels (Firepod-8, Art-2) at once without an external word clock... but if I get hiss/dropouts I should add an external word clock.
Click to expand...
Yes... The clock source in hardware settings will have to be set for SPDIF...
 
You'll simply need to determine if either of the pieces (both may be able to) can be slave' to the other set as master.
Sajs posted; 'Note: When using SPDIF In, you must select SPDIF in your FIREPOD Hardware Control Panel.' It looks like the Firepod can slave to the SPDIF input.
However I glanced at the ART manual and it seems to say it can be master in 44 or 48k' modes only, but only salve to ADAT in or WC- (not SPDIF?) Does Firepod have WC out?
In any case only one can be master'. Presumably it might be better for 10 track' recording to have the most tracks' A/D converters on internal clock (it's the master), and make the ART master when using just it.
 
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