Topic: Wordclock cable and length.

[gratefulphish]

It looks like I may be shifting setups, and potentially linking a V3 wordclock out to a 722, when I am doing four channel recording.  As I understand it, you can just use a normal RCA cable, with BNC connectors as the wordclock cable.  Please correct me if I am wrong.  What I was wondering is what the maximum recommended length of that cable can be.  If I have the two decks separate from one another, can I run a long RCA/wordclock cable, or do I need to keep the decks close together, and bring the source cables there?  Any help would be appreciated.  TIA

[Todd R]

Yep, I've done some testing on this and it seems to work great.  You can feed a normal digital audio signal (either spdif/coax or AES) from the V3 to the digital-input 722 -- no need for using the wordclock out (though I imagine you can if you have it, my V3 has been opti-modded and has not word clock out).  If you decide to clock the 722 using a digital audio feed rather than wordclock out, you'll need to use the menu to select analog input on the 722, not auto-select.  If you've got the clock synced from the V3 correctly, you'll see a "D" on the 722 screen, on the right hand side.


From my quick search, it looks like your digital cable can be up to 10m long, though I suppose that depends on the quality of the digital cable and how well the 722 handles jitter.

[TNJazz]

I'm a little confused here.  Wordclock I/O does not carry audio (only a digital clock) so you'd still need to connect the V3 and the 722 with some other means.  So you really don't need to use wordclock at all.  If you're connecting analog, there's no need to sync the devices.  If you're connecting digital, both AES and SPDIF from the V3 also contain a clock signal.

[it-goes-to-eleven]

I'm a little confused here.  Wordclock I/O does not carry audio (only a digital clock) so you'd still need to connect the V3 and the 722 with some other means.  So you really don't need to use wordclock at all.  If you're connecting analog, there's no need to sync the devices.  If you're connecting digital, both AES and SPDIF from the V3 also contain a clock signal.

One scenario:

source1 > v3 > microtrack
source2 > 722

With the 722 clock sync'd to the v3, both sources can be post matrix'd without the clock sync hassles.

[Todd R]

I'm a little confused here.  Wordclock I/O does not carry audio (only a digital clock) so you'd still need to connect the V3 and the 722 with some other means.  So you really don't need to use wordclock at all.  If you're connecting analog, there's no need to sync the devices.  If you're connecting digital, both AES and SPDIF from the V3 also contain a clock signal.

I'm not sure this will answer everything, but:

-The 722 can sync itself to other devices without using a wordclock.  It has a wordclock input, so that is one option.  But it also can sync itself to a digital audio stream (spdif, AES).

-Just because two devices are set to say 44.1k sampling, doesn't mean they will be synced.  Different word clocks will drift slightly from one another, so if you want to match up the audio from 2 recorders perfectly, you'll want to "sync" clocks -- in essence, use one word clock for both devices.

-You definitely want to sync devices if you are using analog, see above.  For instance, you might want to run soundboard (analog) > V3 (digital out)> recorder for one rig, and then run mics> 722(analog in) for the other rig.  If you want to matrix these two sources together in post, it'll be a snap if they are both using the same clock (otherwise you will have clock drift).  So if you run the second rig as mics > 722 (analog in) and V3> 722 (digital in) and select the "analog in" on the 722 menu, then the 722 in this case will use the V3 for it's clock (you'll see a "L" on the screen, sorry I mistakenly said "D" in the post above).  In this way, both your soundboard feed (into the V3) and your microphone/ambient feed (into the 722) will use the exact same clock, making post-production matrixing a snap.

[Todd R]

Also, I'm not sure if this is causing your confusion, but just because 2 different devices use, for example, 44.1k sampling doesn't mean they are both in sync and sampled the same.

One device, rather than having an exact 44,100 samples per second sample rate may actually sample at 44,115 samples/sec (off by 0.03%, not perfect, but not bad).  The other device could be just as off, but in the opposite direction, so it samples at 44,085 samples/sec (again only off by 0.03%).

These two devices thus vary from each other by 30 samples per second.  Over 30 minutes (60sec*30minutes = 1800 seconds) these two devices will now be off by 54,000 samples (1800sec * 30 samples/sec off = 54,000 samples).  At a rate of 44,100 samples/sec, these two devices are now off by over 1 second in the course of 30min of record time.  Not at all good for matrixing!

Granted, this is far worse than I've generally seen, but over 30min two different sources are often off by 200 or 300 milliseconds, from my own experience.  Still too much to just allow the sources to be matrixed together without correction.

[gratefulphish]

I'm a little confused here.  Wordclock I/O does not carry audio (only a digital clock) so you'd still need to connect the V3 and the 722 with some other means.  So you really don't need to use wordclock at all.  If you're connecting analog, there's no need to sync the devices.  If you're connecting digital, both AES and SPDIF from the V3 also contain a clock signal.

One scenario:

source1 > v3 > microtrack
source2 > 722

With the 722 clock sync'd to the v3, both sources can be post matrix'd without the clock sync hassles.

You hit the nail right on the head.  The exact setup I was going to use with four track.

[gratefulphish]

I'm a little confused here.  Wordclock I/O does not carry audio (only a digital clock) so you'd still need to connect the V3 and the 722 with some other means.  So you really don't need to use wordclock at all.  If you're connecting analog, there's no need to sync the devices.  If you're connecting digital, both AES and SPDIF from the V3 also contain a clock signal.

I'm not sure this will answer everything, but:

-The 722 can sync itself to other devices without using a wordclock.  It has a wordclock input, so that is one option.  But it also can sync itself to a digital audio stream (spdif, AES).

-Just because two devices are set to say 44.1k sampling, doesn't mean they will be synced.  Different word clocks will drift slightly from one another, so if you want to match up the audio from 2 recorders perfectly, you'll want to "sync" clocks -- in essence, use one word clock for both devices.

-You definitely want to sync devices if you are using analog, see above.  For instance, you might want to run soundboard (analog) > V3 (digital out)> recorder for one rig, and then run mics> 722(analog in) for the other rig.  If you want to matrix these two sources together in post, it'll be a snap if they are both using the same clock (otherwise you will have clock drift).  So if you run the second rig as mics > 722 (analog in) and V3> 722 (digital in) and select the "analog in" on the 722 menu, then the 722 in this case will use the V3 for it's clock (you'll see a "L" on the screen, sorry I mistakenly said "D" in the post above).  In this way, both your soundboard feed (into the V3) and your microphone/ambient feed (into the 722) will use the exact same clock, making post-production matrixing a snap.

I understand the mechanics and concept.  I was primarily wondering about the safe length for a wordclock signal cable, in case I wanted to separate the decks vs. bringing both signals to the decks together.  If I understand you correctly (have not received the 722 yet) I can simultaneously connect the V3 both analog and digital via AES/EBU, and if the 722 is set to analog, it will still pick up the clock signal via the AES input.  Is there any advantage/disadvantage to doing it via those outputs/inputs vs. the wordclock out/in, as the V3 has not been optimodded?  Than you for your responses.  +T to all.  (Waiting patiently for gear. . . . .)

[Todd R]

I did a quick search on the max length of spdif audio cables, which turned up the 10m I mentioned.  Since increasing jitter is the primary concern with length, I'd imagine that this would be a fairly representative number whether you're talking about a digital audio signal (with clocking info), or just the straight wordclock.

I'd be sure to get a high quality BNC coax digital audio cable though for long lengths.  I wouldn't imagine that it would be a good idea to be using a cheap cable in this application/

As to whether to use the wordclock signal or the digital audio signal, I don't know.  I wouldn't imagine it will make a difference, but it might be worth a call to SD tech support.

[gratefulphish]

Are BNC cables the normal 75 ohm or the 110 ohm for AES/EBU?

[SparkE!]

Since increasing jitter is the primary concern with length, I'd imagine that this would be a fairly representative number whether you're talking about a digital audio signal (with clocking info), or just the straight wordclock.

Actually, jitter is not necessarily increased by increasing the length of cable, but it does increase the dispersion of the frequency components that make up the logic level transitions that occur on the cable.  It also increases the amount of delay in the reflected waveforms from impedance mismatch at the other end of the cable.  Reflected signals do increase jitter.  If you have a perfectly matched load at the other end, there is no reflection.  Most cable terminations are terminated well enough that reflected signals don't affect the amount of jitter.  The thing that limits the usable length of cable is the amount of filtering due to the bulk capacitance of the cable.  You're driving it from a 75 ohm load and the bulk capacitance appears as a shunt capacitor to the signal path.  The longer the cable, the higher that capacitance and the slower the voltage transistions are.  When the rise and fall times get too slow, that can cause an apparent increase in jitter if the driving side impedance changes, depending on whether it is sending a high logic level or a low logic level.  Unfortunately, this data-dependent variation in driving point impedance is fairly common.  (CMOS, short for Complementary Metal Oxide Semiconductor, is not always exactly complementary when it comes to maintaining the same driving point impedance for a high logic level as for a  low logic level.)

So, from a practical point of view, lengthening the cable does cause an apparent increase in jitter, but it is not the fault of the cable itself.  Instead it is the fault of (possibly data-dependent) mismatches in the driving point and termination impedances.

[TNJazz]

Sorry, I apparently misunderstood the original post...

I thought he wanted to connect the V3 and 722 using ONLY a wordclock cable.

In addition, my reference to "syncing the devices" was only in regard to the V3 and the 722 if he were connecting them via analog cables.  Of course if he has two decks then syncing them to a common clock would be ideal.  Somehow that part of the story escaped me.

I think the whole part about using regular RCA cables with BNC connectors is what threw me off.

Word clock cables are 75 ohm BNC.  I have always beeen told that you should only use these for WC - there are 50 ohm BNC cables available as well so make sure to get the correct ones.

[Todd R]

Yep, I kinda figured you just had some underlying confusion.

I really added the part about wordclock syncing for others who might be reading the thread.  It seems from my reading that a number of people assume that just because you use the same sample rate on two different machines that they will be synced.  Oh that it were that easy.

[thepassionofyonder]

Word clock cables are 75 ohm BNC.  I have always beeen told that you should only use these for WC - there are 50 ohm BNC cables available as well so make sure to get the correct ones.

looks like i'll be testing the resistance of my bnc cable when i get home tonight.
i wonder how many ohms my cable is

[SparkE!]

It seems from my reading that a number of people assume that just because you use the same sample rate on two different machines that they will be synced.  Oh that it were that easy.

Ha! No kidding.  It doesn't take much clock skew to cause some pretty severe sync problems.  Most people try to keep the skew between two sources under 10 milliseconds.  I find it becomes noticeable at around 2 milliseconds or even less.  Think about it. If you misalign two equal amplitude 250 Hz signals by exactly 2 milliseconds and add them together, they completely cancel each other!  You get silence.  I don't know about you, but my hearing goes well above 250 Hz! To keep if from happening up to 20 kHz, you'd have to align the two sources to less than 25 microseconds.  At a 44.1 kHz sampling rate, each sample represents a little over 22 microseconds.  So to be good to 20 kHz without any phasing effects, you actually have to align the two sources within 1 sample. (!!!)  That's just how the math works out.

Now, consider what happens in the real world.  Even if you use really high spec crystals, you usually don't get even close to that type of performance when you are relying on two separate crystals.  High spec crystals are typically spec'd at +/- 10 ppm.  (Commonly available crystals are +/- 100 ppm.)  With just 10 ppm skew between two crystals, that works out to about 36 milliseconds of skew per hour. (Common crystals with 100 ppm will work out to 360 milliseconds per hour.  Yikes!!  That's over 1/3 of a second!)

So yeah, it's rare to get two recorders whose clocks match well enough to do matrix mixes in post.  I'm very, very lucky to have a couple of UA-5's that match each other within 1 millisecond per hour.

[live2496]

Are BNC cables the normal 75 ohm or the 110 ohm for AES/EBU?

BNC cables are 75-ohm.

RE: cable length
I assume that the same rules regarding cable length for carrying SPDIF data should apply to word clock as well.