BNC vs AES connections for clocking

Word clock was an advantage with the first generation of incompetently designed digital transceivers but not with recent designs.

thanx pal, not yet, but i'm positive it's getting closer, i've had contact with him regularly, i will be sending it all right over to JW for preventative maintenance when i get them haha 8)

Yep, I have some input.

BNC is 75ohm TTL(typically 5V) level with "wordclock" being the primary signal used for clocking. Wordclock operates at the frequency of sampling (I.E., 44.1K, etc), which is considered low frequency. The high amplitude and low frequency make it relatively immune to low amplitude noise from the outside world. Coax cables are used for all kinds of signals up into the terahertz range, so coax itself is not a problem at all. Where problems occur is the multiple connections (daisychaining) of the BNC through multiple devices. Each device can add noise in the form of ingress (noise that couples to the conductor through EMI/RFI) or through noise figure (NF is noise added to the signal from amplifiers and active components) or through other means. Termination with 75R is a requirement on the last piece of gear to "sink" signal currents and keep reflections low. Just as with a wave of water in the pool bouncing back from the side, electrical signals do the same thing without a termination resistor sucking the energy out of the reflection.

For wordclock, the jitter on the original signal is the baseline for the jitter for the system.



AES3 is typically differential 110R, up to 7V (at transmitter output) with the clock signal embedded into the datastream. The encoded clock is a "biphase" clock typically at 128x-256x the sample rate. The clock must be digitally extracted from the signal and divided down to a wordclock (Dividing clocks also divide the jitter). Since the signal can be in the low MHz range, the differential data is more for allowing long runs of signal without rounded edge transition problems, than for noise immunity. The large amplitude of the signal is rather immune to most noise sources, much like coax.

AES3 can also be single-ended as well as 75R.

For AES/SPDIF, some receiver ICs have internal PLLs which will lock to the extracted clock on the incoming signal and reproduce a "cleaner" version. Some refer to this as "jitter cleaning". Some designers believe this is just wishful thinking, and some claim that it helps. What does help is the division of the incoming clock signal, which divides the jitter down by the same amount the signal is divided.

"Superclock" was at one time the same idea behind the extracted clock of AES, but over a wordclock-like setup. It would have been a 128X or 256x clock signal (I.E., 11.289600MHz for 44.1Khz x 256 sample rate) that was over BNC, so that it could be divided down by each unit internally, for superior jitter performance. Unfortunately it never really caught on and the increase in costs caused customers to balk at upgrading.

So for me, I would choose AES simply because of the clock dividing. The physical connection doesn't make a terrible difference unless going more than 25ft or so.

really great stuff guys, thanx! i'm interested to hear from svart what he thinks of that link? and what is the deal with "AES black"? i've never heard of that until reading that thread....hmmm.

It's an interesting take.  

I'm sure he's done testing and is aware that the preambles are for syncronization of the decoder, not necessarily for clocking.  

In fact, the preamble is not BMC (biphase mark coded), which means that the clock is not encoded into this portion of the signal..  In layman's terms, the preamble does not contribute to the clocking of the signal.  Only the audio data is BMC coded, so in more layman's terms, the audio data is the only portion that contains clocking information.  For him to suggest that the preamble affects clocking directly is questionable.  However, if he meant that it could affect the receiver and have the potential to cause a glitch, then he's correct in theory.  Most AES/SPDIF chips are now mature devices and it's highly unlikely there is any form of glitch or clocking issues.

In fact, there are only 3 variations of preamble, X,Y and Z as specified by AES3 standard (taken from wikipedia so I don't have to type it all):

X (or M) : 111000102 if previous time slot was 0, 000111012 if it was 1. (Equivalently, 100100112 NRZI encoded.) Marks a word for channel A (left), other than at the start of an audio block.


Y (or W) : 111001002 if previous time slot was 0, 000110112 if it was 1. (Equivalently, 100101102 NRZI encoded.) Marks a word for channel B (right).


Z (or B) : 111010002 if previous time slot was 0, 000101112 if it was 1. (Equivalently, 100111002 NRZI encoded.) Marks a word for channel A (left) at the start of an audio block.

So his claim that preambles vary depending on audio data is making me scratch my head a bit..


Anyway, for some anecdotal evidence, I placed an oscilloscope on the recovered clock of a modern Wolfson digital receiver IC and it was as clean and glitch free for 5+ minutes as you would expect any clock to be.

I suspect that Dan was mainly speaking of older technology on all this.  I mean, I gotta give him the benefit of the doubt.

No money in the software even less in  the hardware! 
Besides most care more about how many songs can it hold, remember when it was about how reel it sounded?

I wish I knew more but my knowledge comes out of conversations I've had with people in a position to know at AES conventions. I'm a nerd who attended technical papers even when they were way over my head because I wanted to understand digital audio as well as I understood analog from hanging out in studio and radio station shops. For example dither was/is an integral part of our digital telephone network that was developed in the 1950s. Today we have people ignorantly claiming that it should be optional while it should just be handled with nobody needing to know or think about it.

Bell labs died with deregulation, My Uncle spent 3 years being trained for the first digital telco switch install, he like others were swept away when Siemans and others entered the market and offered insane money that the U.S. Telcos couldn't match. From the winters in Frozen Rice Lake WI barely making ends meet to Sunny Boca and another digit on your pay check!
The old days of Bell Labs meant you could be working on something for NASA and your own pet project now it's just make it smaller and cheaper!