Can someone explain the clock sync?

Hey OP,

The idea behind a master clock comes from the pro world where you might have many digital sources that had to be kept in synchronization. Especially true for movie production. You can’t have skew.

The only time skew is really an issue for home AFAIK is in a situation where you have an external CD player and separate DAC. A master clock will ensure the CD player gives the DAC bits just in time.

The thing audiophiles and vendors try to do by spending more money is reducing jitter figures. That is, ensuring that the width of each step in the signal remains exactly the same. Not something external clocks were meant to do, but they have some possible uses.

The lowest phase error is now going to be by putting a very good (femto-) clock right next to the DAC. The longer the distance between the clock and the DAC the more error you can have, so having a clock and cable plus internal distance travel means the internal clock MAY outperform any external clock. Benchmark has a good white paper on this and Mytek also wrote about this.

Probably the best idea for home would be to use the DAC’s clock as the master, have a buffer, and sync external sources to it. Asynch USB DACs do this, but some also have clock out capability, allowing the DAC clock to control an external CD spinner. Combination DAC/Streamers do a similar thing but usually have up to several seconds of buffering. Roon does something like this in their server.

Before Async USB DAC’s attempted to fix these issues by guessing the incoming clock rate and buffering the input. They still do this when needed such as for S/PDIF inputs.

Also, lastly, in the last 10 years or so chip clocks have gotten SO much better than they were before. Unless your DAC is old the internal clock is probably going to be the best reference.

Thank you @kairosman ​@audphile1 ​​​​@erik_squires for your responses. 

The sync clock for digital isn't the same as the syncing to movie's 29 frames per second or TV's frame rates together such that the sound and visuals are synced. That is called SMPTE timecode. When digital audio came to being in the mid 70's and one of the big issues with any digital signals is a when you try to mix two or more sources together - the summing together of digital signals is how to sync them together such that you don't get drop outs or aliasing issues. So a word clock was developed so that every pro digital equipment could follow one word clock. The easiest way to bypass that is to place it back to analog and re-sample the analog signal back to the digital domain. But if you want to keep it in the digital domain then work clock is the most preferred way. There are quite a few standards of digital audio - S/PDIF, AES/EBU, MADI ( subset of midi) ADAT and TDIF. They all use word clocks to sync thing together, but there must be only one word clock. Then there are units that will sync together digital signals from machines that don't have the capabilities to sync (or adjust their internal clocks) such that the signal will get reprocessed to sync into the main stream digital audio. 

Home audio isn't as complicated, and for the most part isn't really needed. Your cd collection once placed into your PC will follow your PC word clock, or if any of your equipment has a word clock, it will convert it's clock over. Cd players that have word clocks are mostly masters. It gets quite expensive to have adjustable clocks on such devices - you need a fairly large buffer to hold the data before syncing. In the studio these days most of the work flow are on PCs, so you just download the data into the PC and away you go. 

If you have a good CD collection, it's just better to download the data into a dedicated PC and play from there. Far better that any CD player on the market that I know of. 

The sync clock for digital isn't the same as the syncing to movie's 29 frames per second or TV's frame rates together such that the sound and visuals are synced. That is called SMPTE timecode.

PS - Never meant to imply it was the same thing.  Just that when mastering a multichannel movie you may have even more devices to keep in sync than stereo mixdowns.

@kairosman  I missed your question about the Terminator 15th being a rebadged Terminator Plus. It seems to be the case from all that I can gather-it is the same other than the faceplate. It is also my understanding that the 15th engineer team did some programming enhancements through the FPGA, but I am not positive. It is surely about the Terminator II level which is now supposedly the new Venus 15th.