jitter

BTW, recording studios use Master Clocks to synchronize multiple ADCs or DACs together. Not to reduce jitter.

As I understand it, while external clocks are excellent the distance due to the cable lengths actually can increase jitter vs. an internal clock of the best DACs today.  The internal femtoclocks used today sit right next to the DAC so don't have the same distance issues.  This is similar to the need to keep RAM right next to the CPUs.

It’s quite possible that an external clock can merely change or worsen jitter performance.

I think it's generally agreed that modern A/D converters sound significantly better than the earliest ones. Whether this is due to jitter is open to debate but something changed for the better. I have a few early digital recordings that sound atrocious and it isn't just the EQ. They are shining examples of poor digital transfer. It doesn't matter what kind of DAC I play them through - they still sound gritty and brittle. 

This phenomenon was one of the reasons that the remastering business took off fairly soon after the CD was released. One example is HDCD releases. Pacific Microsonics made the A/D converters that allowed HDCD encoding as an option. These converters were considered a substantial leap forward and were felt my many studios to be the best available at the time (they are still used by some studios). Some HDCD collectors, myself included, feel that one of the main reasons HDCD remastered discs sound better than their original releases is that the converters were so much better. The HDCD encoding is icing on the cake.

@erik_squires 

Your statement on master clocks is somewhat contestable: interestingly leading dac manufacturers like DCS, MSB or Esoteric offer master clocks for their top of the line stacks. See my comment on the cost/benefit aspects of built-in vs. external clocks.

@antigrunge2 Yes they do. And some have published papers explaining the difference in error between a built-in clock and an external unit is different. You may get a better clock in an external unit, but by the time it’s made it inside the DAC the delta in performance may vanish.

Mytek, for example, recommends an external clock only when needed in a studio setting, but still offer the connection.

My personal feeling is, you should listen for yourself and determine if it’s even different or not and whether that difference is actually better for your tastes.

Further, in the last 20 years the performance of internal clocks has improved so much I can’t justify an external clock anymore.  Redbook performance (44.1kHz/16 bit) has gotten so good that if a DAC sounds significantly better with high rez music I blame the DAC and not the bit rate.

That Esoteric and other offer external clocks is not, IMHO, a sign of superiority of the idea. One could argue that they put an inferior clock in to begin with and then make you pay for the difference you’ll undoubtedly hear.

I have heard dcs Vivaldi with and without masterclock and the difference isn‘t small.  Similarly adding Cybershaft or Antelope clocks to a number of top end dacs (where pos) yields significant improvements. The effect is particularly pronounced in soundstaging and separation of instruments in terms of attack and decay. Sigital is all about timing and the Ed Meitner argument on cable loosses isn‘t born out in my experience.