Wednesday, February 21st, 2018

Apogee Electronics : Clock Distribution Theory and Practice

By editorMarch 31, 2012


Co-founded by Betty Bennett in 1985, Apogee initially made its name designing filters that solved many of the early problems associated with digital audio. Later, Apogee’s complete conversion systems pushed the envelope of digital audio quality and established Apogee as a key innovator in the field of professional digital audio.

Here’s what Apogee’s design team had to say about the importance of clock distribution, both theory and practice, as part of designing hi end audio converters:

Master Clock and Conversion

“To understand the importance of the clock signal during the A-to-D and D-to-A conversion, it’s useful to examine the similarities between film and digital audio, two media which capture reality by recording and reproducing small slices of that reality.

“Much in the same way that a film captures fluid motion with discrete, regularly timed still pictures, digital audio technology captures audio waveforms with regularly timed snapshots called samples. In each case, the timing regularity of the slices is paramount to successfully reproducing the original event.

“For example, just as a hand-cranked projector will cause a perfectly normal film to appear jerky and unnatural, timing inconsistencies (known as jitter) in a digital-to-analog converter add inaccuracies to an otherwise good digital audio signal.  Worse, if a hand-cranked film camera were used to capture a performance, even the most stable projector couldn’t remove the instability of the movement recorded on film. Likewise, jitter anomalies encoded during the analog-to-digital stage can never be removed further down the digital audio chain.  The overall quality of a digital system is determined by the quality of the A-to-D and D-to-A converters; the most stable clock possible ensures the most consistent timing, and therefore the highest quality, for these stages.”

Master Clock and Device Synchronization
“A Master Clock also ensures the synchronization of all devices in a studio, allowing any possible digital connection to be made seamlessly without clicks, pops or digital hash.  To understand the requirements of a clock signal, consider the analogy between the digital studio and a musical ensemble.

“In order for a musical ensemble to play together successfully, each member must perform his part at a common tempo. Likewise, each device in a digital studio must transmit and receive digital bits as dictated by a common clock signal.  The larger the ensemble, the more difficult it is to play together without a conductor – imagine the resulting cacophony if each player in an orchestra followed the tempo of his neighbor.  A similar situation arises in the digital studio when each device receives a clock signal from its neighbor – timing differences between devices can cause digital bits to be dropped when the studio must “play together”, for example while transferring or mixing digitally.  On the other hand, when each member of the orchestra follows the tempo set by a conductor, even a large group of musicians can play precisely in unison.

“Likewise, when each device receives a robust and precise clock signal directly from Big Ben, even a large digital studio can operate in perfect synchronization.”

 Word Clock Termination
“As the frequency of a word clock signal increases, termination requirements become more critical to precise transmission. In order to ensure the integrity of the clock waveform, each connection should be terminated by a 75 ohm load impedance.  Over-termination (a load impedance lower than 75 ohms) will attenuate the signal excessively, while under-termination ((a load impedance greater than 75 ohms) introduces overshoot and other waveform distortions.  Both conditions compromise clock accuracy, and are indicated by Big Ben’s termination sensing LEDs.”“From the preceding information , these four Rules of Clock can be established:

  1. Ensure that all A-to-D and D-to-A converters are clocked by the most precise source possible, to ensure the highest quality for the entire digital audio system.
  2. Each digital device should be connected directly to the Master Clock Source with the shortest possible cable, in a “star” configuration.
  3. Each Word Clock line should be terminated with a 75 ohm load. When the word clock input of connected devices is un-terminated, it’s acceptable to chain a few devices with a BNC “T” connector on the word clock input.
  4. Don’t delay the clock signal to any device by chaining a signal through input/output clock circuitry. Every device should receive the digital “downbeat” simultaneously.”

You can view Apogee’s design team talking about the thinking behind their flagship interface, Symphony I/O here

Thanks to Apogee for their contributions to our converter feature. You can check out the whole Apogee range at www.apogeedigital.com

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