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The External Clock Debate – A
Call For Examples
This weeks installment of the audioMIDI.com Pro Tools Corner
is to open up a discussion about using an external clock
with Pro Tools, and to help people better understand what
all the hubbub is surrounding this particular issue.
If you are a regular on one of the many user forums such
as Gearslutz (www.gearslutz.com) or the Digidesign User Conference
(duc.digidesign.com), a thread discussing the merits of using
an external clock with the192 I/O will surface every now-and-then.
The debate seems to be whether or not using a high-quality
external clock can actually improve the recording, playback,
monitoring and final deliverable of the session.
This debate has garnered so much attention, that Digidesign’s
own Gannon Kashiwa prepared a technical white
paper to study
and present his findings on clocking, jitter and the Digidesign
192 I/O Interface,
and true to forum form, deliberations are already underway.
Before one can truly appreciate some of the techno-speak
of this topic, a few basic explanations are in order (of
course, you can also read the entire white paper for a more
detailed look at the technical aspects, it’s not a
difficult read).
When recording an analog signal into a digital system, the
electrical signal is converted into binary data (bits), which
are just 1's and 0's using an analog-to-digital converter
(A/D). The process that an A/D converter uses to capture
an analog signal is called sample-and-hold. Sampling is like
taking a snapshot of the sound wave at regular intervals.
The sampling rate determines how many snapshots, or samples
of the sound wave are taken in one second. The higher the
sampling rate, the more samples taken per second; and the
lower the sampling rate, the less samples taken per second.
A typical sampling rate is 44.1 kHz, which is the sampling
rate used for Audio CDs. This means that 44,100 samples are
taken each second.
A key element to this process is making sure that the samples
you record and play back, are done at a steady pace over
regular intervals without variations in speed. The sample
clock is what is used to provide that reference. If there
are any variations in the interval between samples, called
jitter, then the integrity of the signal maybe compromised
which can lead to distortion and noise, which is problematic
at the conversion stage. Although no sample clock can be “perfect” (meaning
absolutely zero jitter), it is important to understand that
using a reliable, high-quality clock will make for the best
analog-to-digital conversion.
All digital audio systems employ sample clocks for capturing
and playing back audio, and the 192 I/O is no different.
If you have only a single 192 I/O connected to your Pro Tools
HD system, then you are using the internal clock of the 192
I/O for recording and playback. However, you are able to
connect an external clock, such as the Big Ben (http://www.audiomidi.com/Big-Ben-P3918.aspx)
by Apogee (http://www.audiomidi.com/manufacturer.cfm?mid=180),
to the Word Clock input on the 192 I/O and set the clock
source in Pro Tools to the external reference. Does this
improve audio quality? That’s what is up for discussion.
Regardless of whether the Master reference is derived from
an internal or external sample clock, it is passed onto a
frequency synthesizer in the 192 I/O that has a very low
jitter VCO (Voltage Controlled Oscillator) and multiple clock
dividers, which is to say that the sample clock is not simply
passed through the 192 I/O, but used to produce a stable
sampling clock. As a result, and for various other regions
(many of which are illustrated using FFT plots) Gannon finds
that for, “Pro Tools systems with a single interface
that are not required to lock to external sources such as
a timecode or video reference have the lowest amount of jitter
when it is set to internal.”
However, this is where it really gets fun, as Gannon goes
on to conclude:
It’s clear from these examples and from discussions
with audio engineers from around the world that it is difficult
if not impossible to come to a single conclusion when subjectively
identifying the audible effects of jitter. On the bench,
the 192 I/O compares favorably with all of its similarly
priced competitors producing low jitter across the spectrum.
In fact, it has far less jitter than many dedicated clocks
and yet many prefer the sound of these clocks over the 192’s
internal clock.
Controlled listening tests involving a cross section of participants
using established standards need to be done in order to more
fully understand the issue. Marketing materials from many
manufacturers use anecdotal evidence and testimony from highly
regarded individuals to promote their products and while
that is powerful product endorsement, it needs to be recognized
as such. Subjective reactions and uncontrolled listening
tests should not be used as a substitute for science and
it’s clear that more study needs to be done to fully
understand the audible effects of jitter on digital audio.
Sitting down with an Apogee for the last couple of weeks,
I wanted to dispel some of the myths I have personally heard
(or read) floating around, and turn to the readers of this
column to help provide examples that we can all listen to
and learn from.
I would first like to start by saying that Apogee makes some
of the best high-end hardware in the industry – pristine
pre-amps and transparent converters. Big Ben is no exception
to this, it is a rock-solid, well thought out synchronization
unit that is a welcomed addition to my studio. I would recommend
it in a heartbeat for those looking to synchronize multiple
clocks; such as when needing to synchronize your Pro Tools
rig to other audio and video gear.
The Bounce Myth
Just connecting an external clock to Pro Tools and changing
the clock source to external will automatically improve the
quality of your bounce to disk or internal layback.
I wish I could tell you that I found this to be true, but
sadly I did not. However, this is not something that I have
found manufacturers of external clocks claiming, making it
more folklore than corporate messaging. To test this I simply
did a Bounce to Disk and an Internal Layback once using Big
Ben and once using the internal clock of the 192 I/O. In
all cases, the bounce cancelled – which means there
is no difference; good or bad.
In order for an external clock to improve a recorded (converted)
signal, it would have to go through the conversion process
again using the external clock as the clock source, otherwise
the external clock is only affecting playback (monitoring).
However, if you are bouncing through an external summing
device, and back into Pro Tools, then yes, an external clock
may make a difference that you prefer.
The Call For Examples
As Gannon points out, there are people who feel that in
their own listening experience that using an external clock
improves
monitoring and/or recording quality. Rather than restate
the arguments made on either side, I would like to ask
readers who have examples comparing the two configurations
to share
their audio files so that we can post them in a follow-up
article that everyone can learn from. I look forward to
hearing from you, and sharing the collective experience.
Pro Tools Tidbits
If you have a Pro Tools related question that you would
like me to take a shot at answering or have any feedback,
please send it to scott@audioMIDI.com. I will do my best
to select the questions that seem to stump the most people.
As always, stay tuned to the audioMIDI.com eNews for the latest reviews and new product info.
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