by Roger Nichols
So, you're interested in 20 bit analog to digital converters. Eh? Well,
you've come to the right place. Today we are going to talk about the Symetrix
620 20 bit A/D converter.
At first glance, you will notice that the 620 front panel has more than
its share of controls for an A/D converter. Starting from the left of the
front panel, you will find a pair of input level controls and a switch for
selecting a fixed input level. This feature is nice for times when you want
to temporarily alter levels without disturbing your preset record levels.
Next we find a surprise, a switch labeled "Input Select" that
switches between analog and digital inputs. It turns out that the 620 is
more than just an analog to digital converter. The next selector is for
sample rate. The choices are 48kHz, 44.1kHz, 32kHz and the never before
seen 22.05kHz, which we will discuss later. Output word size is selectable
with the next control. Choices here are 20 bit (20), dithered 16 bit (D16),
noise shaped 16 bit (NS16), dithered 8 bit (D8), and noise shaped 8 bit
(NS8). Finally we find an output mute button and the left and right channel
headroom meters.
The rear of the 620 is, of course, populated with the input and output connections.
Analog inputs can accommodate either balanced 1/4 inch jacks or XLRs. The
digital input consists of two connectors, an XLR for AES format and an RCA
jack for S/PDIF. There is a push button switch to select the desired input.
The digital output also allows for S/PDIF or AES connections with a switch.
In this case, both outputs are always hot. The switch selects the output
impedance and digital data format that goes to both connectors. The only
thing left on the rear panel is the AC connection via standard IEC electrical
cord.
The 620 has three uses. First, as an external 16 bit converter for 16 bit
recorders such as DAT machines, modular eight track digital machines, and
hard disk recording and editing systems. Second, 20 bit mode allows high
resolution conversion for systems that allow for 20 bit data storage, or
as a 20 bit feed digitally into high resolution DSP processor. Third, the
digital input allows you to use the 620's DSP for word size conversion and
sample rate conversion (44.1kHz to 22.05kHz) for multimedia audio uses.
Since the 620 is primarily an A/D converter, I thought that the first thing
to do would be to listen to how it sounds. First, I listened to the 620
in 20 bit mode. I used my Apogee 20 bit D/A converters for the conversion
back to analog. I have not yet mastered the art of listening directly to
the digital audio bit stream. The tonal quality of the Symetrix 620 was
superb. I listened to low level audio and then goosed (very technical term)
up the gain in the digital domain so I could hear the way in which the 620
quantizes low level signals. The analog to digital conversion was very smooth.
With state of the art Delta/Sigma conversion and a Total Harmonic Distortion+
Noise figure of -104dB, you would expect nothing less.
In 16 bit mode, you can hear the increase in granulation of the low level
signals, but the 620 continued to perform well even when operating with
crutches. It is interesting to change modes between 16 dither and 16 noise
shape. It depends on the content of the music as to which one is the best
to use. If you want the least distortion but can accept a higher noise floor,
then dithering is the way to go. If the noise floor is the most important
parameter, then noise shaping wins out.
Then there is 8 bit mode. Imagine how much a CD would store if all of the
audio was 8 bits. And they would store even more if the sample rate was
22.05kHz. The reason for the 8 bit mode is that it lets you hear what audio
is going to sound like when you hear it on a CD-ROM. Finally you can actually
hear the results when you mix, and then compensate for the lack of fidelity.
In the "good old days" a producer would make a separate mix for
the single to compensate for the reduced quality over the radio. Now you
can hear how bad your mix will sound, and you can turn up the solo, or the
vocal, or whatever you need to turn up to get your message across.
As far as the 22.05kHz sample rate goes, I have not been able to find a
hard disk system that will slow down far enough to allow me to directly
input the 22.05kHz signal. The Apogee converters would not track that low
either, so I can't tell you what the 8 bit 22.05kHz sounds like. But, You
know what? I don't care what it sounds like. If I ever mix to 8 bit 22.05kHz,
please have me shot.
In summary, the Symetrix 620 A/D converters performs well, and supplies
useful features that you won't be able to find elsewhere. In the "Bang
For The Buck" ratings I would give the 620 a very good rating. If you
don't have a 20 bit converter yet, then this one would be a good place to
start. If you already have a 20 bit converter, the added word size and noise
shaping features of the 620 may make it worth the purchase anyway.