Recording - March 2016 - 61

I talked last month about how accurate conversion could be spoiled by irregular sampling times, or jitter. Converters invariably include internal clocks to control when the
sampling takes place; some converters can also be synchronized to external clocks, via
specialized clock inputs (sometimes called word clock). Boxes designed specifically to
clock digital audio devices include the Antelope Isochrone 10M and the Apogee Big
Ben; most external clocks provide multiple outputs to feed and synchronize multiple digital devices in a complex studio. Most audio engineers who have done the comparison
will tell you that converters fed by an external clock can sound considerably clearer
than converters operating on their own internal clocks (free-running). If you want a more
detailed discussion, Google "digital audio clock" and read the results.
Converters used in audio recording fall roughly into three categories: Internal
conversion systems, external conversion systems, and interfaces that incorporate
mic and instrument preamps, with some hybrids that make the lines fuzzy. There
are also a few formats that aren't widely used anymore, like digital tape decks
and standalone digital hard disk recorders; these almost invariably have A/D and
D/A converters built in. While there are still loyal users of desktop systems out
there (and companies that still make them, like TASCAM), in most home and pro
studios these formats have mostly been replaced by computer-based recording systems, and the converters for those fall into the three categories I mentioned.
Fancy on the inside: sound cards, etc.
Most computers have some audio recording and playback circuitry mounted
on the motherboard, and most of the time it's good enough for basic gaming,
watching movies, and such, but the audio quality isn't really up to the standards
of serious audio recording. A good rule of thumb: if the input and output are
minijacks, and it's not an Apple device or a purpose-built portable recorder, the
built-in audio will probably be run-of-the-mill-mediocre.
Internal pro audio cards seem to be a fading breed; newer external-converter
formats seem to be supplanting them. However, there were and are professionalquality sound cards made for use inside desktop computers, and they can be
truly excellent. I have a Digital Audio Labs CardDeluxe that's lived in three successive computers, and it's cranked out fine-quality audio in all of them. (This
card is now out of production, but shows up regularly on Ebay, which is where I
got mine.) M-Audio's Delta 44 and Delta 66 cards are also out of production but
common on Ebay, and provide more-than decent audio. Current offerings, however, provide world-class audio; Lynx makes a couple of excellent in-the-computer
soundcards, and other companies like ESI, MOTU, and RME make them as well.
Some of these cards can be cascaded to form multichannel input/output units.
These cards plug into PCI or PCIe slots inside the PC; balanced line-level inputs
and outputs are via 1/4" Tip-Ring-Sleeve (TRS) jacks, or multipin connectors that
attach to XLR-input snakes or breakout boxes. The inside of a computer seems like
a dubious place to do high-quality audio-it's aswarm with electrical noise-but
these manufacturers seem to have pulled off minor miracles, and their converters
are clean and quiet despite the storms raging around them.
Fancy on the outside: external converters
External converters do much the same thing as internal sound cards: they take
a line-level balanced signal (the professional standard, with nominal level = +4
dBu) and convert it to numbers. Often the same box will include digital-to-analog
converters, so it can handle both ends of the digital audio process. Some of the
manufacturers that make external converters include Antelope, Apogee,
Benchmark, Burl Audio, Focusrite, MOTU, Lynx, Prism, and RME. These converter
boxes include internal clocks, or they can be synchronized to a dedicated clock
(more in a moment). They don't usually include other functions, though some will
pipe MIDI signals into and out of the computer.
Since they're outside the computer, external converters need to send the numbers they generate to the computer as a digital bitstream. There are several standards for digital communication, including various flavors of USB, FireWire and
the recent Thunderbolt; these can carry many varieties of data, including audio.
There's also a standard created by the Audio Engineering Society and the
European Broadcasting Union (AES/EBU) strictly for audio data; it carries a twochannel digital bitstream via a standard XLR plug through what looks like a regular
microphone cable, but isn't-AES digital audio cables and their connectors need to
meet stringent standards for impedance. An older but still common system uses the
multichannel ADAT Lightpipe standard, which carries up to 8 channels over a cable
made from optical fiber. ADAT refers to the way data is encoded for transmission,
and comes from the Alesis ADAT digital tape deck popular in the 1990s; the physical format for this fiber-optic audio is called Toslink, with the "Tos" from Japanese
developer Toshiba. The consumer S/PDIF (Sony/Philips Digital InterFace) standard

What An A/D Converter REALLY Does
In last month's article on how analog signals are sampled and quantized to make
numbers, I kind of implied that the encoding
was a direct conversion process, and that first
the analog signals were filtered to remove frequency components higher than the Nyquist
frequency, which might cause aliasing.
That's how it was done in the early days
of digital audio, but the results weren't
always good. The A/D converters of the
time were highly nonlinear, which meant
in practice that the numbers they produced
weren't accurate measurements of the analog signal they were sampling, particularly
at low levels. This created audible distortion of a particularly nasty sort. Also, the
anti-aliasing filters needed to be extremely
steep, which created serious phase problems at high frequencies.
Instead, later generations of A/D converters have used an indirect strategy. (Degreed
engineers, please pardon the drastic oversimplification to follow.) In modern converters, the analog signal is sampled in a specialized way at an extremely high rate
(some 10 to 25 million samples per second).
Basically the circuit is asking the question "Is
this sample point in the signal higher than
the last sample point, or lower?" Note that
this is still a "yes-or-no" question, and the
converter puts out a binary data stream that
is essentially 1-bit, but very fast. This scheme
of encoding analog signals into numbers is
called delta modulation, and because the
sampling rate is very high, the anti-aliasing
filter can be set at a high frequency, high
enough to not mangle audio signals.
That very fast 1-bit data stream gets fed
into a calculating circuit which converts it into
a PCM-encoded signal at whatever effective
sampling rate you chose; 10 megasamples/
sec. at 1 bit will be recalculated into a PCM
data stream of, say, 44,100 samples per
second at 24 bits. To do that, the converter
still has to apply an anti-aliasing filter just
above 20 kHz, but this can be a digital filter
rather than analog, and it can be configured
to be non-mangling. The process of calculating a multi-bit PCM signal from a 1-bit is
called decimation.
What if you left the digital information as
a 1-bit delta-modulated data stream and
recorded that, avoiding one step of messing with the signal? Could you do that? In
fact you could: in hyper-simplified form,
that's what's going on in a Direct Stream
Digital (DSD) recording system. You can
actually buy DSD systems (from manufacturers including TASCAM, Merging
Technologies, and Korg), but they haven't
made as big a dent in the recording field
as their developers hoped, probably
because editing and signal manipulation
are harder in DSD, and PCM already had
a big footprint in the studio market. I've
heard some DSD systems, though, and the
sound coming from the playback was
always very impressive.
RECORDING March 2016

61



Table of Contents for the Digital Edition of Recording - March 2016

Contents
Recording - March 2016 - Cover1
Recording - March 2016 - Cover2
Recording - March 2016 - 1
Recording - March 2016 - 2
Recording - March 2016 - 3
Recording - March 2016 - 4
Recording - March 2016 - 5
Recording - March 2016 - Contents
Recording - March 2016 - 7
Recording - March 2016 - 8
Recording - March 2016 - 9
Recording - March 2016 - 10
Recording - March 2016 - 11
Recording - March 2016 - 12
Recording - March 2016 - 13
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Recording - March 2016 - 15
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Recording - March 2016 - Cover3
Recording - March 2016 - Cover4
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