Choosing the right tools to bring analog audio to the digital realm�and vice versa�can make a major difference in the quality of your recording.
By Rich Tozzoli
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Top facilities like Airshow Mastering rely on quality analog-to-digital converters for accurate sound.! |
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Crane Song's Hedd 192 offers sound-shaping tools along with high-end analog-to-digital conversion at up to 24-bit/192kHz resolution.! |
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What The Pros Use |
Ask a handful of people in the music business what they use for A/D/A conversion and you're sure to get a handful of different answers. GRAMMY-Award winning producer/engineer/ mixer John Holbrook (Natalie Merchant, Brian Setzer, B.B. King) has several different converters in his arsenal. "For multichannel I/O, I have Lynx Aurora 16s," he says. "Respect-able sound... and a great value for money. I haven't personally A/B'd them with Digidesign I/Os, but word on the street seems good and they sound great to me."
For stereo mastering A/D and D/A, Holbrook currently uses a HEDD192 by Cranesong. "Excellent sound quality and nice options with the process control," he says. "Of course with a lot of the 'overloud' music coming down the pike these days we could probably get by with 8-bit converters and just use the top 4 bits! How much more loud could this be? The answer is none. None more loud."
Engineer/producer Paul Orofino (Blue Oyster Cult, Leslie West, Golden Earring) notes that he chooses converters based upon the needs of each project. His list includes a Universal Audio 2192 AD/DA, Crane Song Hedd, Genex 9048 16 channel AD/ DA PCM and DSD option, three Benchmark DAC-1s, a Manley Labs Slamm with AD/DA stereo converter option, four Radar IIs with Classic converters and a Radar V with super Nyquest conversion.
"All things being equal, each one of these products imparts its signature sound to whatever you pass through them," Orofino explains. "As a rule, I do change to whatever unit suits the music I'm tracking or mixing at that time. Simply put, these to me are as important as the console, preamps or the mics you choose at the start of a session. I, like so many, originally thought a 'converter was a converter.' I would strongly advise trying a few quality units before mixing, as they can make such a difference in width, imaging, and depth, as well as the overall sound. The difference can be astounding, especially with the high-end converters available to us today. Renting a two-channel unit for mixing is relatively affordable and can make a significant change in the quality of the final presentation of a song."
Speaking of final presentations, the last person to touch your mixes is usually a mastering engineer. "My main converter is a Pacific Microsonics Model 2 because it sounds the best" says Dave Glasser, the multi-GRAMMY award-winning engineer from Airshow Mastering in Boulder, Colo. "I've compared it to Prism, EMM, DCS and it's deeper, wider and more resolved�both with the D/A and A/D sections. It also has a very good clock section which I use as my clock master, and it does 192k which is needed every now and then. I also use a Prism ADA-8XR� It's very flexible� Mixes that sound good on this seem to translate well."
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There's an old computer saying that applies equally well to recording: "Garbage in; garbage out." With digital recording, there are many factors that can turn an otherwise good sound into trash. Good mics, great preamps, and quality cables may be obvious areas of concern, but the quality provided by these tools will be diminished if the gear getting the sound to your digital recorder�and from your recorder back out to your monitor speakers�isn't up to the task.
Picking the right analog-to-digital and digital-to-analog converters has always been a formidable task for those in the business of music production. As with microphones, speakers and headphones, there are countless brands to choose from and personal taste will weigh heavily in the final selection. But before we move forward and examine some products, lets first step back and take a basic look at what converters are�and how they might help improve your sound.
Think about how many products you currently use that feature analog-to-digital (often referred to as A/D or ADC) and especially digital-to-analog (D/A, DAC) conversion. Your laptop plays music through one. Your cell phone speaker plays your voice through one. Your CD player, DVD player, Blu-Ray player and iPod all have D/A's. The list goes on and on.
What They Do
Typically, an A/D converter is an IC (integrated circuit) that converts an analog signal to discrete digital numbers, which are then used for processing.
An analog signal is continuous, but the digitizing process turns it into a set of numbers that are like little super-quick snapshots, called samples The converter's sampling rate (or frequency) determines how many individual samples are created per second of analog audio. The higher the sample rate, the more samples, and therefore, all things being equal, the more accurate the sound.
Conversion is not a perfect science, however, and various errors can compromise the sound. Quantization error is a common one. This imperfection is happens a signal's quantized digital value differs from actual analog value. Aliasing is a problem that occurs when the input signal changes faster than the sample rate, and it can showup a the output of the DAC as distortion. In order to avoid aliasing, the input to the ADCmust be processed with a lowpass filter is called an anti-aliasing filter.
A/D converters also often use dither, which introduces a small amount of random white noise to the input signal before it's converted. This slight increase in noise prevents the filters from cutting off the signal in a harsh way. Non-linearity and aperture error are other error types involved in conversion. Combined, they can make a signal sound noisy, grainy or harsh.
We've come a long way since the early days of DAT machines, TASCAM DA-88s and Alesis ADATs, when 16-bit converters first appeared on a mass scale. Many thought the sound of these products was "harsh, brittle and nasty," but they were the first generation of digital recording products to become affordable to the everyday music maker. As time progresses, so does the quality of the A/D and D/A process. Companies such as Analog Devices, Cirrus Logic, Fairchild Semiconductor and National Semiconductor make the actual chips that do the conversion in our gear.
Hi Resolution
Both ADCs and DACs can work at sample rates of up to 192kHz. While the standard Red Book CD is still a paltry 16-bit/44.1 kHz and TV/broadcast still uses a 48kHz signal, higher resolution conversion�especially 88.2kHz and 96kHz, rates that are available on a wide range of today's gear�can provide a cleaner sound with more headroom. Some studios and producers even prefer to record at up to 192kHz or with the Direct Stream Digital (DSD) format at even higher resolutions. What audible difference does the higher sample rate make? For one thing, the filters don't have to work as hard removing distortion.
In today's working studio, the process of taking your analog signal, converting it to digital, processing and mixing it, then converting it back from digital to analog is more important than ever. Many engineers and producers choose to use outboard converters (instead of the stock ones built into audio interfaces and mixers). this can be both for the gear's inherent sound�many converters have the kind of sonic personality one might associate with an EQ, preamp or mic�and also because of improved master clocking, which many claim delivers a "tighter" sounding signal by reducing A/D jitter (or phase noise). Fundamentally, jitter causes converters to sample at the "wrong" place, reducing accuracy and compromising sound quality. Fortunately, today;s market offers an extensive range of ourboard converters that can range in price from a few hundred to several thousand dollars.
These devices are designed to plug into an existing interface's digital ports. And like mics, preamps, and effects, there's no right answer for everyone.
Apogee Electronics has a full line of products available. From their $495 portable stereo Duet to their extensive Symphony System, users can customize a setup to fit their budget. While the Duet features stereo mic preamps and a FireWire interface, the Symphony can go up to 96 Channels using any combination of their XSeries (AD-16X/DA-16X) or Rosetta Series converters. Unlike smaller systems, Symphony connection takes place using a PCI card, which slips directly into a desktop computer for easy I/O routing.
Benchmark Media also makes a series of converters, ranging from two to four channels. Their popular Benchmark DAC1 PRE is a 24-bit/192kHz stereo pre-amplifier, headphone amplfier and computer audio playback device that has six stereo inputs and four stereo analog outputs. Running just over $1500, it's right at home in a personal production studio operating around a computer DAW.
RME makes a line of products ranging from the $799 2-channel AD/DA ADI-2 (which converts to/from SPDIF, ADAT and AES) to their top-of-the line 192 kHz 8-channel ADI 192 DD. The 192 DD is a format/sample rate converter and supports up to 192 kHz with ADAT, AES/EBU and TDIF (TASCAM) connections.
Universal Audio's 2192 is a two-channel AD/DA converter with 24-bit/192kHz resolution and AES, SPDIF and ADAT I/O. It has a Class A signal path with no capacitors and lists for around $2999.
The $3600 Crane Song HEDD 192 is a bit different in that it features triode, pentode and analog tape compression sounds.
Solid State Logic's Xlogic Alpha-Link AX AD/DA converter cost about $2600, and boasts 24 Balanced Analog Inputs, Lightpipe (ADAT) digital connectivity, Word Clock I/O, MIDI I/O and even analog metering. Lynx's Aurora 16 is a 16-channel 192 kHz capable A/D D/A converter costing around $3000. Aphex's 141 is an eight-channel ADAT-to-analog converter with and ADAT output and a $399 street price.
The choices over which converters to use are almost endless. It can certainly be argued that the quality outboard units on the market today can deliver clarity and detail beyond even a relatively high-end "stock" auio interface.
While specs alone can't really tell you how good or bad the music going to sound, the objective of using a great A/D-D/A system is to get an accurate reference for your audio. Some pieces of recording gear will use inexpensive converters in order to keep their costs down. Take that into consideration when making your next purchase of anything (which is almost everything!) involving A/D and/or D/A conversion. One good outboard unit can improve the sound of an inexpensive interface, mixer or effects unit, but the only way to really know what's right for you is tp use your ears.
