Is USB a Suitable Interface for High-Quality Audio?

Absolutely … if:
1. Jitter is adequately attenuated
2. The interface supports 24-bit audio
3. The interface is “frequency agile” so that it can respond to sample-rate change requests originating from the computer
4. The interface supports high sample-rates

It has been said that USB is “inadequate for audiophile use,” or “not suitable for high-resolution audio.”  At Benchmark we strongly disagree with these statements.  USB can be a highly transparent interface, achieving exactly the same performance as S/PDIF.  Audiophile performance can be achieved when all four of the above requirements are met.

Benchmark’s USB-equipped DAC1 converters meet the four requirements listed above.
1. The Benchmark UltraLock™ system eliminates jitter-induced distortion and noise.
2. The Benchmark USB audio interface eliminates truncation by providing a 24-bit data path.
3. The Benchmark USB audio system is always slaved to the computer clock and can instantly respond to sample rate change requests.
4. The Benchmark Advanced-USB™ system supports sample rates up to 96 kHz without special drivers.

Why these four requirements are important:
1. Jitter:  USB interfaces have high jitter levels.  The specifications for USB 2.0 actually require that jitter be added to the interface clock to reduce radio interference.  This high-jitter interface-clock is not suitable for driving an audio D/A converter sampling circuit.  There are three common solutions to the jitter problem that are equally capable of reducing jitter to levels that are well below audibility:
   
   1. A low-jitter clock can be recovered from the high-jitter clock using a well-designed “phase locked loop” (PLL).
   2. A low-jitter conversion clock can be isolated from the high-jitter interface clock using certain “asynchronous sample rate conversion” (ASRC) devices.
   3. Clock transmission can be reversed so that the low-jitter conversion clock also supplies the clock to the USB system.
2. 24-bits:  Audio data passes through several cascaded digital volume controls inside the computer.  If any digital volume control is not set to exactly unity gain, the word-length of the audio will increase.  16-bit audio output hardware will cause truncation whenever a computer-based volume control or DSP process is active.  Sometimes it is impossible to set these controls to a setting that will achieve unity gain.  A slight digital gain change is a sonically transparent process if enough of the output bits are preserved.  Truncation at the 16^th bit produces distortion at -96 dB FS, a level that is high-enough to be audible in many playback systems.  In contrast, truncation at the 24^th bit produces inaudible distortion at an insignificant -144 db FS.  24-bit hardware is necessary to achieve consistent performance - even when playing 16-bit files.
3. Frequency agility: Some computer operating systems will apply low-quality sample-rate-conversion if the audio playback hardware is unable to respond to sample rate change requests issued by the operating system.  This low-quality OS-based SRC is only avoided when the audio hardware is designed to respond to sample rate commands issued by the computer’s operating system. Frequency-agile audio hardware is required when playing a music library that contains a mixture of sample rates.  A typical library may contain CD rips (44.1 kHz), video (48 kHz), and high-resolution downloads (88.2 kHz or 96 kHz).  Audio playback hardware should be capable of quietly changing sample rates within a few milliseconds.
4. High sample-rates:  High-resolution (88.2 kHz and 96 kHz) downloads are becoming more common.  If the playback hardware cannot operate at these sample rates, low-quality operating-system based SRC will degrade the playback quality.  When this occurs, the “high-resolution” files will sound worse than the “low-resolution” files. If custom audio drivers are required to support high sample rates, these drivers may eliminate the ability to respond to sample-rate requests.  Custom drivers also add CPU overhead, and tend to reduce the reliability of any operating system.