Test and Measurement: Analyzing computer playback systems

The age of computer-based media storage and playback has brought innumerable conveniences and features to the music-lover. With inexpensive hard drives and configurable media players, your entire music catalog is never more then a few clicks away.

Unfortunately, computer-based playback comes with its own set of unknowns and variables that can be very difficult to analyze and trouble-shoot. Beyond the typical vulnerabilities of computers, problems may exist where you least expect them. For these reasons, Benchmark is providing leading research to determine the limitations of computer-based audio playback systems and the effects of different configurations within these systems. This article discusses the parameters that impact the quality of computer-based audio playback.

When R & D began for the DAC1 USB, Benchmark began investigating the performance of popular computer-based media playback systems. The software and operating systems were objectively tested for performance qualities such as sample-rate and word-length capabilities, volume control performance; bit transparency; bandwidth; and distortion. The most common obstacles for high-quality computer playback are sample-rate conversion, word-length truncation, and various other DSP functions. Results from this research are documented on Benchmark’s ‘Audio Wiki’ (http://www.benchmarkmedia.com/wiki).

Sample-Rate Capabilities

Sample-rate capabilities are a function of the media player, the operating system, and the hardware being used to deliver audio from the computer. Severe distortion may occur if proper considerations are not given to the handling of sample rates. This distortion is a result of poor-quality sample-rate conversion.

The quality of a system’s sample-rate conversion (SRC) services can range from very-high to very-low quality. Very-high quality SRC can be virtually inaudible when up-sampling. When down-sampling, SRC should and usually will apply a low-pass filter at the Nyquist frequency. Therefore, some of the high-frequency information will be lost during a down-sample SRC. Very-low quality SRC will cause significant distortion of the audio. Unfortunately, many computer-based playback systems operate with very-low quality SRC.

Word-Length Capabilities

Word-length considerations for a media player include several factors. It is important to know the maximum possible word length of the connection to the output device. It is also is important to consider how the software will handle longer word lengths. Does the software truncate longer word lengths? Will longer word lengths be dithered to shorter word lengths? Will the entire word length of the digital audio remain unchanged?

Truncating to 16 bits should be avoided when possible as it significantly reduces audio quality. The best media players maintain the entire word length of the digital audio, while other media players truncate 24-bit audio to 16 bits. Certain media players will not even play 24-bit audio files.

The word length of the connection to the audio hardware is equally important. The Benchmark DAC1 USB establishes a full 24-bit USB connection with the computer. Many output devices are limited to 16-bit USB connections to the computer, even if the device has other 24-bit capabilities. Check the manufacturer’s specifications to determine the maximum word length of a device’s computer interface.

Output devices that are limited to 16-bit connections to the computer can often experience distortion due to 16-bit truncation by the playback system. The media player must dither to 16 bits with 16-bit output connections to avoid truncation and maintain low-distortion performance.

Output devices with a 24-bit connection to the computer are much more likely to maintain high-quality performance. In these cases, the media player should ideally implement a 24-bit connection to the output device. Several media players currently available will employ a 24-bit connection to the output device.

Volume-Control Performance

The performance of a digital volume control is directly related to the word-length capabilities of the system. If a volume control truncates to 16 bits, the audio will incur severe distortion. 16-bit volume controls will even degrade 16-bit audio. To maintain high-quality audio playback, 16-bit audio should be played through a 24-bit volume control, followed by a 24-bit data path to a 24-bit output device.

If the volume control dithers its output to 16 bits, truncation distortion will be avoided. However, the noise from the dither may increase the noise heard during playback depending on playback level and the system’s overall signal-to-noise ratio. Dithering to 24 bits is ideal. Truncating to 24 bits is usually acceptable because the resulting artifacts are usually well below the threshold of hearing.

Effects and Other DSP Functions

We do not test non-essential DSP functions, such as ‘sound enhancers’, ‘volume normalizers’, etc. We discourage using these functions, as they generally contradict Benchmark’s goal of truly transparent audio playback.

More Information and Updates

After testing a playback setup, we post all results on Benchmark’s ‘Audio Wiki’ (http://www.benchmarkmedia.com/wiki), which can also be found on the BenchmarkMedia.com website under the ‘Support’ menu. These pages also include recommended setups for particular systems.

It is important to understand that these tests have a ‘shelf-life’. The behavior of computer systems is incredibly dynamic because of the interaction between all software and hardware involved. Furthermore, automatic updates may bring unexpected changes. Without the user’s knowledge, their computer-based audio playback system could have sudden changes in performance!

Each time a media player or operating system is updated, the previous test results and recommended configurations may be obsolete. Do not assume that the newer versions are better then their predecessors, or that any problems with the previous version were corrected. We will try to keep the ‘Audio Wiki’ (http://www.benchmarkmedia.com/wiki) up to date as new versions of software are released.