ASUS Xonar D2 Sound Card

While some sound cards may differ more substantially in audible ways from other sound cards, the only way to truly quantify the most subtle differences between high-quality sound cards is to measure and analyze their output. While in a laboratory setting, expensive lab-grade testing equipment may be used, in our experience a far simpler and almost as telling measurement method is the ‘loopback’ test. In loopback testing, the output of the sound card is routed through its ‘line in’ input, and recorded simultaneously with the test signal playback.

A minor disclaimer: The resulting measurements from loopback testing may be affected by other factors, such as the quality of the input ADC circuitry, since the entire input/output signal chain of the card is tested. However, this limitation only limits our ability to provide absolute measured values – the results from loopback testing are still plenty useful for comparing sound cards to each other. The measured values you see here may not be the numbers that the sound card is capable of under ideal conditions as measured by high-dollar testing equipment, so don’t be alarmed if the numbers we give here aren’t the same as the numbers that appear on the box.

For comparison testing, we’re using two other sound cards that we’ve got on hand, the popular 24-bit/192kHz-capable Creative Sound Blaster Audigy4 (113dB SNR) and the recently reviewed Razer Barracuda AC-1, formerly my personal reference sound card. The Razer Barracuda AC-1 is also based on the C-Media CMI8788 OxygenHD chipset and rated for the stratospheric SNR of 117dB. However, the Xonar D2 card uses higher-quality DACs and analog signal components than the Barracuda AC-1. This selection of cards makes for quite an interesting match-up, indeed.

Test System
• AMD Athlon 64 X2 4200+ EE CPU
• 1GB A-Data Vitesta PC2-6400 RAM
• ASUS M2NPV-VM Motherboard (NVidia GeForce 6150)
• Western Digital 160GB SATA 3.0GB/s HDD
• OCZ StealthXStream Power Supply
• Cooler Master iTower 930 Case

RightMark Audio Analyzer 6.0.5

For the audio measurements, we’re using the latest version of RightMark Audio Analyzer, version 6.0.5 (newer than the 5.6 version included with the Xonar D2 bundle). Despite its quirks, RightMark Audio Analyzer is the only game in town when it comes to powerful benchmarking functionality for sound cards. The full battery of tests was performed on all three sound cards, in three audio modes: 16-bit/44.1kHz, 24-bit/96kHz, and 24-bit/192kHz. However, in some cases, the swept-frequency THD+N and IMD+N tests produced some strange results, no matter what we tried. For your identification, we’ve flagged the suspect measurements in red.

RMAA, 16-bit/44.1kHz Mode

RMAA, 24-bit/96kHz Mode

RMAA, 24-bit/192kHz Mode

ALT Test, RMAA, 24-bit/192kHz Mode

Frequency Response
Throughout testing, the ASUS Xonar D2 delivered a smoother frequency response with less overall variance than either of the other two cards. The maximum variance range displayed by the Xonar D2 card was 0.11dB, however at a 192kHz sample rate, the range was only 0.1dB. The Razer Barracuda came in second place here, with a variance of 0.14dB. The Audigy4 card holds its own here, but still comes up last with a maximum variance range of 0.36dB observed at the 44100Hz sample rate. For all three cards involved, these are very ‘flat’ frequency response numbers.

Signal-to-Noise Ratio (SNR)
The greatest signal-to-noise ratio in each of the three tests consistently belonged to the Xonar D2, especially at 24-bit resolutions. The minimum SNR observed for the Xonar card was 96.8dB, though the maximum SNR topped out at 115.9dB. The Razer Barracuda AC-1 came in second place again, maxing out at 106dB SNR in loopback testing. Keep in mind, however, that this testing method tests the entire signal path, including the quality of the input analog-to-digital conversion. Last of all was the Audigy4 card which delivered a top SNR of 90.8dB, a figure that’s possibly also hamstrung by its input stage.

So, what’s the significance of SNR? Lately, it seems to be the specification that gains the most traction in marketing a card. Signal-to-noise ratio describes a ratio of the decibel level of a unity-gain signal over the ‘noise floor’, beneath which audible signals seem to disappear in a mess of hiss and noise. A high SNR is responsible for the relative ‘blackness’ of musical silences. ASUS’ Xonar card claims a high 118dB output signal-to-noise ratio, while its output DACs are capable of a 123dB SNR. Therefore, the limiting factor here may be the AV200 audio chipset itself (CMI8788).

Dynamic Range
This parameter refers to the relative decibel level between the quietest sound the card can reproduce and the loudest sound it can reproduce. Typically, this tends to coincide with the overall SNR of the card, which means that the quietest sound that RMAA could detect was a sound that was just above the noise floor of the card. Naturally, the ASUS Xonar D2 was found to exhibit the greatest dynamic range of the three cards.

Total Harmonic Distortion (THD)
Of the three cards compared in testing, the ASUS Xonar card delivered the lowest level of total harmonic distortion (a mere six ten-thousandths of a percent). For measurement purposes, THD levels below the noise floor cannot be accurately measured, so the two values are commonly composited into another parameter, THD+N.The Razer Barracuda AC-1 again came in second in this test, with a minimum THD of 0.0036% at the 44100kHz sampling rate. The minimum THD value for the Audigy4 card was 0.0059%.

Perhaps even more than SNR, Total Harmonic Distortion is an important parameter for describing the overall ‘musicality’ of the sound card, because it describes the sound card’s ability to deliver a pure tone without added harmonic content. A high THD figure would manifest itself as a sort of ‘hash’, much like the distortion of an electric guitar. Lower total harmonic distortion means that the card’s output is more ‘pure’ and uncolored by distortion products.

Intermodulation Distortion (IMD)
Intermodulation distortion occurs when two waveforms sum constructively and destructively, resulting in an additional third distortion product that contains frequencies that are in neither of the two original signals. Intermodulation distortion also can have a destructive effect on overall clarity. The lowest ‘intermod’ distortion once again was turned in by the Xonar D2 card. In a reversal, the Xonar’s low IMD figure was followed closely by the Audigy4 card, with less than 0.014% IMD+N. Bringing up the rear was the Razer Barracuda card, with a minimum of 0.035% IMD+N.

Stereo Crosstalk
This parameter describes how much of one stereo channel ‘leaks’ into the other stereo channel throughout the signal chain on the sound card. This parameter is given as the overall level of the signal that has leaked into the other channel. These are all multichannel sound cards, but typically the front left and right signals are routed together through the card. One potential way of reducing crosstalk through a stereo signal is to route stereo pairs of signals through separate DACs and preamplifier circuits, but this is a board design thing, and not something we can adjust.

ALT Recording
This test measured the overall benefit of the ALT recording feature of the Xonar D2 card with respect to the traditional loopback cable method, using a short loopback cable. Of particular interest here are the SNR and crosstalk ratings. The use of ALT mode resulted in 0.7dB greater SNR, a small improvement. However, stereo crosstalk in ALT mode was nearly 10dB lower than it was inside the loopback cable. This means that ALT is not only a more convenient solution than a loopback cable – it’s a better-sounding solution as well for preserving your recordings in the most pristine form possible.