By John Siau
April 4, 2014
High-Resolution Audio systems offer the promise of an extended high-frequency range. High-Resolution digital systems now operate at 2 to 4 times the sample rate of the standard CD. This means that these systems have the capability of extending the playback frequency range well above the 22 kHz limit of the standard CD. Does this added high-frequency range improve our listening experience? How high is high enough? Do we really need anything over 20 kHz?
In the previous post I stated that "High-Resolution Audio will only truly arrive when the sum total of all defects in the audio chain become inaudible". In that post I discussed the need for low noise components. In this post I will look at frequency response. How much bandwidth do we need to satisfy the 20 Hz to 20 kHz limitation of the best human ears? Is a 20 kHz bandwidth good enough for High-Resolution Audio? If not, why not?
Let's assume we have a playback system consisting of a CD player, a preamplifier, a power amplifier and speakers. If each of these has a 20 Hz to 20 kHz frequency response, is this sufficient to reproduce all of the frequencies we can hear? The short answer is no. Here is why:
The frequency response of a system can be determined by summing the frequency response of each component in the audio chain. If two components are - 3 dB at 20 kHz, then together they are - 6 dB at 20 kHz. If we look at our example system, we have four components in the chain: CD player, preamplifier, power amplifier, and speakers. If all are -3 dB at 20 kHz, then we have a system response that is -12 dB at 20 kHz. Worse yet, this system will typically measure -4 dB at 10 kHz. This system will not come close to meeting the performance of our ears! Each individual component was well matched to the limitation of our ears, but as a system, these components cannot achieve anything resembling High-Resolution Audio.
If we want to accurately reproduce 20 kHz audio, the frequency response of each component must extend well beyond 20 kHz. For this reason, Benchmark products are engineered to achieve a 200 kHz to 500 kHz bandwidth. Is this excessive and unnecessary? To answer this, let's replace each of the four components in our sample system with components that have a 200 kHz bandwidth. The combined system now measures - 4 dB at 100 kHz, - 0.8 dB at 50 kHz, and about - 0.2 dB at 20 kHz. This simple 4-component signal chain achieves a 100 kHz bandwidth and is well-matched to the 96 kHz bandwidth of a 192 kHz digital sample rate. We could legitimately argue that the region between 20 kHz and 100 kHz may offer little musical content, and even if it does, we may not be able to detect its presence. The real benefit is that we have preserved the entire 20 Hz to 20 kHz bandwidth after passing through four audio components in a typical playback system.
Professional audio systems often have very long analog signal chains. These applications place difficult demands on the frequency response of each analog component in the chain. A chain of 16 analog components, each with a bandwidth of 20 kHz, will produce an overall response of about - 3 dB at 5 kHz. This is telephone quality at best! If the same system is built with 200 kHz components, the overall response will be - 3 dB at 50 kHz, and about -1 dB at 20 kHz. Benchmark's founder, Allen H. Burdick, identified this problem when investigating the poor performance of analog audio distribution within television facilities in the mid 1980's. Performance of these analog systems was improved significantly by increasing the bandwidth of the analog distribution amplifiers. More details on this subject can be found in chapter 3 of "A Clean Audio Installation Guide" by Allen H. Burdick. Modern video systems use digital audio distribution and are not subject to these cumulative effects.
In summary, very high bandwidth is required at each link in the audio chain if we want to assemble a High-Resolution system. Audio components and digital formats that just meet the requirements of the human ear may be entirely inadequate when connected together in a chain. The audio chain is nowhere near as strong as the weakest link!
At Benchmark, listening is the final exam that determines if a design passes from engineering to production. When all of the measurements show that a product is working flawlessly, we spend time listening for issues that may not have shown up on the test station. If we hear something, we go back and figure out how to measure what we heard. We then add this test to our arsenal of measurements.
Benchmark's listening room is equipped with a variety of signal sources, amplifiers and loudspeakers, including the selection of nearfield monitors shown in the photo. It is also equipped with ABX switch boxes that can be used to switch sources while the music is playing.
Benchmark's lab is equipped with Audio Precision test stations that include the top-of-the-line APx555 and the older AP2722 and AP2522. We don't just use these test stations for R&D - every product must pass a full set of tests on one of our Audio Precision test stations before it ships from our factory in Syracuse, NY.
Paul Seydor of The Absolute Sound interviews John Siau, VP and chief designer at Benchmark Media Systems. The interview accompanies Paul's review of the LA4 in the December, 2020 issue of TAS.
"At Benchmark, listening is the final exam that determines if a design passes from engineering to production. But since listening tests are never perfect, it’s essential we develop measurements for each artifact we identify in a listening test. An APx555 test set has far more resolution than human hearing, but it has no intelligence. We have to tell it exactly what to measure and how to measure it. When we hear something we cannot measure, we are not doing the right measurements. If we just listen, redesign, then repeat, we may arrive at a solution that just masks the artifact with another less-objectionable artifact. But if we focus on eliminating every artifact that we can measure, we can quickly converge on a solution that approaches sonic transparency. If we can measure an artifact, we don't try to determine if it’s low enough to be inaudible, we simply try to eliminate it."
- John Siau