Head-Direct HiFiMan HM-602 Digital Audio Player Manufacturers' Comments

Sidebar 3: Manufacturers' Comments

Editor: Thanks a lot for Stephen Mejias's excellent writing. We released new firmware at the end of February to solve the noise problem when the power runs out. I am very sorry to have this bug in the early-version firmware. The reason for the noise is that when the player is almost out of power, the battery voltage becomes very low. In the new firmware, we improved the voltage a little bit so that the player will be automatically shut down before the noise begins.

About the measurements in general: Before we designed the HM-602, we tried many modern low-cost D/A converter chips, such as the CS4398, CS4334, CS4344, and ESS9023. Most of these DAC chips measure much better than the TDA1543. However, there are two very important reasons we did not use them: 1) Most entry-level DACs do not do well with an active low-pass filter running at a low voltage. 2) None of them has a good squarewave spectrum, which is actually very important to sound quality.

According to our results, the NOS TDA1543 design (bypassing the digital filter) might be a solution that can provide an analog-like or musical sound signature in low-voltage and low-cost situations. However, an NOS DAC usually measures worse than a normal DAC design. It was a difficult decision for a low-cost portable solution: better measurements, or a better subjective listening experience? Finally, we decided to follow our ears, not the measurements. That is a very important reason why Stephen Mejias had such positive things to say about the HM-602's sound quality (eg, better than the iPod), but John Atkinson got overall negative measured results (eg, worse than the iPod).

About high jitter in USB DACs: According to the measurement result, when the HM-602 is used as a USB DAC, the jitter is much higher than when it is used as a player. This is actually very common in portable USB DACs: The USB receiver chip in a portable device normally gets its power directly from the computer's USB socket, along with a lot of high-frequency noise from the computer's power supply. Because a laptop computer is a very compact design, its simplified power supply creates even more noise than a desktop's. One of the solutions is to use a desktop with a good power supply; for example, a fanless power supply.

Further thinking: As someone with more than 15 years of experience in scientific research, I have noticed that there is always something that cannot be verified by current scientific theory. There are many weird observations in the lab, and some of them become the beginning of a new theory or technology. For example, blackbody radiation cannot be explained by Newton's classic mechanics, but the research into blackbody radiation become the beginning of modern quantum-mechanics theory.

I believe that to reproduce sound is to actually reproduce a part of the real world. A 100% perfect clone of a natural sound is theoretically impossible, but reproductions of that sound will become more real as the technology develops. Modern sound-measurement instruments have been developed for about 100 years, and are definitely helpful in designing audio products. On the other hand, human ears are more important, because they have been developed over billions of years, and continue to evolve. In addition, the goal of designing audio products is to satisfy human ears, not instruments. That is why audio design is still somehow a state of art, not a state of technology. However, as a scientist, I am confident in the development of science. I believe that, sooner or later, scientists will discover at least part of the solution of why subjective listening sometimes does not agree with the measurements.—Fang Bian, Head-Direct

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