Released in August 2012, Accudio™ is an iOS music player developed by Goldenears. In addition to its powerful 9-band parametric equalizer, this app also comes with brilliant acoustic enhancement features called "Reference/Simulation mode". As long as headphone measurement data are stored in the database consisting of frequency response measured from over 300 headphones, users can tune any of their headphones to match Goldenears' proprietary diffuse-field target, or simulate them into nine different kind of headphones from various brands.
As many of inexpensive portable headphones lack in quality, such features shall definitely improve the user experience by a great degree. However, the legitimacy of Goldenears' claims is yet to be known. By analyzing Accudio™'s sound enhancement features in detail, their technological validity will be assessed.
The version of Accudio™ used in this analysis is 1.1.1, and an iPhone is used to drive headphones. The frequency response deviation from the reference setup is less than 1dB from 20 Hz to 20 kHz.
An occluded ear simulator complying ITU-T Rec. P.57, IEC 60318-4, ANSI S.325 is used, with the measurement procedure conforming IEC 60268-7.
A parametric equalizer
Accudio™'s 9-band parametric equalizer is one of the equalizer apps of the highest quality available in the iTunes app store. This function that utilizes IIR allows users to set equalization parameters at any nine points within the frequency spectrum from 20 Hz to 20,000 Hz in -15 dB to 15 dB scale. The bandwidth can also be adjusted from 28.9 to 0.2 in Q factor, or 0.05 to 5.0 in octaves.
However, despite its high resolution, regrettably, a touch-to-set function, which is commonly implemented in other equalizer apps, is missing, leaving non-expert users out of the range of usage.
Above is the result of amplification/attenuation of 15 dB in Q of 28.9, at 20 Hz, 200 Hz, 2 kHz, and 20 kHz, and clearly indicate Accudio™'s equalizer operates exactly the way it is supposed to.
Moreover, even with an extreme filtering, the equalizer does the job very precisely. As the signal processing is able to handle highly detailed equalization target, this parametric equalizer is recommended for even quite demanding purposes.
A reference emulator
One of the notable features of Accudio™ is its "Reference mode". By calculating the difference that the headphone of choice has over the Goldenears' reference response, the app applies the differential gain to the user's headphone to closely match the designated target. Although technical justifiability of the target is highly disputable, it is no doubt the feature may be able to dramatically improve sound quality of some low-quality earbuds.
The reference setting can be adjusted into 21 levels in 5 categories, leaving the users with great control over what they'd like to hear. However, how Goldenears set the frequency and bandwidth of each parameters, which are conceptually vague to define, is highly questionable.
Nonetheless, Goldenears firmly believe the concept of their reference emulation does work, so Etymotic Research ER-4P, which is officially rated 4/5 for its equalization adaptability by Goldenears, has been used to assess the effect of the process.
And the result shows the sign of amplification below 100 Hz, and above 3 kHz, which is quite far from either flat or even their own target.
A headphone simulator
The idea of simulating a headphone using another set of headphones has been around ever since Voinier and Briolle (1992), but has never been able to be successfully commercialized due to lack of dsp technology in portable devices, as it involves with a heavy amount of calculation for accurate reproduction. One commercial implementation of such technique is BDNC's HMT(Headphone Modeling Technology), but their technology never made it to the market.
Currently there are total of nine simulation targets, which are indeed of very high quality. If a cheap earbud can be turned into any of these products, Accudio™'s "Simulation mode" will totally revolutionize the user experience in portable audio.
However, as seen above, a simulated ER-4S lacks a good amount of bass compared to the actual ER-4S, indicating the simulation model is far off in this case.
Compared to the actual Sennheiser HD600 & HD650, simulated models are overly emphasized in bass, not to mention the characteristic deviation between HD600 and HD650 is nowhere to be found. Thus, even with Goldenears certification in adaptability, it is clear that accuracy of the simulated result is not guaranteed at all.
As it is possible that the headphones with higher adaptability might perform better, their certified headphones, T-PEOS H-100 and LG HSS-F450 Quadbeat have been analyzed.
Unlike what the official data suggests, the problematic dip @ 8 kHz and the peak @ 10 kHz have not been neutralized with "Reference Mode" setting. The high frequency bandwidth does not improve either.
Upon analyzing Accudio™'s acoustic enhancement features in great detail, the developer's claims have been fully assessed. While the parametric equalizer is indeed of a professional quality, it turns out "Reference/Simulation mode" highly lack in accuracy. It is especially disappointing, that the developer has not implemented a high resolution filtering process, which is critical for accurate simulation, in "Simulation mode". One way of doing so is to fully utilize the time-domain characteristics of their measurement data, which requires a lot of processing power. Maybe the current mobile technology may be still too far from achieving the means of true simulation.
All in all, the idea furnished in designing Accudio™ is innovative and creative, yet the developer's execution has proven to be quite poor. And with the price tag of $4.99, which is rather expensive for a music player app, it is doubtful that the app could give a strong impression to the potential buyers.
T. Voinle and B. Francoise, “Transfer Function and Subjective Quality of Headphones: Part 1, Transfer Function Measurements,” presented at the 11th International AES Conference, Portland, Oregon (May 29-31, 1992).