Eilex Binaural Pro

  • Binauralize Standard Stereo Source (such as CD) in Real-Time
  • Sound Stage is set to 60-deg (Standard for Studio Monitors)
  • Proprietary HRTF Derived from Impulse Responses at Both Ears
  • Precise VIR Filtering of HRTF
  • E-Notch Installed
  • Applicable on All Headphones/Earphones
  • Preservation of Headphones/Earphones’ Tone Character
  • Hi-Res Audio Compatibility

Eilex Binaural Pro is a process which converts recorded materials intended for stereo speakers to Binaural Audio ideal for reproduction with headphones/earphones. It maximizes the performance of any headphones/earphones while maintaining their tonal characters.

The sound stage of Eilex Binaural Pro is set to 60 degrees which is the standard of studio monitors layout. The HRTF (Head Related Transfer Function), which are necessary for binaural process, are derived from the impulse responses recorded at the both ears. The binauralized stereo source materials using such HRTF provide sound similar to the speaker sound.

Eilex’s proprietary VIR Filters are used to process the HRTF. VIR Filter processes the complex HRTF frequency responses using only small MIPS.

E-Notch (refer to section 5) prevents the typical hissy headphone sound by removing the perceptual 7.5 KHz peak.

Eilex Binaural Pro maximizes headphones/earphones performance while preserving the tone character. Since it has a virtually flat frequency response, the tone characters are kept unchanged. Only the tonal improvements by the binauralization are heard.

Eilex Binaural Pro is for high quality audio including Hi-Res, but also suitable for 2-ch movie soundtracks and internet audio reproduction.

1. Problems of Headphone Sound Reproduction

Headphones/earphones always deliver an artificial sound image, regardless of their quality. Compared to speakers, the disadvantages of headphones are:

  • A vocal in the center moves to the middle of your head.
  • Panned sound shifts to the extreme left or right.
  • Details are over-emphasized, damaging the tonal balance.
  • The amount of reverb is exaggerated.
  • Different parts of the sound from the same instrument come from different directions.
  • Background instruments move from behind a singer to the sides.
  • The placement of instruments in an orchestra get mixed up.
  • 7.5 KHz peak causes dry sound. (Refer to the section 5. E-Notch.)

These problems do not happen with speakers. The cause of the problem is reproducing materials meant for speakers with headphones.

When listening to a stereo system, both ears can hear both the L and R speakers. When listening through headphones, each ear only hears one channel. This is the reason why the headphone audio produces a strange sound image and an uncomfortable feeling. (See REFERENCE at the end of this paper.)

2. Binaural Recording

There is a recording technique called Binaural recording, which is only intended to be reproduced by headphones. To mimic the physical auditory condition, the Binaural recording is performed using a dummy head and torso with microphones buried in the ears. Thus the Binaural recording automatically includes the subsequently mentioned cross-feed, HRTF and time delay. (The Binaural recording is the most suitable for reproduction using headphones, but not for speakers.)

3. Eilex Binaural Pro

Eilex Binaural Pro converts regular stereo materials into Binaural format in real-time.

The key elements of binauralization are cross-feed and unique HRTF (Head Related Transfer Function) of direct and cross-feed paths.

Cross-feeding is a technique to let the ear hear the opposite channel by feeding in the opposite channel’s sound. This is performed on both channels.

HRTF is the frequency response between the speakers and both ears.

The sound stage of Eilex Binaural Pro is set at 60-deg which is the standard layout of recording studio monitors. HRTF are derived from the impulse responses measured at both ears receiving the signals from the reference speaker’s location (30-deg from the center line and 0-deg elevation).

Fig. 1 and 2 show the impulse response at the ear from the same-side and opposite-side speakers.

Fig. 1 Impulse response at the same-side ear of speaker
Fig. 1 Impulse response at the same-side ear of speaker
Fig. 2 Impulse response at the opposite-side ear of speaker
Fig. 2 Impulse response at the opposite-side ear of speaker

HRTF are calculated by converting the impulse responses into frequency domain. Such HRTF are processed by Eilex’ proprietary VIR filter. VIR filter executes complex HRTF at a very small MIPS.

The time delay, which comes from the difference in the distance between the ears to the reference speaker, is added to the cross-feed in addition to HRTF.

The following waveform pictures illustrate the effect of cross-feeding. Fig. 3 and 4 show the actual operation of the cross-feed. The source is the first 40 seconds of The Beatles’ “Here Comes the Sun”. The upper and lower traces are L and R channels, respectively.

As seen in Fig. 3, the song starts with a guitar introduction on the L-ch, while the R-ch is totally silent for 15 seconds. The guitar from the left speaker sounds just fine for both ears. But on the headphones the guitar is heard only by the left ear, which creates an odd feeling because this never happens in the natural world.

Fig. 4 shows the result of cross-feeding. A signal of almost the same shape and strength appears on the R-ch, as it does when listening to a speaker. An appropriate time difference is added to the cross-feed to preserve the directionality and spatial information.

Fig. 3 'Here Comes the Sun', Original
Fig. 3 “Here Comes the Sun,” Original
Fig. 4 'Here Comes the Sun', with Cross-feed
Fig. 4 “Here Comes the Sun”, with Cross-feed

Fig. 5 shows the original waveform of the whole tune of “Flying”. Fig. 6 is after cross-feed is applied. The L and R waveforms become very similar, yet a normal stereo image is maintained.

Fig. 4 and 6 contain direct and cross-feed HRTF.

Fig. 5 'Flying', Original
Fig. 5 “Flying”, Original
Fig. 6 'Flying', with Cross-feed
Fig. 6 “Flying”, with Cross-feed

4. Sound Improvement by Eilex Binaural Pro

The Eilex Binaural Pro process makes headphone/earphone sound identical to reference monitor speakers. The reference speakers are equalized to a flat APVD (Acoustic Power Volume Density) frequency response by Eilex PRISM. The flat APVD speakers reproduce a true-to-source sound in the room.

The Eilex Binaural Pro process is calibrated by multiple experts to match the sound of reference monitor speakers.

The sound difference between the headphone/earphone models mainly comes from the difference of high frequency sensitivity and characteristics. Eilex Binaural Pro process has a High Frequency Level control to handle the wide variety of sound character of unspecified headphones and earphones.

Listeners can adjust (boost or cut) the high frequency level to what suits their taste. The sound should be closer to the APVD flat speakers. (Note: This process is unnecessary for speakers because listeners hear exactly the same sound. Headphones and earphones provide a different sound to each individual’s ears, so adjustment is necessary.)

As with speakers, the tonal character and taste of each brand’s headphones is often as important as the sound quality itself. The compensation coefficients of Eilex Binaural Pro are carefully composed to preserve the brand’s intended character for their products.

5. E-Notch (7.5 KHz)

In the process of evaluating headphone sound, Eilex team discovered an abnormal 7.5 KHz boost. No such boost occurs when speakers are used. This means our ears inherently have a 7.5 KHz notch which disappears when we wear headphones or earphones. As a result, a perceptual 7.5 KHz peak appears in the headphone sound.

This phenomenon occurs regardless of the quality of headphones or earphones. Pink/white noise gets accompanied ringing, vocals become husky and the smooth high-end of strings in the orchestra become coarse.

It is common knowledge that our ears have a relatively wide V-shape dip between the 6 and 8.5 KHz range. Such a dip is normally included in the headphone/earphone HRTF. But, even so, we still hear the annoying high frequency ringing.

Further study at Eilex shows that the notch resides in the narrower range between 7.3 and 7.8 KHz, having a 7.5 KHz center. Its depth is as deep as more than -20dB. We named the notch “E-Notch”. E-Notch must be included in the HRTF to compensate for the high frequency ringing.

The above mentioned V-shape dip doesn’t match with the E-Notch spike pattern. It is difficult to add a precise mechanical notch to the headphones, but not hard to accomplish that electronically in the amplifiers or Binaural process. Eilex Binaural Pro has an E-Notch built in.

6. Movie Reproduction

Headphones and earphones are typically used when movies are played on a PC, laptop, notebook, tablet, smartphone or mobile phone. Such movie soundtracks are normally 2-ch stereo. Eilex Binaural is ideal for reproducing 2-ch soundtracks; it improves the sound quality and restores the sound image.

Eilex Binaural Pro improves the sound quality by matching the frequency response of headphones and earphones to the user’s ears. It improves sound clarity, and details are more easily heard. Speech intelligibility improves and dialogue becomes clearer. Sound effects are more vivid.

2-ch soundtracks, down-mixed from multi-track materials, tend to reproduce uncomfortably for headphone users. The components which were originally a part of the surround sound, such as explosions with continuous mid-low frequencies, ambient noise, rain, thunder, and applause sound unnaturally close to the ears. Such close sound to the ears masks more important sounds, including dialogue. Also, the sound image for sound effects expands too wide, creating location discrepancies between the sound and picture. Eilex Binaural Pro corrects these types of errors and restores the sound image as if it originated from two front-firing speakers.

7. Internet

Internet audio is normally 2-ch stereo. PCs, laptops, notebooks, tablets, smartphones, and mobile phones are used for reproduction. The content consists of news reports, TV channels, radio, videos, movies, music, commercials, games, etc. Headphones/earphones are more commonly used for audio than speakers, and exactly the same audio problems arise as with movies.

Headphones do a much worse job of reproducing internet sound than speakers; the inferior sound quality of internet audio is exaggerated by headphones. This is because the noises and distortion caused by compression errors of low bitrate signals are heard very close to the ears. Eilex Binaural Pro pushes such noise and distortion away from the ears, leaving only the clearer, smoother sound. Eilex Binaural Pro very effectively improves all internet audio.

8. Compatibility

Eilex Binaural Pro has compatibility with the following:

  • Regular 44.1/48KHz and Hi-Res 88.2/96/176.4/192KHz sources.
  • All lossless and lossy digital compression formats, including MP3 and AAC [Note: Refer to Eilex Harmony and Eilex Focus for sound recovery]
  • Down-mixed materials from multi-channel recordings
  • Monaural Materials
  • Noise-cancelling headphones
  • Tone control
  • Equalizers (parametric and graphic)
  • Preset EQ (classic, jazz, pop, etc.) [Not recommended to use other than for marketing purposes.]

Compatibility is not guaranteed in the following cases:

  • On-board 3D or surround post-processing within the MP3 player
  • Headphones with internal (electronic or mechanical) 3D or surround processes
  • Headphones for the hearing impaired
  • Use with hearing-aids

No compatibility (Eilex Binaural process must be turned off):

  • Binaural recordings
  • Headphones with electronic or mechanical (acoustic) cross-feed


Speaker and Recording Reproduction
Problems in Headphone Reproduction
Eilex Binaural Headphone