PC4uMusic & a head-related transfer functions

 

This post briefly covers various topics that may seem unrelated, but have a common denominator. This was previously discussed on the Polish language AIMP forum, but is no longer available. It is about how the ear receives sound from a point in space, and such an explanation is not very clear. There is an entry on Wikipedia on this topic: Head-related transfer function. Microsoft has been working on this topic for many years, and one of the clues is raised in their presentation Spatial Audio, where you can read, among other things: The temporal and spectral cues used by our auditory system to determine the direction of arrival of a sound source can be expressed in head-related transfer functions (HRTFs). HRTFs are measurements that capture the directivity patterns of human ears, that is, the way sound, arriving from a certain direction, reaches the left and right ear. HRTFs are a function of source azimuth and elevation, distance, and frequency.  

In PC4uMusic configurations, this primarily concerns practical applications with the Microsoft system driver for the sound processor built into motherboards to play stereo, quadraphonic, and home theater recordings

The configurations are for optimal setup with e.g. the free AIMP program, to provide an analog signal for 4 speakers, which is converted from digital with 32-bit float precision at a frequency of 192 kHz. The possibilities are even wider, but for ambiphonic playback they are not fully available in AIMP based on the Windows Audio Session Application Programming Interface.

There is already a Spatial Audio plugin for foobar2000 that is not technically perfect, but points in a direction with impressive potential. It is an alternative to plugins that allow this program to work with WASAPI or ASIO (Audio Stream Input/Output)

  

Microsoft drivers have automatic room calibration (correction) and were used for testing with AIMP and foobar2000. 

It is very useful for faithful reproduction of music at home with 4 speakers and not only with their arrangement: left, center, right and back. It also works well with the traditional arrangement with a pair of speakers in the front and back.

Practical comparative tests were, among others, with analog solutions for quadraphony in the Sansui QS-1 decoder, which is mentioned in Wikipedia: in Wikipedia: In 1971, Sansui introduced the Quadphonic Synthesizer QS-1, which could make simulated four-channel stereo from two-channel sources. Sansui developed the QS Regular Matrix system, which made it possible to transmit four-channel Quadraphonic sound from a standard LP. The channel separation was only 3 dB, but because of the human way of hearing it sounded relatively good. In 1973, Sansui introduced the more advanced QS Vario Matrix decoder with 20 dB separation. Wikipedia also states: SQ records could give some amount of quadraphonic effect when played through a QS Regular Matrix decoder, although only accurately decoded playback is via a correctly designed SQ decoder.  Calibration for 4 speakers was the biggest problem from the very beginning, but there was little awareness of this and at that time there was no suitable digital technology. As a result, other solutions with analog technology were sought, which can also be found on Wikipedia:: The early SQ decoders could not produce more than 3 dB of separation from front to back. Early "Front-rear logic" circuits were introduced to enhance separation to 12 dB and later "Full logic" circuits 20 dB, but both provided poor performance, very noticeable gain-pumping and an unstable 'swaying' sound field.

Another interesting topic here is the analog solution for playing music through speakers from binaural recordings, which are intended for listening with headphones.  Japan Victor Company (JVC) offered such solutions under the name Biphonics. The flagship product was the BN-5, which was mentioned in the text entitled: Ambiophony and Rendezvous on SR2 with Roger Moor. 

Often advertised together with the BN-5 were headphones with built-in microphones, namely the JVC HM200E artificial head, which has as yet untapped measurement potential for calibrating 4 loudspeakers placed in a music lover's room.

Even with the most renowned equipment, measurements are taken with one microphone, and we have two ears, which is worth considering to ensure faithful reproduction of music. What is important is what we want to measure. The quality of the microphone is of course important, but what is most important is in laboratory measurements in soundproof chambers, to create curves for advertising materials of loudspeaker manufacturers. At home, the most important thing is that the loudspeakers work optimally in a given room in relation to our ears.

MathAudio Room EQ is a professional studio software and there is also a free version for foobar2000. It has a left and right microphone measurement function, but not for 4 speakers and does not include a dummy head measurement either.

In offers with exclusive audio equipment for home cinema, the Swedish calibration program DIRAC is becoming popular. It is very advanced, but despite this, there is no mention in this offer that a person has a pair of ears.

 In the field of surround sound, the emergence of the Immersive Audio Model and Formats (IAMF) standard may bring a lot of benefits, and a text about this novelty was published in 2023 entitled [Interview] Movie-Quality Audio From the Comfort of Your Home: Meet the Leaders of Next-Generation 3D Audio Technology.