.. a reply to stuff on the "Anyone had experience with AllRAD decoding to physical loudspeakers?" thread where Aaron brought up the topic of NFC.
" A practical problem is that forward NFC filters have infinite gain at DC. Jerome Daniel's solution is to have a reference decoding distance for Ambisonic program material, say 1 meter -- In the encoder, you encode and then decode for speakers at 1 meter (which produces finite gains at DC). [1] J. Daniel, "Spatial Sound Encoding Including Near Field Effect: Introducing Distance Coding Filters and a Viable, New Ambisonic Format ," *Preprints 23rd AES International Conference, Copenhagen*, 2003. [2] J. Daniel and S. Moreau, "Further Study of Sound Field Coding with Higher Order Ambisonics," *Preprints from 116th AES Convention, Berlin*, no. 6017, 2004. " Daniel introduced this into a shark feeding frenzy of gurus & pseudo-gurus debating HOA encoding. I wanted to join in and introduce Yet Another Encoding but beach bum issues intervened. Besides I was seriously intimidated as I neber wen 2 skul. ___________________________ MY SOLUTION TO LOADSA LF GAIN FOR HOA ENCODING Ignore it. ___________________________ Making a HOA soundfield, we deal with 2 type of sources/objects. The first is naturally recorded stuff from a well calibrated soundfield mike and IMHO, these are the most important. It was the incredible sense of REALITY playing an original Calrec Soundfield recording on a properly set up Ambisonic system that pulled many of us into the arcane world of Ambisonia. As a(n Ambisonic) mike designer, you immediately come up with a serious problem with correct Ambisonic encoding .. the huge LF boost at close range. Even the First Order Ambisonic (FOA) Calrecs & TetraMics have 30dB/8ve LF filters to alleviate their facility as seismographs, air-con, slamming doors, distant thunder .... detectors. And you will find even more LF filtering required at times. The pretenders w/o such filters WILL have serious problems. A practical HOA mike has additional problems as it must use cancellation & mixing of 1st & 2nd order microphone capsules + LF boost to get the high orders which give serious S/N problems at LF. You see this with full force in Eigenmike which uses omnis which need even more LF boost. Angelo Farina has several reports & papers showing how useful (??) this is. The solution is to drastically HP filter the higher orders AND MATCH THE PHASE OF THE UNFILTERED CHANNELS. You can do this with the method in the appendix of BLaH3 http://www.aes.org/e-lib/browse.cfm?elib=14705 In essence, a practical HOA microphone will give you as much HOA signal AS IS USABLE (mostly determined by S/N) while keeping the correct relationship between the components. ______________________ But you will argue, "But that means the Ambisonic order will be frequency dependent???" Why is this a bad thing? Ambisonic playback is also frequency dependent. An Ambisonic recording is the DEFINITION OF SOUND AT A POINT. But this POINT is frequency dependent! For a FOA recording, at 20Hz, an IEC listening room is a 'point'. But at 1kHz, the 'point' is about the size of a Mk1 Human Head. What HOA does is define the 'point' to be larger at any frequency. We don't need a larger 'point' at LF so FOA is usually sufficient at LF. At mid & higher frequencies, HOA encoding allows a larger 'point' to be defined. So a practical HOA mike as described above degrades in exactly the correct fashion to be USEFUL. A PRACTICAL decoder can make use of as much info as is available in the recording without worrying about EXACTLY what is available at each frequency. (Actually this isn't strictly true but that's a whole new field of research in itself which may lead to Yet Another HOA Encoding and/or Decoders :) ) ____________________________ Back to HOA encoding. The 2nd type of source/object are artificial. HOA is most useful for shoot-them-up games and other virtual reality stuff. You pan the (mono) bad guy to someplace in your HOA space BUT THIS STUFF CAN USUALLY BE SET AT AN INFINITE DISTANCE SO NO NFC IS REQUIRED It's very rare that proximity adds anything to the virtual reality gaming experience. (In practice, 'infinite distance' can be considered anywhere outside the 'circle' of speakers.) If da bad guys do come closer, you can use the same drastic HP filters to keep the LF boost within limits. ie treat the sources as though they were being picked up by a REAL LIFE HOA MIKE. There is NO NEED FOR SET DISTANCES/FREQUENCIES etc. You only need to MATCH THE PHASE. And usually you don't need to add NFC to the HOA signals for a realistic effect. FOA (with NFC) is certainly good enough for the Spitfire flying low overhead .. or even Peter Lennox attempting to run over Grand Vizier Malham's Mk3A Soundfield with his motorcycle. (one of the very first Soundfields with the correct NFC encoding) So in most cases, just use LF boost on the FOA signals for NFC ____________________________ You can apply this to any of the generic HOA encodings. When I last looked seriously at this, Furse-Malham was favoured up to 3rd order. I gather some other Encoding is now 'standard'. :o Can some of you can enlighten me on which 'standard' has won out and is commonly incorporated into HOA Decoders ? _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit account or options, view archives and so on.
