On 15 Dec 2011, at 17:00, sursound-requ...@music.vt.edu wrote: > > Message: 3 > Date: Thu, 15 Dec 2011 14:57:06 +0000 > From: Fons Adriaensen <f...@linuxaudio.org> > Subject: Re: [Sursound] UHJ decoding and shelf filters > To: sursound@music.vt.edu > Message-ID: <20111215145706.ga29...@linuxaudio.org> > Content-Type: text/plain; charset=us-ascii > > On Thu, Dec 15, 2011 at 07:19:40PM -0000, Richard Lee wrote: > >> Fig 3.3 shows them in the form that Ambdec uses; matched gain at LF. >> >> So 'order gain' at LF is 1.0 for both. >> >> At HF, 'order gain' is >> >> 3.7dB for W >> and >> -2.1dB for X & Y > > So what this amounts to is that for normal B-format the difference between > the > HF gains for resp. W and X,Y is 3 dB, and for UHJ it should be 5.8 dB. > > The actual shelf gains (the difference between LF and HF gain > for any component) don't tell the real story. Changing the LF > or HF gain for all components by the same amount doesn't modify > the decoding, it just results in some EQ. > > The Ambdec presets actually normalise for pressure at LF > and for energy at HF. That means that the shapes of the > equivalent shelf filters depend on the number of speakers. > > For example, the square decoder has order gains > +3, 0 at HF, while the octagon first order one > has +6, +3. In both cases the difference between > W and X,Y is 3 dB, this makes the decoder max rE. > > The balance between the HF and LF parts (that is, > the actual slopes of the 'shelf filters') can be > changed at run time using the LF/HF balance control. > > Ambdec doesn't use shelf filters, it uses crossover > filters and two independent matrices.
That is still a shelf filter in its composite response! We used such bandsplitting shelf filters right from the earliest experimental Ambisonic decoders in the 1970s as it was much easier to change the cross-over frequency this way. I have always built experimental decoders this way. Its really only when you seek to reduce costs in analogue implementations that shelf filters of the all-pass type become attractive. Geoffrey > So all the > above is assuming that the two matrices are the > same. They usually are for regular layouts, but > not in the case of the 5.1 one. This complicates > matters. > > Looking at the HF order gains for the 5.1 decoder > it seems the ratio is close to 5.8 dB, but this > doesn't mean anything as the matrices are quite > different. > > Ciao, -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20111216/ac70a025/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
