> 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.
Du...uh! I stand corrected. Shelf filters change the ratio of Pressure (W) vs Velocity (XY) so the different gains must be incorporated in the matrix.[1] Does anyone have Fons' 5.1 matrices handy that they could send me? I can't promise anything quick as its 35C in the shade and 100% RH in Cooktown and my small brain is boiling. It's also 3 decades since I did this stuff. > Ambdec doesn't use shelf filters, it uses crossover filters and two > independent matrices. 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. Shelfs.zip describe how to use crossover filters for Shelfs and attribute this to Fons. BLaH3 does the same but attributes this to Geoffrey Barton cos he first suggested it. We are going to use the term "Shelf filters" for any phase matched filters, whether IIR or FIR, used to adjust the ratio of Pressure vs Velocity in an Ambi decoder. I can't see any point in using FIRs these days unless you have a soft spot for them like Angelo. 8>D Fons' approach having separate LF & HF matrices is "purer". You have to do this for irregular layouts like the evil ITU 5.1 The ideal HF matrix for 5.1 is still open to debate. There is at least Fons, Bruce Wiggins, the Huddersfield Choral Society[2] and BLaH4. BLaH4, "Design of Ambisonic Decoders for Irregular Arrays of Loudspeakers by Non- Linear Optimization" Heller et al, preprint 8243 AES San Francisco nov10, discusses the various trade-offs and presents our favoured method. Roger, you might want to use a rectangle, other diametric-opposite, or regular (pentagon, hexagon, octogan etc) layout and sit in the centre. The LF & HF matrices will then be very similar and it will be easy to work out what needs to be done to get the 5.8dB difference. ________________ IMHO, the surround community needs to have another look at UHJ, especially simple 2 channel BHJ. The expanding music market is simple 2 channel stereo driven by iShops & other iThings, not multi channel surround. Any dedicated compressed Ambi format needs to make efficient provision for UHJ. My Ambisonic Surround Decoder (due out before the end of the millenium) will decode UHJ properly as well as Dolby Digital zillion.1 better than their native decoders. ________________ [1] Double Du...uh! "order_gain" DOES change the matrix. > If you look in the preset file, there are two matrices defined, one for LF > and one for HF. There are also 'order_gain' values for each matrix. Only the > first two of these matter (the others are for higher order). The first is > applied to the W column of the matrix (the first column), the second to X and > Y (the second and third column). If I read this correctly, you CAN use "order_gain" for a Shelf filter effect. I'll suggest trying an extra 1.95dB for the 1st (W) order gain and an extra -0.85dB for X & Y with Fons' 5.1 decode at HF [2] Moore D, Wakefield J "The design of ambisonic decoders for the ITU 5.1 layout with even performance characteristics" 124th AES Convention May08 _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
