I think Patrick's point is that by selecting gain coefficients that are multiples of 1/256, no bits are thrown away when truncating to 24 bits. I think my patch above ensures that after multiplication, no bits need to be thrown away when truncating to 24 bits. With these coefficients, the gain multiplication is reversible, the original samples can be recovered because no bits were thrown away.
Steinar dean Wrote: > On Oct 30, 2005, at 1:39 AM, Patrick Dixon wrote: > > > > > > >> It's not clear that there is an issue at all. We'd need a > >> reproducible case. > > > There is definitely an issue, and it's easy to demonstrate/reproduce > - > > the only question is whether you can hear it or not! I say I can, > and > > jhwilliams has done blind testing which indicates that he and his > > girlfriend can too. I have now reverted to Firmware 15, which I'm > > happy with sound-wise. > > Well, internally on the player, the old volume control and the new > volume control both change the same gain coefficient. > > The old-style is a 1.7 fixed-point encoding and the new one is a > 16.16. So, depending on the value of the gain, you may find rounding > > errors, since the resultant value is truncated at 24-bits, but the > trade off is that you can have much smaller volume increments. > > I can't argue that you can't hear the difference (folly in this > forum), but I do believe that it's mathematically correct and for the > > vast majority of users the new volume curve is a benefit. > > p.s. I found this article on dither very interesting: <http:// > stereophile.com/features/705dither/> > > > > > > > > The issue is that previously the volume control used an 8-bit > > multiplier coefficient which meant that the full resolution of > 16-bit > > audio (which is of course what CDs are) was always retained within > the > > 24-bit datapath of the SB2. The new log volume control uses a 16x16 > > bit multiplier which means that rounding errors are 'always' > > introduced, and these are unconcealed by the addition of any dither. > > It's surprising to me too that I can hear these artifacts, but I > think > > it's the character of the noise (coupled with oversampling in the > > DAC?) > > that makes it stand out. > > > > Even with the addition of dither (which may well be useful for the > > replay gain situation anyway), there is an audiophile arguement that > > says you should always preseve the S/N ratio of the original 16 bit > > signal where you can. Therefore I am simply suggesting that you > > 'tweak' the coefficients of volume control settings 20-39 to use > 8-bit > > resolution rather using the full 16-bit resolution of the new > > multiplier. Much below vol 20, 8-bit coefficients are not enough to > > resolve the volume steps required, but at these levels the artifacts > > are unlikely to be an issue. > > > > My calculations gave something like this below - so the volume > changes > > would be negligeable with the adjusted coeffs. Just ANDing the > 20-39 > > coeffs with 1100hex would probably give acceptable results. > > > > vol 8-bit coeff 'new' dB 'old' dB > > > > 20 14 -25.2 -25.4 > > 21 16 -24.1 -24.1 > > 22 18 -23.1 -22.8 > > 23 21 -21.7 -21.6 > > 24 25 -20.2 -20.3 > > 25 29 -18.9 -19.0 > > 26 33 -17.8 -17.8 > > 27 38 -16.6 -16.5 > > 28 44 -15.3 -15.2 > > 29 51 -14.0 -14.0 > > 30 59 -12.7 -12.7 > > 31 69 -11.4 -11.4 > > 32 80 -10.1 -10.2 > > 33 92 -8.9 -8.9 > > 34 107 -7.6 -7.6 > > 35 123 -6.4 -6.3 > > 36 143 -5.1 -5.1 > > 37 165 -3.8 -3.8 > > 38 191 -2.5 -2.5 > > 39 221 -1.3 -1.3 > > 40 256 0.0 0.0 > > > > > > -- > > Patrick Dixon > > > > www.at-view.co.uk > > -- sbjaerum _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
