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New crossfeed algorithm for Rockbox: "Meier" crossfeed

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Emulates the basic "Meier" crossfeed (2 capacitors, 3 resistors)
as discussed in
http://www.meier-audio.homepage.t-online.de/passivefilter.htm

This crossfeed blends a bit of low-pass filtered L signal into
the R signal (and vice versa) while adding about 300 us delay
to the crossfed-signal. A difference with the crossfeed already
present in rockbox, is that this algorithm keeps the total
spectrum flat (the one currently in rockbox accentuates
low-frequency signals, making it sound a bit muffled).

This implementation is quite lightweight, just 3 multiplies per
left-right pair of samples. Has a default C implementation and
optimized assembly versions for ARM and Coldfire.

The crossfeed effect is quite subtle and is noticeable mostly
one albums that have very strong left-right separation (e.g.
one instrument only on the left, another only on the right).

In the user interface, the new crossfeed option appears as
"Meier" and is not configureable. The existing crossfeed is
renamed to "Custom" as it allows itself to be customised.

There is no entry for the user manual yet.

Change-Id: Iaa100616fe0fcd7e16f08cdb9a7f41501973eee1
This commit is contained in:
Bertrik Sikken 2012-05-01 03:58:27 -04:00 • committed by Thomas Martitz
parent 08f5224b1b
commit afc96087f8
11 changed files with 259 additions and 27 deletions
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44
lib/rbcodec/dsp/dsp_arm.S
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@ -8,6 +8,8 @@
* $Id$
*
* Copyright (C) 2006-2007 Thom Johansen
* Copyright (C) 2010 Bertrik Sikken
* Copyright (C) 2012 Michael Sevakis
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
@ -246,6 +248,48 @@ crossfeed_process:
ldmpc regs=r4-r11
.size crossfeed_process, .-crossfeed_process
/****************************************************************************
* void crossfeed_meier_process(struct dsp_proc_entry *this,
* struct dsp_buffer **buf_p)
*/
.section .text
.global crossfeed_meier_process
crossfeed_meier_process:
@ input: r0 = this, r1 = buf_p
ldr r1, [r1] @ r1 = buf = *buf_p;
ldr r0, [r0] @ r0 = this->data = &crossfeed_state
stmfd sp!, { r4-r10, lr } @ stack non-volatile context
ldmia r1, { r1-r3 } @ r1 = buf->remcout, r2=p32[0], r3=p32[1]
add r0, r0, #16 @ r0 = &state->vcl
ldmia r0, { r4-r8 } @ r4 = vcl, r5 = vcr, r6 = vdiff
@ r7 = coef1, r8 = coef2
.cfm_loop:
ldr r12, [r2] @ r12 = lout
ldr r14, [r3] @ r14 = rout
smull r9, r10, r8, r6 @ r9, r10 = common = coef2*vdiff
add r12, r12, r4 @ lout += vcl
add r14, r14, r5 @ rout += vcr
sub r6, r12, r14 @ r6 = vdiff = lout - rout
str r12, [r2], #4 @ store left channel
str r14, [r3], #4 @ store right channel
rsbs r12, r9, #0 @ r12 = -common (lo)
rsc r14, r10, #0 @ r14 = -common (hi)
smlal r9, r10, r7, r4 @ r9, r10 = res1 = coef1*vcl + common
smlal r12, r14, r7, r5 @ r12, r14 = res2 = coef1*vcr - common
subs r1, r1, #1 @ count--
mov r9, r9, lsr #31 @ r9 = convert res1 to s0.31
orr r9, r9, r10, asl #1 @ .
mov r12, r12, lsr #31 @ r12 = convert res2 to s0.31
orr r12, r12, r14, asl #1 @ .
sub r4, r4, r9 @ r4 = vcl -= res1
sub r5, r5, r12 @ r5 = vcr -= res2
bgt .cfm_loop @ more samples?
stmia r0, { r4-r6 } @ save vcl, vcr, vdiff
ldmpc regs=r4-r10 @ restore non-volatile context, return
.size crossfeed_meier_process, .-crossfeed_meier_process
/****************************************************************************
* int lin_resample_resample(struct resample_data *data,
* struct dsp_buffer *src,