From: Bertrik Sikken <bertrik_at_sikken.nl>
Date: Sat, 09 Oct 2010 16:26:21 +0200

Replying to myself again.

On 30-8-2010 7:29, Bertrik Sikken wrote:
> On 28-8-2010 5:26, Bertrik Sikken wrote:
>> I made a simple proof-of-concept to make this delay configurable.
>> Please let me know if you want to try it.
>
> I created a task on the tracker with a patch so you can experiment
> a bit with changing the delay yourself:
> http://www.rockbox.org/tracker/task/11577

While looking into the cross-feed stuff, I found the "Meier" crossfeed
circuit. This is a deceptively simple passive crossfeed circuit
designed by audio engineer Jan Meier, described here:
http://gilmore2.chem.northwestern.edu/projects/meier_prj.htm

I made an software implementation of his electronic circuit and
put it in this patch: http://www.rockbox.org/tracker/task/11559
(meier_v2.patch)
The patch changes crossfeed from an 'on/off' setting to a choice
setting, with choices 'off', 'meier' and 'custom', where custom
is our current rockbox crossfeed algorithm.

Features of this crossfeed:
* frequency neutral
  (unlike our current crossfeed which makes the audio a bit more bassy)
* it adds a delay of approximately 300 us, mimicing the delay between
  one ear and the other with speakers set up 60 degrees apart
  (like our current crossfeed algorithm).
* the crossfeed only affects audio that is actually different between
  the L and R channel, Jan Meier calls this "natural crossfeed".
  (unlike our current crossfeed algorithm)
* relatively light-weight to calculate, 3 multiplications for one
  (L,R) sample pair plus some adds and subs.
* designed by an audio engineer :)

The Meier crossfeed is a bit more subtle than our current crossfeed,
you'll notice it mainly on audio that has hard-panned instruments.
A few people on the anythingbutipod forums like it so far.

I believe this patch is ready to go into SVN, except for the update
to the manual (to be written) and (optionally) an assembly version
of the algorithm.
Please try it and let me know what you think.

Kind regards,
Bertrik
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Received on 2010-10-09