Re: software that emulates Echoplex DP

[Jeff Larson <Jeffrey.Larson@Sun.COM>]

Per Boysen wrote:
> The big challenge in software looping is to get around the AD/DA  
> latency, IMHO. The only way to do that is to use a software that  moves 
> every new loop layer recorded a bit forward in time, to  compensate for 
> the latency of the computer hardware. No such live  looping software 
> seems to be available today (please correct me if  I'm wrong).

Mobius does latency compensation as you describe.  It was probably
the single most difficult thing to implement because it impacts so
many internal calculations besides just lining up overdubs.

For example, when you receive a MIDI event that ends a recording,
you can't end it immediately, you have to wait until all of the
input buffers have been received, otherwise you'll chop off a little
of the end of what the user thinks they recorded.  At ASIO latencies,
this is difficult to detect, but it's really obvious with large buffer
sizes.

Similarly, whenever you make an abrupt playback transition such
as going into reverse or triggering a loop, you have to skip
over a bit of the beginning of the next loop because the output
buffers are already full of content from the previous loop
and they still have to drain.  This is especially important
when you're synchronizing.

It's easier when you're quantizing because we know ahead of time
when the transition will occur and begin output buffering of
the next loop before we actually get there (does that make
any sense :-)

I certainly can't claim to have all the bugs worked out, but
it seems to work reasonably well.  The one thing I haven't been
able to get my head around is how to do continuous playback
rate changes (speed up, slow down) and at the same time
do latency compensation "correctly".  I finally decided
not to bother because if you're changing the playback rate
then you're almost certainly out of sync with any external
devices anyway.  So the minute dealignment caused by incorrect
latency compensation is impossible to hear.

Sorry for all the techo-babble, but it's something I love
to talk about :-)

Jeff