refer to related question at
http://audacityteam.org/forum/viewtopic.php?f=12&t=6316
I do know how to fix your live signal contamination problem, but there is another factor you need to consider when working with your old projects.
In Audacity 1.2, there is substantial signal path latency that causes noticeable time skew between the old canned and live new tracks in the Audacity project. Manual de-skewing is unsatisfactory. Audacity 1.3 has a latency de-skew parameter. I recommend that you use Audacity 1.3 to record your new tracks.
In any case, you still need to be able to monitor your canned tracks without contaminating your new live track. I recommend that you disconnect your external monitor patch path and try one of these methods instead.
Note that in any of these methods you still have lots of flexibility to re-route effects sends and returns with patchmix or your external mixer so be careful and use your imagination. I am giving bare-bones instructions only.
Method 1) Quick and dirty but low quality, use Audacity to monitor the canned/live mix
external mixer instructions:
send the live in to the line in of the sound card or some other input such as adat etc. if you have the capability
pass through the line out from the sound card to the amp
patchmix instructions:
right-click on the live input patch bay in patchmix that you want to send from i.e. I/O card in L or R, and select ‘Insert Send (Output to ASIO/WAVE or Physical Out)’ menu choice
the pop-up dialog box will present you with ‘Select send Output’ multiple choice box that contains a pre-selected choice representing the first available resource on the list, probably HOST ASIO IN 1/2 or 3/4 or whatever
left-click on the form field and select HOST WAVE L/R, then click OK
mute the attenuator on the input slot and zero the aux1/2 effects sends – you do not want to hear any live signal from anywhere but Audacity
insert the HOST WAVE 1/2 input slot
adjust main mix and effects sends of the (HOST WAVE 1/2 input slot) canned plus software play-through signal
Audacity instructions:
edit - preferences - audio I/O - check both overdub and software play-through
Advantages:
simply accomplished
preserves flexibility in the external mixer
live effects can be added either in the external signal path or in the patchmix input slot effects bay, and the effects will all be equally present both in the recorded track and in the monitor mix
Disadvantages:
there may still be substantial signal path delay – from the input signal you generate while performing, to the play-through monitor output signal that reaches your ears. The delay might confuse your chops. Use the direct X drivers for record and playback to minimize this delay, but there is no way to completely eliminate it.
there is no separate live input mixer attenuator in Audacity to adjust the monitor mix. Adjusting the input level will corrupt the recording level, and adjusting the separate output levels of prior tracks will corrupt the canned mix. Either way, there is no viable solution to obtain a quality monitor mix without severe trade-offs.
this method might not work with other sound cards. Some sound cards are internally configured to monitor their own live mix and selecting software play-through in Audacity too causes regenerative feedback. there is basically no way to do multitrack overdub with those (usually older) sound cards.
there is no way to independently control the monitor mix with a single external volume control knob. all monitor mix must be controlled via mouse and keyboard.
Method 2) Simple but high quality, use Patchmix to monitor the canned/live mix.
external mixer instructions:
send the live in to the line in of the sound card or some other input such as adat etc. if you have the capability
pass through the line out from the sound card to the amp
patchmix instructions:
right-click on the live input patch bay in patchmix that you want to send from, and select ‘Insert Send (Output to ASIO/WAVE or Physical Out)’ menu choice
the pop-up dialog box will present you with ‘Select send Output’ multiple choice box that contains a pre-selected choice representing the first available resource on the list, probably HOST ASIO IN 1/2 or 3/4 or whatever
left-click on the form field and select HOST WAVE L/R, then click OK
insert the HOST WAVE 1/2 input slot
adjust main mix and effects sends of both the canned (HOST WAVE 1/2 input slot) and the live (I/O card in L or R input slot) signals
Audacity instructions:
edit - preferences - audio I/O - check overdub and uncheck software play-through
Advantages:
simply accomplished
preserves flexibility in the external mixer
live effects can be added either in the external signal path or in the patchmix input slot effects bay, and the effects will all be equally present both in the recorded track and in the monitor mix
Windows software latency from live performance to your ears is eliminated, leaving only trivial hardware DSP latency that is negligible and should not affect your chops at all.
the monitor mix can be saved in a patchmix configuration file for later instant retrieval
Disadvantages:
there is no way to independently control the monitor mix with a single external volume control knob. all monitor mix must be controlled via mouse and keyboard.
Method 3) Quick and convenient to control but inflexible, use your external mixer to control the monitor mix
external mixer instructions:
route an external mixer monitor send via dedicated monitor mix or use a spare external mixer effects send to add live signal directly to your external mixer live/canned mix
fork the live input via an auxiliary output that connects to your sound card input.
follow the instructions for method 1, except use the Audacity set-up instructions for method 2.
Advantages:
gives single-knob control of the live mix directly on your external mixer while preserving the live recording path levels
simple to set up
Windows software latency from live performance to your ears is eliminated, leaving only trivial hardware latency in the external mixer that is negligible and should not affect your chops at all.
disadvantages:
may use up external mixer outputs and effects channels
may complicate the use of software effects on the live channel
monitor mix cannot be saved easily to a configuration file
Method 4) combine methods 2 and 3 to get the best of both worlds
instructions:
left as an exercise to the reader but basically it means routing the live signal from an aux effects or monitor send out of the external mixer then into and out of patchmix then through an external live monitor or effects return in the mixer – while forking post-effects live signal into Audacity with the WAVE L/R plugin
advantages:
simple in-line patching of both hardware effects in the external mixer and software effects in patchmix
preserves effects in both the recorded signal and in the live monitor signal
allows for easily accessible external volume control knob
disadvantages:
too complicated to describe in words without actually having the actual equipment in hand
cannot save the external mixer configuration to a file easily
In any of these methods you can combine both hardware and software effects if you are clever and careful what you are doing.
The best solution to your problem depends on if you are doing any live performance and recording simultaneously, or if you are just doing studio recording. It also depends on how you do your final mixdown and if live input is part of that process or not. Usually, live input will only be incorporated into mixdown if there are equipment limitations driving the decision, for example the effects routing or number of tracks in the recording application.
Recommendation:
Use only Audacity 1.3 and disable software play-through if you have either patchmix or an external mixer. They both have superior methods of monitoring the live/canned mix.
I use method 2 but if I were serious about recording with free software I would use method 3 because it requires the least tweaking to go from live recording to final mix configurations. If using method 2 it can get messy trying to insert external effects during live recording.