I am new to cubase , I have set up an external Synth using “External Instrument” and was trying to record it to an audio track but I found I can not add the input from the synth to the audio track input as its being used by the “External Instrument” . I can’t even find how to add a bus ?
coming from Logic I would just create an audio track and select the input I need
Once your input is being used in the external instrument it cannot be access via the inputs list.
You can use MIDI tracks instead so you can send MIDI data to the synth and record that into Stereo tracks.
Make sure to time align them after since this method is not delay compensated.
You can render in place and the track will play in real time and record it for you.
Logic makes this very easy, as many other DAWs.
I have a few external synths i quit using the Cubase internal " External instrument" because of this.
In this image you can see i have external instrument on track 1 using inputs 1+2 and in track 2 i have an audio track using the same inputs, no problem at all.
I Think is related to how the audio engine in Cubase is built! Just my guess
man I was hoping I was doing something wrong 
next problem is the Midi set up which in Logic I would have more that one synth on a Midi IO , I cant work out how to start multiple midi channel on a single Midi Out , only way I can work this is by saving a track preset .
so Midi out 1 , could have a Moog channel 1 and Korg Channel 2 , but how can I create a Midi Device for each ?
is there a way to route to a buss and record that ?
You can use two MIDI tracks routed to different external synths. From what i remember now, but i´m sure there is another way to achieve that! Maybe someone else can let you know.
