transcoding from input to output

how can i take a input stream and transcode it into the output with audio aac, not take the output ,transcode it and create another output as it currently is as i need to change the transcoding on some channels based on needs. Having to add a channel with input and output , then create a transcoding with a different output would cause me problems to change them.
Example i have stream1 that has audio track ac3 and i want the output aac but i already have the channel defined input and output. i just want to add the transcoding in the middle not have a output ac3, then i need to create another output from the transcoder

You can add same input several times in transcoder scenario and made different outputs. It will be decoded one time anyway so it will not waste resource to decode same stream twice. Also if you have some filters in a middle, you may add asplit filter to have several identical outputs.

i mean. here is a more concrete example

i have stream 1 defined as mpegts in name stream1in, defined in mpegts out as stream1out, app name appout. When i make a transcoder scenario video input and audio input i must put in stream name and app name which is stream1out and appout, i dont have the option to tie the transcoder to stream1in . the scenario is i want to activate the transcoder whenever i need without needing to use a difefrent output name so if i have appout/stream1out.mpegts without transcoding active, i want to be able to have appout/stream1out.mpegts with transcoding active, not have to create some other output or the workaround i could only think of is when i start the encoding i name the output something else like stream1out2 then start the transcoder with the input set to stream1out2 and the output with the previous output name stream1out

Does that make sense?

You may setup republishing of stream1in by RTMP or RTSP to same server (e.g. rtsp://127.0.0.1:554/appout/stream1out), and pause republishing while transcoder is active - so it will be either copy of source stream or transcoded stream.

i have same issue the system is nice but seams is made on mess and hard to understand ...... in my case i have SRT in from vmix and i setup udp output get stream out all fine .... but this stream iun is 6mbs i need to make 3mbs i setup transcode scenario to get 3 mbs but how to add this to my UDP output instead of raw srt input

1. You need to configure the MPEGTS Out Settings for your Incoming SRT Stream.
Then you will see a Stream in your Live Stream list with the Source Protocol "MPEGTS".
2. Open Transcoder and set this MPEGTS Stream as Video and Audio Main Stream.