Since I only have a four track, busses 1-4 would be mainly left for the tape return but available for tracking too. Busses 5-8 would be mainly for the monitor mix.
The goal is to use less cabling mainly... I can assign the channel that I’d like to compress to the PGM channel and use that insert
Okay. I think I understand what you are trying to accomplish, but a couple pieces need teased out here.
You refer to “busses 1-4” and “busses 5-8”. I think you’re talking about the Monitor mixer section. Those aren’t busses. A “buss” in terms of audio mixing is a signal pathway upon which multiple other signal pathways (aka channels or other individual inputs) can hop on board and go to the same place. It is a summing pathway. If I’m correct that your references to “busses 1-4” and “busses 5-8” are the Monitor mixer channels, those are channels, not busses.
The Monitor mixer is like a mini version of the main part of the mixer with the 12 channel strips. Think of the Monitor mixer as 8 mini channel strips. But they are channels, not summing busses. Why? And who cares about the distinction? It’s important. Each Monitor mixer channel has two possible inputs:
1. An individual tape return (each Monitor mixer channel numerically corresponds to a tape return jack...Monitor mixer channel 1 can source tape return jack 1, channel 2 jack 2, etc.) -OR-
2. A PGM buss, post-fade and post-insert...monitor mixer channels 1 & 5 can source PGM 1, channels 2 & 6 source PGM 2, etc.
But it’s an either/or situation...a Monitor mixer channel can source either the tape jack or the post-fade, post-insert PGM group sum. There’s no summing there. The Monitor mixer channels are not busses, they are channels.
And then on the output side those Monitor mixer channels have how many places they can go? On the M-312 the Monitor mixer outputs can go up to two places (kind of three) simultaneously: an AUX buss (AUX 3 for channels 1-4, and AUX 4 for channels 5-8), and the L-R main stereo buss (and that’s sort of two places...there’s two channels in that summing buss, but it’s a ganged stereo buss so we consider it one place, but you have control over how much in each L or R buss with the PAN control).
In a live FOH setup you could use the monitor mixer as combination of effect returns via the tape jacks to the house, and monitor feeds for the stage via the AUX busses (leaving the ON switch unlatched so your feed doesn’t go to the L-R buss and the house), and in recording the monitor mixer is also a powerful tool as you are using it...a combo mix engineer monitor matrix for tape returns and tape sends it however you are using the PGM groups.
Remember, in the big picture, a mixing console is simply a device that takes multiple inputs, and sums them and/or routes them to one or more outputs. As we drill down from the big picture we see the typical mixing console typically has inputs organized into channel strips, and often also has strips for output summing busses. Your M-312 has primary inputs organized into 12 channel strips, then there’s eight more mini channels (the monitor mixer) that have a mix of primary and secondary inputs, and then there are primary summing outputs organized into six strips: the four PGM groups, the L-R main buss. The MONO buss *is* one of the console’s summing busses and it’s there and can’t be ignored, but there’s no direct access to it from any of the primary inputs (except for the L-R SUB IN jacks), so it is otherwise a secondary summing buss...a sum of a sum.
So back to your question about using a PGM group insert for your dynamics processor, yes you can do that, just keep in mind that’s a summing buss, and if you send multiple signals to that buss and insert the compressor, you can’t separate that back out later. Usually a compressor is used as an insert effect: the effect is applied 100% and there’s no dry signal (although there’s a hybrid approach with parallel compression...topic for another day...Google it). So if you want to compress your kick drum and bass guitar, and you assign them both to PGM 1 and compress, your kick track and bass track are now married...no divorce possible. I don’t know about you but I set my compressor settings very different for those two types of sources. That just wouldn’t work for me. However, If you are only sending one source to that PGM group and are simply wanting to leave the compressor inserted on that group so you can, at will, assign individual sources to that group and compress without having to hop the compressor from input channel insert to input channel insert, and I think this is what you are talking about and wanting to achieve, then absolutely yes. That is a good use of the PGM group with insert...flexible and convenient control surface fingertip management of signal routing and processing without having to plug and unplug.