Summing, Bussing, Digital, Semi-Digital?

[adamlloyd]

Hey Guys.. I'm sure this has been addressed somewhere on the forums but I'm having trouble finding anything...if there's any other threads you could direct me to that address the topic, please do! :-D

I am curious as to what happens differently in Pro Tools when I assign each track to the SPDIF L/R outs and record the mix through my mytek converters back into a 2-track vs. assigning all the tracks to a stereo track and recording it directly ITB to that 2-track...

I know the ITB route is just that...ITB digital summing, but what I don't get is why the other option sounds different (which it does)... considering I only have 2 outs and 2 ins of DA/AD, I know this isn't like classic "OTB" summing which requires more DA channels, etc.. It seems to me everything is still being "summed digitally" to combine all tracks for the DA L/R stereo SPDIF channels...

So can anyone tell me what is happening that is different? (besides the extra pass of DA/AD)

PS>>> I tried recording the ITB sum out of the DA and back into the AD just to level the playing field, so both options had that extra DA/AD pass...and they still sound different.

Thanks!

[Jack Ruston]

1. SPDIF is a digital connection. There will be no difference passing out and in again via digital (not sure if you'd pick up jitter or something...i'm not that technical)

2. Passing a stereo mix out through certain types of analogue circuit, particularly the more coloured transformer based circuits, like classic Neve designs, or tubes (eg culture vulture) provide a very subtle distortion which serves to limit ever so slightly, make the mix more harmonic, and widen the stereo image perception.

3. Summing a mix in analogue using stems does very much the same thing but it also allows you to be a bit fast and loose with headroom in the box, and get out of jail free at the end of it. If you're very disciplined wth your gain structure and run through a stereo chain you can achieve much the same result. If you squeeze the box too hard it gets weird quickly. This is why people find multiple stems more forgiving.

4. The summing in the box is perfect. There's nothing wrong with it. At all. In any of the major DAWs. But software doesn't knock the corners off the transients, doesn't saturate the bottom end or widen the image. It's perfect. So if we want those side effects that we've grown to love, we need to add them.

J

[adamlloyd]

Hey Jack. Thanks for the reply. I don't think I explained my question properly though. I'm definitely familiar with #2 from your post. My question is really what is "happening" differently in Pro Tools when the tracks are all bussed internally to a stereo aux or recorded to a stereo audio track as opposed to when each track is sent directly out the main outs. Both options sound different to me, but like I said, sending the digitally in-the-box summed 2-track out the DAC and back into the ADC should have made the comparison of both options variably equal (as both are passing through the conversion process).

My question pertains to the signal flow of Pro Tools, probably in regard to the mix engine or something... everything being sent out and "summed" at the main outs/spdif vs. everything being summed in the DAW via an aux bus or stereo audio track...
Any ideas?

[raal]

Originally Posted by adamlloyd83

HMy question pertains to the signal flow of Pro Tools, probably in regard to the mix engine or something... everything being sent out and "summed" at the main outs/spdif vs. everything being summed in the DAW via an aux bus or stereo audio track...
Any ideas?

for starters if you're using different converters than you have been using previously, things are likely to sound different.

even when you sum ITB, you have to be hearing playback from a set of converters, probably included in whatever interface you have at the moment. so if you go out S/PDIF thorugh another set of converters and back in again, things can easily sound different. if the outboard converters are higher quality than what's on the previous interface, the assumption is that things should sound better through them.

[kittonian]

I think I know what you're asking about. Let me try and explain.

When you combine tracks ITB by setting their outputs to a stereo aux (whether this is groups of tracks for use like bussing OR using a stereo aux or master fader for mix purposes) the digital algorithms are summing those tracks together. Thus, when you hear the output, you are only hearing a single, already summed stereo mix. Also, bear in mind that digital is unforgiving and exact. Sometimes this is just fine, sometimes it's not. All depends on what you're looking to accomplish with your sound.

Likewise, when you send one track at a time to an individual output into an analog summing device not only are you using the d/a conversion on your interface for each signal but that analog device is putting the tracks together (i.e. summing) and giving you a resulting stereo mix.

Analog is very different than digital because there are always going to be distortions in analog. The audio is traveling through wires, components, etc. By distortion I am not talking about a ripping guitar sound but rather the general definition which is anything different than the original untouched waveform (your d/a converter and summing device will play a part in some type of distortion of the signal).

Analog distortion is very pleasing to our ears because it's not always flawlessly in phase or absolutely pristine in the resulting signal. This allows us to hear spacial depth, wider stereo imaging, etc. and is why websites such as this are so incredibly popular. People are always trying to capture the magic of using high end gear because they like the distortion to the audio signal.

[adamlloyd]

Hi Kittonian... So you're saying everything is getting summed in the DAC, if i'm not assigning the tracks to any aux busses, etc? If that's the case then I'm still kind of confused, because why would people spend money on summing boxes if you can just use a stereo DAC and ADC to sum together all your tracks from Pro Tools?

I guess my question is whack or something because no one understands what I'm asking...lol.

[Jonkr]

Originally Posted by adamlloyd83

Hi Kittonian... So you're saying everything is getting summed in the DAC, if i'm not assigning the tracks to any aux busses, etc? If that's the case then I'm still kind of confused, because why would people spend money on summing boxes if you can just use a stereo DAC and ADC to sum together all your tracks from Pro Tools?

I guess my question is whack or something because no one understands what I'm asking...lol.

Because what you are doing is not analog summing. It is digital summing. Pro tools is digitally summing the at the master bus. The output of the master bus is what is sent out through your interface. Your mix is summed before it ever hits your D/A.

Analog summing boxes take multiple stems and send them through multiple, separate outputs. These output hit the D/A before going to the summing unit. The actual summing occurs in the analog domain and then the resulting analog 2-track is converted back to digital. In this case only the stems are being summed digitally due to the fact that most setups in smaller studios only have 8-16 A/D D/A converters so every single track cant have its own output.


If you like the sounds you're getting from running the mix through your Mytek that's great! But, there is definitely a big difference between analog summing and what you are doing.

[dubrichie]

i'm a little confused. are you saying that:

1) you send all of your tracks to a Master Fader, where they are summed, then out to your DA and back in through your AD.

2) you send all your tracks to an Aux Buss, where they are summed, record this onto an Audio Track, then send this Audio Track out to your DA and back in through your AD, using the exact same DA / AD chain as in scenario 1) above.

if this is a correct description of what you are doing you should not hear any difference.

so either:

a) you are doing something wrong and changing something else in the signal path, somehow.

b) you are hearing things that are not there, for whatever reason.

[adamlloyd]

ahhhh... i retract my question... sorry dudes.

i know what distortion is and what analog processes do, i know what analog summing is, what i dont understand is why sending each track to a direct out as opposed to sending them all to an aux and THEN to the direct outs sounds different. I don't use a master fader in PT, I either watch my levels on the DAC (with each track sent directly to the spdif main outs) or I send everything to an aux faux "master fader" (it sounds different from the PT "master fader" for some reason, again, I have no f-ing clue why). I noticed that each way (directly routed or stereo-auxed to the direct outs) sounded different for some reason. Both methods involved combining all tracks into a two-track digitally, I presumed.

It really doesn't matter... I'm just a little obsessive about knowing *why* I'm hearing what I hear when I work... it seems to help me make better decisions in production than just using my ears alone because even trained ears are subjective depending on the listener's mood, the position of the planets, etc etc.

[dubrichie]

unless by routing everything to an Aux Track you are causing some of the tracks to have different relative amounts of delay, from plugins or such, i can't really understand what is going on.

you should not be hearing any difference unless something else is changing.

summing signals on an Aux Track or a Master Fader in PT sounds the same.

a Master Fader is just a graphical representation of ANY Buss in PT. you can use a Master Fader to control the level of a Buss that you send to an Aux Track for example.

either something wacky is going on with delay / latency somewhere in your routing, or you are hearing things.

i don't know what else it could be.