USB to USB-B or Line Out to Line In for playback from PC to Qu-24

[gilly]

Hi Folks
Just wondering which way you guys connect up from PC to QU-24 (or 16 or 32) desk for playback sounds (from Youtube, i-tunes, videos etc) to Qu-24 desk. There are a couple of ways (I’m sure there are more but lets go with these 3) to playback music from PC to Desk:
1) Line Out (or headphone) on PC to ST1 Input on Qu-24
2) USB on PC to USB-B on Qu-24.
3) USB on PC to ST1 Input on Qu-24 via external USB to Jack DCA converter

I have heard that Option 1 can have mains hum issues from the PSU transformer proximity to th PC sound card, have anyone experienced this?

[Anonymous]

Yes – although it’s often worse with laptops…

Personally I’d use 2 if I could, but 3 is valid, don’t buy another card until you’ve tried 1 though – if you’re thinking youtube quality then I’d not get het up about it…

[JD]

I often run iPod to ST-1 ’cause it all fits in my pocket – not had a problem with hum, and it sure is easy. FWIW, my audio files on the iPod are mostly lossless imports from the CD, and ‘quality’ has in general not been an issue.

[Andreas]

If PC stands for desktop, I’d go through a line isolator in any case, since stationary computers are likely to be earth grounded and will produce a ground loop with other gear (if not already with the mixer). Portable units are normally isolated and should work either way.
Option 1 is obviously worst, not only because of the non-symmetric connection, but also since most integrated sound chips are not well enough designed to produce a clean signal at all. If the output itself is clean enough (to get amplified without noise in order to be sent to a PA), a line isolator with symmetric connection to the Qu (not ST3!) may work very well though.
If the integrated sound chip isn’t clean enough, an external audio interface along with line isolator may be fine (Option 3). This must not necessary be USB, many modern PC/Mac have an optical S/PDIF output as well, so a simple S/PDIF to Audio interface will do and you’ll get line isolation for free.
For Option 2 you should be aware that even if you only need two channels, the USB-B connection will always be a multichannel audio interface and require a decent quantity of system performance to run reliably. And it may be difficult to send your audio to the correct pair of channels, if your applications are not multi-channel aware.
You’ve missed Option 4 to use AirPlay… 😉

[Anonymous]

Why run the iPod to ST1 rather than ST3?

[Andreas]

Why run the iPod to ST1 rather than ST3?

I also prefer ST1/2 over ST3 for the only reason of getting rid of the additional cable on the front. I’m not a big fan of these mini TRS.

[gilly]

On our old analog GL2000 desk we always connected it as per option 3 with a small external DCA but when we got the new Qu-24 one of our sound guys (who’s works in computers) set it up with USB to USB-B to use as playback from PC to desk and also to record from desk to PC (using Audacity), but he couldn’t get the playback to work this way, but it works fine for recording the talks. I thought it would just be a matter of patching ST1 to USB-B (i.e. change ST1 input source from “LOCAL” to “USB”).
He also tried option 1 (PC Line Out to Qu-24 ST1) for playback audio from PC to desk and said he was getting a distorted sort of under water type sound and said this was probably due to mains hum from PC PSU transformer due to proximity to sound card. Go figure (as ye say in the US 🙂 )

[gilly]

Yes Andreas, by PC I meant fixed desktop.
Re ground loops, the Qu-24 and PC are wired from the same 230V mains earth via 230V socket wired back to MCB/Fuse Board earth bar, so why would you get ground loop currents here?

“For Option 2 you should be aware that even if you only need two channels, the USB-B connection will always be a multichannel audio interface and require a decent quantity of system performance to run reliably. And it may be difficult to send your audio to the correct pair of channels, if your applications are not multi-channel aware.”
Don’t understand what you mean here, for Option 2 I’m just talking about playing back audio (e.g. from youtube or i-tunes) to ONE channel (ST1) via the USB B streaming?

Re Option 3, this is an interface box with USB connection (which connects to PC USB port) and a mini-jack Line Out connection (which connetcs to Qu-24 ST1 input). I am calling this interface box a DCA, Digital (USB) to Analog (Line Out) converter, is this correct?

[Anonymous]

I have a right angled mini Jack lead, so the iPod sits under the desk, and the cable is no higher than the right angled USB lead I have next to it…

A USB soundcard is just a sound card, it just happens to be external, which offers potential benefits in terms of getting the sound card your PC manufacturer was too cheap to give you…
The desk is basically a multichannel soundcard, even if only two channels are in use the PC has to track all of them.

[gilly]

Bob, but is what you are calling a USB soundcard not a DAC as it converts the digital from USB into analogue to go into ST1 analogue input.
And there is a soundcard in the PC, connected to the green Line Out connector, so Option 1 is viable, albeit with ‘underwater’ type sound…

[Andreas]

Ok, some technical clarifications needed… 😉
The USB-B connection realizes a multi-channel, bidirectional audio stream of 32in-to-32out on a Qu32 (less on Qu24/Qu16). This is great for multichannel DAW operation but may be a nightmare to operate from non-multichannel-aware applications (i.e. iTunes), since it may be hard to get the audio streams to the channels you want them to go. And, for the same reason, if you need one or 32 channels, the physical link and driver always streams 32+32 channels (Qu32), and this is where the technical performance is required.
If you think you do DAW recording and and playback through the same USB-B connection but from different applications, this may easily fail, since the driver can not merge two audio streams (which requires some digital mixing which we purists do not want at all).
Consequence: Record via USB-B to the PC and choose something different for playback (i.e. stereo USB audio interface).

And if both, mixer and computer, are grounded on the mains side and have an additional ground connection on your audio link, you do have a perfect ground loop which is sensible to any kind of EM fields (i.e. transformators, switching power supplies, mobile phones…). I highly suggest to use some sort of line isolator in this setup (or check if your PC has a S/PDIF output).

You may try to use a pseudo differential connection between your PC line out and ST1/2 with connecting shield from the PC side to both rings on the Qu side and leave shield on the ST1/2 connectors open (or via some resistor to GND). Could help sometimes.

[Anonymous]

“Bob, but is what you are calling a USB soundcard not a DAC as it converts the digital from USB into analogue to go into ST1 analogue input.
And there is a soundcard in the PC, connected to the green Line Out connector, so Option 1 is viable, albeit with ‘underwater’ type sound…”

Only as much as the soundcard in the PC is a DAC (converting digital from the PCI bus to Analogue) – they both *contain* a DAC, but they do a whole lot more (much of which you probably don’t care about)

Maybe it’s semantics, but I think you’ll find the search for a USB card much easier than a USB DAC 😉

[gilly]

Yes Bob I realise the sound card in PC does more than convert from Digital to Analog, so do you reckon this (see attached) small box we have on top of sound desk cabinet is in fact an external ‘sound card’ rather than just a converter ? There is no manufacturers name or model number on it?
USB IN (BTYPE) connects to PC USB
FRONT OUT connects to Qu-24 ST1

[gilly]

Bob I realise the sound card in PC does more than convert from Digital to Analog, so do you reckon this (see attached) small box we have on top of sound desk cabinet is in fact an external ‘sound card’ rather than just a converter ? There is no manufacturers name or model number on it?
USB IN (BTYPE) connects to PC USB
FRONT OUT connects to Qu-24 ST1

[Andreas]

If you have USB->Box->Audio then the Box does contain a USB-to-Audio-DAC (=”USB Soundcard”).
…and even it is external this does not break your ground loop, since neither (bus powered) USB nor audio is normally ground-lifted.

[Author]

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