John Birbeck from Rawmarsh asks for some technical advice:
“We want to play backing tracks from our laptop computer through the PA system. So we want to go from usb on the laptop to an XLR microphone socket on the mixer. We are using Windows Vista on the laptop but will probably play the tracks through Itunes. We had thought about buying a new mixer desk for about £500 which has a usb input. Then we wondered if we could buy some kind of digital to analogue interface device for less money and without disrupting our current physical set up, we have a lockable rack system. We use a 12 channel analogue mixer (12 channel Spirit E by Soundcraft). I’ve tried to output using the headphones socket on the laptop and connecting to the mixer through ordinary jack plugs but the sound quality is very poor. Please can you advise us?”
Tim Horton, Project Manager at SFL Group, replies:
There are two ways to play sound from a laptop:
Stereo Mini Jack into the desk input
This is the most common way of doing it. The problem with going straight from the PC to the desk is you pick up the background noise (a “whirring” sound). There are two ways of getting round this background noise problem. Either disconnect the PSU (if it’s a laptop) or use a DI box which has the ability to get rid of unwanted hum & earth problems.
USB Interface Device
The alternative is to invest in a USB pre-amp / interface device, in which you link the PC and the interface via USB (or indeed FireWire) and then XLRs out to the desk.
The USB route is technically a better quality audio; however unless you are using a top quality sound system you will not notice this quality difference. In fact, in all of our events work we run our PC feeds through a DI box – despite the fact we use top quality audio kit. The only scenarios in which we are using USB interfaces or the like is in environments when we need the highest quality audio possible; such as when commissioning an audio system. Even then, it is rare as it is far easier to use a decent CD player with high quality audio tracks.
I would suggest that looking to get a desk with a USB input is not the most appropriate route. Certainly not for the sake of being able to play sound from a PC. A far more cost effective solution is the humble DI box. There are many cheaper options out there, some are even designed to handle phono feeds from “AV” devices, such as laptops; the Radial PRO AV DI range is one such example.
The “poor quality sound” John refers to is almost certainly the “noise” generated by the PC. If you cannot fix this with a DI solution, then it may well be that there is something wrong with the computer’s soundcard?