[wdmaudiodev] Re: Building an USB audio interface with a PIC18F4550

  • From: Tim Roberts <timr@xxxxxxxxx>
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Mon, 26 Mar 2007 10:55:32 -0700

Elios Elios Elios wrote:
> I'm making my end of career project (telecomunications engineering),
> that consist in a USB interface for capturing audio by 4 channels. We
> are using as IC the PIC18F4550 of Microchip. We have made the PCB, and
> we have comunicated the PIC and the PC with an C# example that we have
> found in the WEB. This example is for low speed, and for audio isn't
> very good...

To do 44.1kHz audio at 16 bits per sample with 4 channels takes 353
kB/sec.  That's double what a low-speed device can do.  You'll need full

> So, what is the better way to make an USB Audio Interface?
> I read the "USB Device Class Definition for Audio Devices 1.0" and the
> 2.0... I think that if we follow the specification, we could to
> control the interface with the driver Usbaudio.sys and programme it
> with an Native API like WDMAudio Driver (winmm.dll, isn't?), or Core
> Audio ...

Yes, the best way to build an audio device is to meet the USB Audio
Class Spec, as you describe above.  The trickiest part is creating the
descriptors to describe your architecture, and even that's not too hard;
there are samples in the documents on the USB web site.  You will want
to do your streaming with an isochronous pipe.

> I read that there is a problem with USB 2.0 (the PIC18F4550 works in
> USB 2.0, full speed) and Usbaudio.sys... Somebody knows it?

Not that I know of.  There are a few discussions of this on the
Microchip web site, but nothing negative.

> Maybe the other solution can be to make a driver with Windriver... I
> have listened that is very easy to do. But in this way only we could
> to control our interface with our own driver... isn't?

Making a simple driver with WinDriver might be easy, but making an audio
driver of any kind is not easy.  Unless you run into some very serious
problem, you should strive to make your device USB Audio Class compliant.

Tim Roberts, timr@xxxxxxxxx
Providenza & Boekelheide, Inc.


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