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Re: [Discuss-gnuradio] Re: FUNCube dongle

From: Alexandru Csete
Subject: Re: [Discuss-gnuradio] Re: FUNCube dongle
Date: Fri, 25 Feb 2011 18:45:29 +0100

On Fri, Feb 25, 2011 at 5:36 PM, Moeller <address@hidden> wrote:
On 24.02.2011 15:46, Patrick Strasser wrote:
> Just like every USB sound interface it does not matter where the signal
> comes, where it is going and how things behind the interface work. It
> makes no difference to your application if you connect a converter via
> cable to your sound interface in your computer or if you have the sound
> interface built into your converter.

But I think it's a big difference in signal quality.
There's no IQ imbalance due to L/R audio channel differences,
no disturbed analog audio frequency response between dongle and PC.
And I suppose in audio you have a spectral gap in the audio bass/LF region,
a gap near the baseband center frequency. This is far more than just a single
FFT bin (DC offset) in direct conversion receivers.

There is still an audio codec - the difference is that it is in the Funcube Dongle rather than in the host computer. Moreover, the IQ imbalance is not only due to audio hardware but also due to the qudrature demodulator, so DC offset and phase error is pretty much a "feature" of all direct conversion type receivers (and transmitters) where you process the I and Q channels in separate hardware chains. The USRP & daughterboards have it and the Funcube Dongle has it too.

I believe there is one exception, namely when not using a hardware IQ demodulator but doing the demodulation in software, e.g. on an FPGA.

The audio codec used in the Funcube Dongle is TLV320AIC3104:

> If it implements the USB Audio Class, its a USB Audio device. A headset
> works the same. That's the nice thing about abstract interfaces.

Yes, I think it's a nice abstract interface.
Do you know the theoretical limits for the sample rate?
Can it fill the full USB bandwidth or does it only accept
"standard audio" sample rates?

If you ask about the Funcube Dongle then it has a fixed sample rate of 96 kHz. It uses signed 16 bit samples, so the required USB bandwidth is ~ 3 Mbps. It can actually work on USB 1.1 hosts, though I don't know who may have such things lying around today.


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