Re: [Discuss-gnuradio] FM Radio USB Dongles?

[Johnathan Corgan]

On Fri, 2006-11-10 at 14:19 -0500, michael taylor wrote:

> :-) Silicon Labs makes one chipset used in some. It has a 8051
> microcontroller and a FM Tuner chip. I think all those chips output
> Left/Right audio, not I/Q.
> 
> http://www.silabs.com/tgwWebApp/public/web_content/products/Broadcast/Radio_Tuners/en/Si4700-01.htm
> http://www.silabs.com/tgwWebApp/public/web_content/products/Microcontrollers/en/USBFMRadio.htm

According to the reference design (AN264), they are taking the analog
out from the Si4700, putting it through a single pole 23 KHz lowpass
analog filter, then using the 10-bit ADC on the 8051 to go back to
digital.  The samples over the USB are at 96 Ksps, 4 bytes each (two
channels.)

10-bit audio doesn't sound fantastic but there is probably room for some
DSP tricks to improve fidelity. They are oversampling by 3 since they
are only starting with 15 KHz audio in the transmitted FM signal.

AN264 refers to a reference design for Windows that implements the "over
the USB" protocol for data and control.  It probably wouldn't be all
that difficult to hack up a GNU Radio driver that did the same, not that
I'm volunteering.  And each actual manufacturer may fiddle with the 8051
firmware to suit their tastes, so YMMV.  On the other hand, these things
are commodity priced ($30-$50 retail), so I'd bet they are barely
repackaged reference designs with some cosmetic fluff wrapped around it,
and no changes to the firmware.

-- 
Johnathan Corgan, AE6HO
Corgan Enterprises LLC
address@hidden