Sampling Card / Sound Card for Linux

Sakari Mattila smattila at
Mon Oct 29 20:07:09 EST 2001


The IQ radios used to use Crystal Semiconductor codecs and dedicated
CPUs. Typical was Motorola DSP56002EVM (ca. 1995) with CS4215 codec
and Motorola 56002 CPU. It went up to 50 kHz sampling frequency, thus
had about 22 kHz bandwidth, just enough for 25 kHz narrow-band FM.
Probably could run as a 115 - 128 kb/s radio modem.

Note: Quadrature demodulator is almost same as IQ demodulator.

CS4232 based sound cards are available. Crystal Semiconductor Corporation 
is now part of Cirrus Logic Company. There are good IQ codecs available
from other companies, mainly for digital mobile phones. I have seen
ones with over 1 MHz sampling rate and 20 bit dynamics.

These DSP URLs may be useful:  (RATS.FI)  (VHF Comm. index) (digital mobile phone chips) (IQ radio)  CS4232 driver for

ALSA or other Linux sound card driver collections may contain
useful code, but most cards are not designed for accurate sampling.

The second issue is latency in Linux. Even the low-latency (<0.5 ms) 
kernels are not fast enough for radio use without some kind of
slave I/O processor or hardware buffer.

Something may come out of these Linux low latency URLs:

Happy building !

Sakari, VK2XIN

> Message: 3
> Date: Thu, 25 Oct 2001 17:32:04 +1000
> To: <wireless at>
> From: Michael Farr <mfarr at>
> Subject: Sampling Card / Sound Card
> I am built a basic software radio modem (IF only at the moment, however it
> wont be hard to make an RF transmitter when I am ready), and am using my
> soundcard as the sampling unit.  The soundcard's sampling unit is slow,
> the sampling frequency is not accurate and it has a huge sample offset between
> left and right channels.  Does anyone know where I can get some kind of
> PCI stereo sampling unit that is both fast accurate and less than $(AU)200?

Sakari.Mattila at	= smattila at  =  VK2XIN / OH2AZG

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