[linrad] Re: sound card requirements?

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Dan,

> What is the preferred architecture for a linrad sound card?
There is no preferred architecture for the soundcrd.

First of all, Linrad is not designed for any particular
hardware, it is intended to fit anything that can transfer
digital data into a PC with Intel 86 architecture.

In a longer time scale, this will be entirely digital units
like the SDR-IQ and SDR-14 (which work with Linrad) but 
in the near future various converter schemes will be the 
most common radio hardware for Linrad.

There are two choices, the filter method, like in an ordinary
SSB transceiver, where you feed a single audio signal into
a soundcard for a single RF channel. Here is some info on
my first system of that type:
http://www.sm5bsz.com/pcdsp/hware.htm (and links on it)
At that time the "standard" was the Soundblaster 16 soundcard.
With this kind of hardware it is probably ok to use any
soundcard because the soundcard is protected from
out-of-band signals by the filter.

One can also use the phasing method, 'direct conversion', 
and feed two audio signals into the soundcard for each 
RF channel. Soundcards that do not have proper anti-alias
filters are useless in this context. I have never seen one
myself but I have read about them. Any 'normal' soundcard
will do the sampling at a high multiple of the output
sampling speed (64 times higher for example) Then the alias
comes at perhaps 10 MHz and the card only needs an RC
filter in hardware to be well protected from primary aliasing.
The soundcard will then have a digital anti-alias filter
that is applied before the decimation process that results
in the 48,96 or 192 kHz output. 

There is no reason to buy a soundcard that has far better
performance than your analog hardware. I have not followed the
market so I do not know what is available today. I suggest you
start by using whatever your computer is equipped with to
start with. It is not difficult to find out if it limits
your system performance.

> How effectively is it abstracted by OSS or ALSA?
????????
I do not understand this question.

> My current card a  Intel Corporation 82801DB/DBL/DBM. This apparently 
> does not support simultaneous input and output. I am not sure if this is 
> a hardware, driver, or configuration issue. Does anyone have any 
> information on this or other cards?
When searching the Internet, I find this is some kind of
ICH4. It seems unlikely to me that it would not support
simultaneous input and output. Have you really tried?
Probably it only has RDWR mode, that means that input
and output have to run with identical parameters.

73

Leif


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