Full-duplex questions

Guenter Geiger guenter.geiger at psp-vie.be.philips.com
Tue Dec 22 01:05:26 CET 1998

> -----Original Message-----
> From: Michael Lechasseur [mailto:mlecha at music.mcgill.ca]
>     Is there a simple way to test for a driver's full-duplex
> capabilities
> outside of pd?  I've tried mixing several inputs sources such
> as line-in, cd
> audio in, and playing an mp3, which can all heard via the sound cards
> output.  This suggests to me that my sound card and drivers
> are full-duplex
> capable outside of pd.  Or is this a naive assumption?

Be careful here. Most of the times, the output you hear is the monitor output
from the soundcard. This means, the cd in for examples doesn´t even go through
the adc - dac section. This is all done  by the analog mixer of the card.

Most of the times full-duplex - not full-duplex is a feature of the card, every
more or
less serious driver would then support that mode (The full duplex mode of
standard SoundBlaster cards is an exception here - full duplex drivers use a
sort of
"crippled" 8 bit mode for either record or playback).

What makes pd that critical about it, is latency. Pd tries to allocate
as small as possible. There are certain limitations for this audiobuffer size.

>     The doc's for OSS/Free suggest that these drivers are
> half-duplex only.
> I didn't actually try it outside of pd before I bought the commercial
> drivers.  Are you getting full-duplex audio outside of pd
> using OSS/Free?

The PCI128 is definitely full duplex :), so yes I get full duplex with OSS/Free
large audiobuffer size.

>     I am unfamiliar with the audiobuffer size parameter.  Do
> I want to set
> this for the driver or loading pd?

just start pd with:

# pd -audiobuf 100

for example, where 100 is the length of the audiobuffer in milli seconds.

>     I have found the OSS email tech support to be excellent
> and really fast.
> Which is why I paid for the commercial drivers.  Why don't
> you get in touch
> with them?

Well, if I really started to comment on this topic ......
Basically, my opinion is, that Linux got that stable because of open source ...
see ?
Still I want to support OSS/Linux too, it´s just not high priority for me ..

>     Hey, I still have my old SB16.  Maybe I can use that as
> an input device
> and use my PCI128 as output.  I doubt that win95 would like that
> configuration, but ALSA supports it.  Thanks for the idea.

Mhmm .. yes could work.

>     I found that the ALSA drivers sounded better than the OSS
> ones, but have
> no testing gear to prove it.

I can only imagine that some filters and/or 3D support are set differently,
otherwise the sound is a feature of the card, not the driver.
ALSA lets you control this settings, whereas I´m not sure if OSS does.
Check out the ALSA mixer.

>     ha!  I have the ambition, but I doubt the technical knowledge yet.
> There are people in the know here at school though.  Maybe
> between I can
> convince them to give a hand for a good cause.
Convinve them to play around with pd, maybe they will start to get interested


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