1 Feb
2011
1 Feb
'11
6:55 a.m.
Hi All,
I am trying to read data from a usb microphone and using the pretty standard
method of using ioctl's to setup the sampling rate, channels, bits and block
size . This all works so the device is correctly setup. I then use "read" to
read samples from the device which shows up as /dev/dsp1. I get a lot more
samples from this read command in one second of recording than the set
sample rate. E.g. if i set 10Khz on one run i got 269312 samples. Looking at
the raw data it looks like there is a lot of duplication of data? is this
common for the audio input device? if so what kind of encoding is it (e.g
with some specific redundancy built in)?
thanks
farhan
1 Feb
1 Feb
8:49 a.m.
On Mon, 2011-01-31 at 21:55 -0800, farhan baluch wrote:
Hi All,
I am trying to read data from a usb microphone and using the pretty
standard method of using ioctl's to setup the sampling rate, channels,
bits and block size . This all works so the device is correctly setup.
I then use "read" to read samples from the device which shows up
as /dev/dsp1. I get a lot more samples from this read command in one
second of recording than the set sample rate. E.g. if i set 10Khz on
one run i got 269312 samples. Looking at the raw data it looks like
there is a lot of duplication of data? is this common for the audio
input device? if so what kind of encoding is it (e.g with some
specific redundancy built in)?
thanks
farhan
OSS has been obsolete for over a decade. Don't use it.
What have you got the sample rate set to? It's possible that your card
isn't capable of reading at that rate so it goes to the nearest sample
rate it does have and then interpolates.
If you really must deal with audio hardware directly, use ALSA. Even
better, use jack and abstract all that messy cruft away into a handy
server.
Gordon MM0YEQ
8:57 a.m.
Gordon JC Pearce wrote:
On Mon, 2011-01-31 at 21:55 -0800, farhan baluch
wrote:
But it's still supported. Of course, this API must be used correctly,
i.e., after setting parameters, one has to check whether the device has
accepted the value or has changed it to something supported.
I am trying to read data from a usb microphone
and using the pretty
standard method of using ioctl's to setup the sampling rate, channels,
bits and block size . This all works so the device is correctly setup.
I then use "read" to read samples from the device which shows up
as /dev/dsp1. I get a lot more samples from this read command in one
second of recording than the set sample rate. E.g. if i set 10Khz on
one run i got 269312 samples.
OSS has been obsolete for over a decade. Don't use it. What have you got the sample rate set to? It's
possible that your card
isn't capable of reading at that rate so it goes to the nearest sample
rate it does have and then interpolates.
In that case, it doesn't interpolate, it just returns data at the supported
rate.
Regards,
Clemens
1:39 p.m.
On Tue, Feb 1, 2011 at 2:57 AM, Clemens Ladisch <clemens(a)ladisch.de> wrote:
Gordon JC Pearce wrote:
Its no longer supported by at least two distributions, AFAIK.
On Mon, 2011-01-31 at 21:55 -0800, farhan baluch
wrote:
But it's still supported. I am trying to read data from a usb microphone
and using the pretty
standard method of using ioctl's to setup the sampling rate, channels,
bits and block size . This all works so the device is correctly setup.
I then use "read" to read samples from the device which shows up
as /dev/dsp1. I get a lot more samples from this read command in one
second of recording than the set sample rate. E.g. if i set 10Khz on
one run i got 269312 samples.
OSS has been obsolete for over a decade. Don't use it. 
4:05 p.m.
On Tue, 01 Feb 2011 07:49:15 +0000
Gordon JC Pearce <gordonjcp(a)gjcp.net> wrote:
On Mon, 2011-01-31 at 21:55 -0800, farhan baluch
wrote:
OSS is alive and well, and actually better than ALSA in some respects.
It does software mixing well, and the API is reasonable.
It's not supported on Linux out of the box though. If you are using
ALSA's OSS emulation layer, that one is obsolete, so you should prefer
the real ALSA API.
Hi All,
I am trying to read data from a usb microphone and using the pretty
standard method of using ioctl's to setup the sampling rate, channels,
bits and block size . This all works so the device is correctly setup.
I then use "read" to read samples from the device which shows up
as /dev/dsp1. I get a lot more samples from this read command in one
second of recording than the set sample rate. E.g. if i set 10Khz on
one run i got 269312 samples. Looking at the raw data it looks like
there is a lot of duplication of data? is this common for the audio
input device? if so what kind of encoding is it (e.g with some
specific redundancy built in)?
thanks
farhan
OSS has been obsolete for over a decade. Don't use it. 
4:16 p.m.
OSS has been
obsolete for over a decade. Don't use it.
OSS is alive and well, and actually better than ALSA in some respects.
It does software mixing well, and the API is reasonable. It's not supported on Linux out of the box though.
If you are using
ALSA's OSS emulation layer, that one is obsolete, so you should prefer
the real ALSA API.
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11:37 a.m.
On Tue, Feb 1, 2011 at 7:55 AM, farhan baluch <farhanbaluch(a)gmail.com>wrote;wrote:
Hi All,
I am trying to read data from a usb microphone and using the pretty
standard method of using ioctl's to setup the sampling rate, channels, bits
and block size . This all works so the device is correctly setup. I then use
"read" to read samples from the device which shows up as /dev/dsp1. I get a
lot more samples from this read command in one second of recording than the
set sample rate. E.g. if i set 10Khz on one run i got 269312 samples.
Looking at the raw data it looks like there is a lot of duplication of data?
is this common for the audio input device? if so what kind of encoding is it
(e.g with some specific redundancy built in)?
You need to use the correct ioctl()'s to set up the data format before you
read from the device. After you've done that, the data will make sense to
you. If you just start reading from the device, you don't know the sample
rate, bitdepth and channel count. I suspect the "duplication of data"
happens because the device is sending data in two channels. (mono in
"stereo")
Sampo
5072
days inactive
5072
days old
linux-audio-dev@lists.linuxaudio.org
6 comments
7 participants
participants (7)
-
Clemens Ladisch -
farhan baluch -
Gordon JC Pearce -
Johannes Kroll -
maex＠firstfloor.org -
Paul Davis -
Sampo Savolainen