Re: [Jack-Devel] JACK Capabilities

On Wed, Apr 13, 2011 at 05:17:22AM +0200, Hampiholi, Vallabha Vasant wrote:

> Hello Adrian,

Hi!

> I have few more queries based on your feedback :)

I just noticed your e-mail address: are we talking about embedded Linux
in cars?

If it has to be good, reliable and fast, hire Paul to do the job. He
knows all the answers, also to questions you don't even know you want to
ask them. ;)


> "There are no different sample rates, all jack clients run at the same
> samplerate."
> 
> Are there any JACK plugins which can do sample rate conversion? Say if
> I get my speech data from the sound card @ 48 KHz but I want to do
> some ECNR on this. But to do that I need to convert the 48KHz signal
> to 8 KHz. Is there a plugin that can do this?

You can use a readily available library to do SRC in your jack client
(running at 48kHz). libsamplerate jumps to my mind.

If you do all processing at 8kHz and only want to interface to the 48k
world outside, you can run jackd in dummy mode at 8kHz and use alsa_in
and alsa_out to resample from/to 48kHz. If you want to do music, then
this is clearly not an option.

> I have another query now. From my project perspective, say if I have a
> JACK client which is doing some equalization. Now while this is
> happening, the user changes the equalizer parameters on the HMI. How
> will this get communicated to the JACK equalizer client?

The jack client usually is a process, same as any other application. In
case of a console programme, you'd use ncurses or whatsoever. If it's a
GUI, you'd use QT/GTK/foo. You can also use sockets, shared memory, OSC
or any other data to get the user input to your client. It's a fully
featured programme, you only have to propagate the changes to the
processing thread. That's simple if you go for a global variable in your
client.

> 1) I read somewhere that a JACK client failure can stop other clients
> from running. If true, is there a way to overcome this without
> introducing major latency/sample drop-outs?

There has been some work for this in jackd2, a client is allowed to be
late without causing too much trouble.

> 2) How easy/difficult is it to write a JACK plugin. I know that this
> could be relative, but consider a comparison with PA in terms of
> effort and testing.

That's the nice thing: pretty easy:

   http://trac.jackaudio.org/browser/trunk/jack/example-clients/simple_client.c


> 3) Is there a command line tool to test the JACK plugins like
> gst-launch for GStreamer?

You don't need any. Your app *is* the command line tool, all GUI
operations are optional.

And yes, you can operate jackd without a GUI: jack_connect connects
sinks and sources, jackd is the daemon, jack_lsp lists ports. That's it.

> 4) So according to your reply for the 1st question I will assume that
> I can create a JACK pipeline that can take input from a source (file,
> mic, internet etc) and send the same to a output sink (file, speaker,
> BT etc). 

Yeah, works.


Cheers

-- 
mail: [hidden]  	http://adi.thur.de	PGP/GPG: key via keyserver

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