Re: [Jack-Devel] JACK Capabilities

Hi David,

Thanks for your inputs.

> You could write a client that would do this, but it would be 
meaningless, since the only potential destination for the converted data 
would be a JACK sink, which is running at 48khz.

Well in my system I get input speech sampled at 48 KHz which needs to be sent to a BT device connected on a I2S line operating at 8 KHz.

> What's HMI?
Communication between clients is probably best done through MIDI or OSC, 
though there's no reason you couldn't use HTTP, or define your own 
protocol on top of TCP or UDP. JACK doesn't define a way of 
communicating between clients except audio and MIDI.

HMI = Human Machine Interface.
In my design all JACK clients would reside in the same system. Probably my question was not framed properly. Say in case of a mobile phone, while the audio is played through a player, the PCM data gets routed through some equalizer (a JACK plugin). Now the user changes the equalizer settings (gain/attenuation for some specific band) through some UI. How can we reset/set the equalizer (JACK client) with the modified values?

> Probably the most helpful thing for you to do at this point is to 
download and install JACK and some example applications like JAMin, JACK 
rack, a synth or two, and just play with them for a while. Once you've 
done that, you'll have a much better understanding of what JACK is and 
what it's for.

Yes that would be my next step :). But unfortunately I do not have the time on my side as the decision needs to be taken within next few days :(

Thanks for your help again.

Have a great day/evening.

-Rgds
Vallabha   



-----Original Message-----
From: David Nielson [mailto:[hidden]] 
Sent: Wednesday, April 13, 2011 9:44 AM
To: Hampiholi, Vallabha Vasant
Subject: Re: [Jack-Devel] JACK Capabilities

On 04/12/2011 09:17 PM, Hampiholi, Vallabha Vasant wrote:
> 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 could write a client that would do this, but it would be 
meaningless, since the only potential destination for the converted data 
would be a JACK sink, which is running at 48khz.
> 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?
What's HMI?

Communication between clients is probably best done through MIDI or OSC, 
though there's no reason you couldn't use HTTP, or define your own 
protocol on top of TCP or UDP. JACK doesn't define a way of 
communicating between clients except audio and MIDI.
> A few more queries:
> 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?
If a JACK client that's part of your signal chain dies, then your client 
will stop processing. This is what you want, since any more data coming 
in might be unsafe to process on. If a JACK client that's not connected 
in any way to your client dies, then your client will continue to 
process as if nothing had happened.
> 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.
"Plugin" implies something like LV2 or LADSPA, which (can) operate on 
top of JACK, but which aren't really part of it.

Writing a JACK client isn't difficult, but it does require you to make 
some design decisions up-front--the thread which does audio work needs 
to be driven by a process callback that you register with the JACK 
daemon, and that thread really, really needs to not do certain things 
like disk access or memory allocation. I'm simplifying here; there is 
really good documentation on the JACK website.

Basically, what you need to know about writing a JACK client is this: 
you can't expect to write a PA, ALSA or OSS-capable program in isolation 
and make it work with JACK later. Your application really needs to be 
designed correctly from the start.
> 3) Is there a command line tool to test the JACK plugins like gst-launch for GStreamer?
Once the JACK daemon is running, you can start any clients you like from 
the command line, and use jack_connect and jack_disconnect to experiment 
with them; however, using qjackctl or patchage to manage connections is 
much, much easier.
> 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).
Not exactly. Jack sources and sinks are either hardware audio interfaces 
(PCI, USB or Firewire, mainly, using either the ALSA or FFADO backends) 
or a client program. A file can't be a source or a sink directly, though 
it would be fairly simple to write a client to read a file and play it 
through JACK, or record JACK data to a file. (I know examples of these 
clients exist, and you can find them. I'll not do your homework for you. ;-)

Probably the most helpful thing for you to do at this point is to 
download and install JACK and some example applications like JAMin, JACK 
rack, a synth or two, and just play with them for a while. Once you've 
done that, you'll have a much better understanding of what JACK is and 
what it's for.

David

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