[LAU] JACK: inputs and outputs

Patrick Shirkey pshirkey at boosthardware.com
Sun Jul 18 12:40:39 UTC 2010

On 07/18/2010 09:46 PM, Andrew Bryant wrote:
> If I use qjackctl to start jackd, the connect window shows two capture 
> ports and two playback ports.
> I can then start a pair of meterbridge dpms, and connect their inputs 
> to the capture ports.  Signal applied to the mic jack then shows up on 
> the meters.  So I deduce that the capture ports are related in some 
> way to the mic jack.

They are capturing whatever is being sent to the internal digital input 
of the device. So anything you have enabled in the mixer settings [line 
in, mic, cd] will be heard in jack.

> If I connect between the capture ports and the playback ports, should 
> I then expect the signal to be present at either the headphone or the 
> line output jack?

Yes, if you have unmuted the master output and turned up the pcm 
channel. Some cards also have additional input controls that need to be 

> And (this may actually be the same question), the soundcard has 
> numerous analogue inputs (mic, line, CD), at least two outputs, not to 
> mention whatever the mechanism in the digital domain is to relay 
> internet radio etc.  Can they all appear as ports in the qjackctl 
> connect window?  Is there some special imprecation needed when I start 
> jackd?

This is dependant on the card. If it is a multichannel device then you 
may see multiple inputs but for most(all) consumer cards the inputs are 
mixed at hardware level and presented as one (stereo) to jack.

Patrick Shirkey
Boost Hardware Ltd

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