[PlanetCCRMA] A general Jack question - which

Jorma R rjorma@sunpoint.net
Thu Jan 15 22:07:02 2004

>> Jack handles audio with a certain sample rate and other parameters.
>  Ardour saves 
>> the recorded audio as 32-bit float. Given that my hardware does the
>  AD conversion 
>> with at 16-bit the audio has to bo converted to 32-bit at some
>  point.
>> At which point does this happen?
> I have not looked at the source but that should happen as soon as the
> samples are received from the soundcard (inside jack's alsa driver).
> Jack's audio ports are (each one) mono and always transmit and receive
> 32 bit floats at the sampling rate selected when you start Jack. 
>> If it is always the same, 
>> is this bit depth defined by the ALSA settings and the hardware or
>  is it something 
>> else?
> It is "hardwired" into Jack's design. If your hardware can do 24 or 32
> bits then jack will open the alsa driver with the proper parameters,
> otherwise it will just use 16 bits. On the outputs you can specify if
> Jack will do dithering to try to minimize noise (when going down from
>  32
> bits to the outgoing 16 bit soundcard). 

Thank you for the detailed reply. This is all good news to me. I have been benchmarking the HD performance of my laptop and thought I am recording at 16-bit and 44.1 KHz that my hardware allows. The old machine  is able to record 8 simultaneous stereo tracks in Ardour. I read that Ardour always records at 32-bit but did not really get the point previously. So when I now upgrade my audio interface to something that does 24-bit my conclusion is that I can record the exact same amount of tracks that I did with 16-bit hardware. Not bad.

Two more questions I have been wondering:

-My hardware does 16-bit and 44.1 KHz. Can I run Jack and record in Ardour at 96 Khz if my hardware is powerful enough for that? 

-Latency: When I run Jack with 16/44.1 and 1024 frames the HW latency is 46.4 ms. Jack gives me the following output: load = 0.1941 max usecs: 46.000, spare = 23173.000. Does the usecs: refer to latency? Am I correct concluding that the latency caused by Jack / other software components is max 46 usecs at that point and hence the total latency is below 50 ms?

Thanks in advance,


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