[LAU] Problems with sample rate and recordings (Mackie onyx > 1640-i < Firewire > Debian )

sub subvertao at inventati.org
Mon Mar 2 23:03:57 UTC 2015

Hash: SHA1

The problem is not the sample frequency of the final audio mix.
What i think is happens here is that most probably the Onyx and the
debian recorder are not using the same sample rate at the time of
recording, so the samples that are taken every second on the mixer
(probably at a frequency of 48Khz?) are being spreaded over a 44.1Khz
file, generating a 44.1Khz audio file that is longer in time, with a
consequent change in pitch. Now, I have no information about what
sample frequency the onyx 1640i is synched at, since the firewire
module used by jackd does not "see" the onyx as an audio card, and
doesn't seem to have the possibility to set the sample frequency of
the mixer through jackctl. What i think it's happening is that jackd
is simply setting its own sample rate, and not synching the mixer or
being synched by the mixer.

In pro-audio environments these kind of problems are normally avoided
by using a word clock coax cable between the two devices, or by using
AES-EBU connections (that carry the word clock on one pin, and
therefore synch all devices in the chain automatically) but i ignore
how this is achieved with the only use of firewire (no word clock coax
connectors on the devices either here). The drivers for Mac OS and
Windows provided with the onyx give at least the possibility to
monitor and set the sample rate the onyx synchs to, so that an
operator can at least choose safely at what sample rate to record
(although this does not, in my experience of digital audio, give the
guarantee of a perfect synch). Here i don't even get a debug line
about the onyx synch to sample rate in the jackctl log file, so what i
can come to is only assumptions.

Playing mp3s (so 44.1Khz files) with mocp through the onyx by using
jackctl has the effect of the songs improvisely changing pitch and
speed, without actually having any change in what jackctl shows as a
sample rate in its panel. This means obviously that the onyx is
switching between different synchs without jackd noticing it.

On 02/27/2015 05:06 PM, Chris Caudle wrote:
>> Date: Thu, 26 Feb 2015 17:18:35 +0100 From: sub
>> <subvertao at inventati.org> Message-ID:
>> <54EF475B.8040601 at inventati.org> i received a mail from the band
>> in which they tell me that the mix is faster and at a higher
>> pitch than the audio they recorded through a video camera at the 
>> concert.
> Video cameras record at 48k rate, so someone will need to rate
> convert. Either the camera audio will need to be converted to 44.1k
> or your audio will need to be converted to 48k. Which you choose
> probably does not matter, but everyone involved needs to know and
> agree on the chosen project sample rate. If the final output will
> be a video then converting every thing to 48k is probably the
> easiest choice.  If the final output will be a CD (or audio only
> files) then probably 44.1k is the easiest choice.  If you will
> have both a video and audio only files, my choice would be working
> in 48k and then sample rate convert the final mix to 44.1k for CD.

Version: GnuPG v1


More information about the Linux-audio-user mailing list