David's notes are definitely germane for sound sources that are close to bo=
th mikes/recorders. It's not a good idea, for instance, to mike a piano wit=
h two separate recorders...the drift between the two clocks may be only say=
a quarter of a second across an hour, but the comb filter effect of the gr=
adual shift will be audible.
In a nature recording scenario, where the microphones/recorders may be sepa=
rated by many feet, the amount of audio that is common to both recordings i=
s tiny. If you already have many milliseconds of delay for sound arriving a=
t Recorder B from Point A and vice versa, the timing shifts due to clock er=
rors are much less noticeable. I did a project for NPR/NGS a while back tha=
t involved five different portable DAT recorders capturing mikes that were =
as much as a hundred feet separate from one another. There was timing drift=
, but little audible effect.
Timing drift has two components: steady drift (when two clocks are each ver=
y stable but slightly off one another) and variability (when a given clock =
has a range of frequencies that it drifts within). If steady drift is your =
issue, then that can be corrected to minimize audibility using time compres=
sion or expansion in a computer DAW. If variability is the issue, it's much=
harder to correct.
If the recording is for scientific analysis, different standards of accepta=
bility may apply than if the recordings are just for personal enjoyment or =
for media release.
> 3n. Re: Simultaneous recording with two digital recorders?
> Posted by: "soundings23" soundings23
> Date: Fri Jun 8, 2012 2:32 pm ((PDT))
> Thanks David, I may just try something without a common source as you sug=
gest. Following on from previous discussions, quiet soundscapes might work =
well. I'll just have a play and see what happens.
> --- In "Avocet" <> wrote:
>> I've used two "toy" Tascams together with two pairs of stereo mics.
>> Syncing the tracks is fiddly and needs more than a clap. With film
>> recording a clapper board gives accuracy to the nearest frame, but if
>> you want to match two sounds with a common source, you need better
>> than millisecond accuracy. That's why I'm saving up for a 4-track.
>> With timeode recorders, the timing accuracy is better than one frame
>> per week or around one 10 parts per million. With two free-running
>> affordable recorders you will get a drift which will produce flanging
>> from a common sound source. If you don't have a common source, exact
>> sync is not necessary anyway - just sync to a passing plane or
>> whatever. :-)
>> o.1 millisecond across a stereo image gives a noticeable shift in the
>> image - that's 5 samples at 44.1 Ks/s. One part per million drift will
>> produce that in around two minutes. Mixed, that produces a comb filter
>> effect based on 1kHz.
>> Point to ponder - how far away can a clapper board be before it is 1
>> frame out of sync? That was one of my questions for trainees. :-)
>> David Brinicombe
>> North Devon, UK
>> Cogito cogito ergo cogito sum - Ambrose Bierce