Kendrix wrote on Thu, 14 October 2004 21:02 |
In the domain of sound waves the generally accepted rule of thumb is the 3:1 minimum ratio of distances for positioning 2 mics to record the same source. As I understand it, this distance/delay is enough to minimize the phase induced "attenuation" by offsettng the signals sufficiently to cause them to be effectively uncorrelated phase-wise for most frequencies/sources. In this case no cancellation occurs. ( real world sources are not pure sine wave generators) |
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If we apply such a rule to the electronic/digital domain it suggests that you might be OK if the differential delay between correlataed channels is greater than X. If you can tolerate X delay from a musical/timing standpoint then the phase-induced attenuation might not be an issue. |
Rick Sutton wrote on Thu, 14 October 2004 22:26 |
I've got a real world example that I'd like to get your opinion on. I'm recording a solo acoustic guitar album and am using 4 mics into 4 channels of pro tools. |
bobkatz wrote on Fri, 15 October 2004 02:49 | ||
Well, in multitracking, there usually are no correlated channels... everything's isolated if it's overdubbed. Or, if not isolated, then the correlation comes from the room acoustics and it falls into 3:1 anyway. Can you give an example of your idea and explain why this is different from the acoustical case, where you generate correlated musical signals at different times and your idea of increasing the delay to avoid comb filtering becomes relevant? I'm trying to think of a for instance... When would you even get two correlated sources into two different tracks of a multitrack except through use of microphones and normal acoustics anyway? |
bobkatz wrote on Fri, 15 October 2004 15:23 | ||
The time delay differences I've observed between different models of A/D (even when synchronized) have been as low as "less than a sample" to as much as a few samples. Very very rarely in the milliseconds. BK |
danlavry wrote on Fri, 15 October 2004 14:31 |
Hi Bob, I appreciate your participation, I learn from your comments and most often I am in agreement with you. |
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Regarding time delay differences, please look at my numbers and you will see that a few samples delay can cause a mess. A one sample delay differance at 44.1Khz is almost 25uSec - 3dB loss at 10KHz, 8.3dB loss at 15KHz... |
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What is the origin of the 3:1 rule? Is it based on experience? |
Rick Sutton wrote on Sat, 16 October 2004 18:56 |
With the "heads up" that this thread has provided, I have done several tests and come up with the following results. Compared to an 888 converter the Lavry Blue is 48 samples behind (@44.1) on the A/D and 22 samples behind on the D/A for a total of 70 samples as it appears back on the analog mixing board. |
danlavry wrote on Sun, 17 October 2004 00:02 |
I see, you are correct. We are talking about different issues. |
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What is the origin of the 3:1 rule? Is it based on experience? |
danlavry wrote on Sat, 16 October 2004 23:02 |
What is the origin of the 3:1 rule? Is it based on experience? |
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That's exactly what you had to do. And for a 3-mike setup I'm sure it will be fine, though I think it would be intuitive (and perhaps redundant) to say that the outer pair of mikes (L and R) should go through one converter and the center mike through another |