Different speeds on different computers

I am recording a podcast with two Podcaster microphones using two different computers, one for each member of the podcast.When I later try to edit them together and sync them up, normally with a clap in the beginning. But there is a slight difference in the speed of the audio, so after an hour there’s a noticable delay on one of the tracks. Both of the computers record at 44100 hz, with the same microphone, with the same program, so it really doesn’t make any sense. I could just sync it up with another clap at the end, but sometimes, the speed changes in the middle, so i have to split the audio every ten minutes just be sure. I.e, if I sync the first clap on both of them, and then change the speed of one of them so that the second clap at the end are synced up, but still, in the middle, there is a very noticable delay. I don’t know if there is something wrong with one of my computers/copies of the program, but this is getting to be very annoying without any logical reason. I really need help. Both of the computers use Windows 7 Home Premium, and as far as I can tell, it’s the same version of the program. If this problem can be caused by different versions of the program, I will get to fixing it.

Both of the computers record at 44100 hz, with the same microphone, with the same program

With different computer clock systems making the 44100Hz. Most clock signals just have to be accurate enough to keep the system or computer from crashing.

There is a provision on some computers to record from two different USB microphones at the same time. They warn you that the computer will use only one of the two data signals as “sync” and the other one will drift over the course of the show.

Pay attention to the direction. If one show is consistently early, that could mean that computer is dropping samples creating a gradually off-time signal. This is common on an underpowered computer, or one that is having trouble managing Skype and Audacity at the same time.

This error usually happens the first time the analog voice gets digitized. That’s the first time you need to make 44100 and that’s the clock signal that gets used in that portion of the show. If it’s your Windows sound card, well, none of those will be winning any awards for quality. Two identical USB microphones should do quite well.

The grownups use master sync signals or expensive dual clock generators and they can go all day with no noticeable sync problems. The movie people have to do that.

By the way, congratulations on covering all the bases (except possibly one). That is exactly how I would do it if I had to marry two different sound files. How are you getting one set of human hands to appear on both computers – assuming they’re in two different locations.


You didn’t get into details about how you’re recording these shows, but one thing I might do is record a clock ticking for some time (20 minutes??) with the show configuration up and running, then do it again with nothing else running except Audacity. If your clicks don’t match after 20 minutes then your computer system is “doing something naughty” to the capture when it’s busy.

If you have provision for bi-directional audio, then send the same clock ticks to both machines. Compare the sound files playback to the actual clock. If they both drift all over the place, you may have two “broken” computers.

Lucky you.


Oh, and Audacity doesn’t do anything to the sound during recording. Whatever the computer provides is what Audacity records. Koz

All right, so for my understanding. There is absolutely nothing I can do to make sure they record at the same speed? I’m guessing boosting its priority in task manager won’t do anything either? I use two computers at the same location (at the time being) to record, but for what you’re telling me, not even if i managed to record both on the same computer would i be able to make sure the timing will be the same? That really, really sucks. :frowning:

not even if i managed to record both on the same computer would i be able to make sure the timing will be the same? That really, really sucks.

If you are using the same computer and the same device (both sides of a stereo soundcard or a multi-tracking interface) it will stay in sync. However, if you use a USB mic and your soundcard, the clocks may not be identical. (No two clocks are ever identical, but they can be very close.) Higher-end interfaces should have more accurate clocks than a typical consumer soundcard. Pro equipment uses a master clock to keep all devices in sync.

That’s “Rode Podcaster USB” is it?

How “noticeable” are you talking about? 30 seconds? 30 milliseconds?

I don’t think that anyone has said that, yet.

The guaranteed way to ensure synchronisation is to throw lots of money at the problem. Depending on how severe the problem is it may be possible to improve the situation without burning a hole in your wallet.

You apparently haven’t done any of the tests yet. You may find that one or both of the computers is broken and damaging the signal. Then there is certainly something you can do. Fix the computers.

You also didn’t respond to the amount of error. If it’s seconds off, you have a broken system.

We’re telling you to do these things for a reason.


Oh, sorry. The delay differs from time to time. It seems to be more severe now than it was when I did the first couple of episodes, and now I’ve done about 10. Usually there is only a few dussin milliseconds, and in the beginning it was an easy fix. But in the later episodes, the timing seems to change during the file. I.e, if I fix the delay at the end, in the middle there is still a delay. And yes, it is the Rode USB podcaster mic.

If you don’t mind me asking again, fixing it so that I can record both of them on the same computer will make it better but not completely fix it? How so? Will there be a constant delay, or will there still be two different speeds. From what I understood from the initial response, plugging them into the same computer wont fix it because it still only goes after the timing on one of the mics. Is it worth trying that, anyway?

Trying to understand your reply.
Are you saying that amount of de-synchronisation is a few milliseconds?

If the drift is much more than a few milliseconds then one or both devices may be broken.

Because the devices are independently “free wheeling” with no master clock signal, it is to be expected that they will gradually drift a little out of synch with each other, just like if two people that cannot hear each other start counting seconds will gradually drift apart, whereas if two people are looking at the second hand of the same clock and then start counting seconds they will be able to remain synchronised.

We need to know if they are drifting “a little” out of synch or “a lot” out of synch.
1 millisecond drift over a 1 minute period is less than 0.002 % drift, so that would be considered “a little” for a consumer level device, but quite a lot for a professional level device. Rode USB podcaster microphones are quite good consumer level devices so I’d not expect much more than a couple of milliseconds drift per minute (though of course the two devices may drift in opposite directions, effectively doubling the “error”).

if I fix the delay at the end, in the middle there is still a delay.

To add a few words, you got the beginning and end to match, and the middle is still off?
It’s very unlikely that the clock signals would change within one performance. They almost always go their own way consistently.

What could do that…?

Now you’re into supernatural territory. I have no good answers for that one. I could make something up like the drive filling up caused one computer to slow for the first half of the show and then a humidity increase caused the other one to start dropping samples.


I agree that clock signals usually run one way or the other, though I have seen a sound card gradually drift one way and then gradually drift back again.
Jack Audio has a neat workaround to this, which is to allocate one device as “master” and the other as “slave”. If the slave becomes out of step from the master, Jack will duplicate or drop one sample from the slave to bring it back into step. The inevitable “glitch” from doing so is tiny and likely to be undetectable. I think that Jack on Windows is still classed as experimental so is perhaps not a viable option for non-geeks.

Heat can certainly have an effect on the clock rate and there is likely to be a greater change in clock speed during the first hour of use (see: Frequency Calibrator)

You might want to change the tempo on one recording to match the other – that way they are at the same speed in the end.

Yes, but the current problem is the ends match but not the middle. Koz

It’s very difficult to say how much delay there is, but normally after an hour there’s about 1/10 of a second to 1/2 of a second delay. I hope that helps.

1/10 second drift over an hour is within what I would expect, and can be managed quite easily with just a little editing.
1/2 second over an hour is toward the maximum that I would thing reasonable, and I agree that is getting a bit awkward to manage, but nowhere near as bad as back in the days of tape recordings where the drift might be several seconds per minute! :wink:

The bottom line is that in the absence of active synchronisation, they will drift apart. What we can do is to try and minimise that drift so that it is more easily managed.
Try switching on the computers and mics at least half an hour before use, and try setting 48000 sample rate in both Audacity (Edit > Preferences > Quality) and in the Windows Sound Control Panel. Is that any improvement?