can someone explain latency and how to compensate for it?

[zen_city]

Hi --

I am importing mp4 tracks into audacity 1.3.5b on my intel macbook pro and then using a KORG pandora USB effects box with my electric guitars to record additional bass and guitar tracks. The pandora gives me lots of special effects and simulates different amps and types of guitar and the tracks sound great except they are playing back at a slightly slower speed, but pitch is fine. That is, the tracks I record are slower and and out of sync with the mp4 tracks. At first that I had overlooked some delay setting on the KORG but I have checked it out and the problem. I thought Granted I am not Carlos Santana but I know I am hitting the major beats with my bass, but thats not how its playing back. I am using 42100 16 bit settings. I have tried messing with the latency correction numbers in the prefernces/audio 1/O pane but that doesn't seem to work. (Maybe someone can explain what those numbers actually do?)

Also, what would cause latency to be different from one project to the next? Or if I got it right on one track would those settings work for all others?

Is there any way to set control points on the mp4 tract or a master track and adjust timing so the sound syncs at those points?

In the effects menu I have tried both the "speed" and "tempo" effects to speed up my instrumental tracks, and this does speed the tracks up a bit but seems like a really inexact way to address the problem. (by the way, what is the difference in those two plug ins?

Can someone help me or link me to a FAQ on this problem.

PS I should also mention that I am short on hard disk space at the moment--down to about 10 usable gigs partly because I have been recording so much. Could this contribute to the latency problem or the tendency of 1.3.5b to crash a lot.

[kozikowski]

<<<or the tendency of 1.3.5b to crash a lot.>>>

No System Drive on a video or audio production machine should ever drop below about 10% free space. Both the operating system and programs start getting really nervous and unstable the fuller your drives are and it shows up noticeably at about the 90% point. No connected hard drive can be that way. If everything in the system is really cool but you have a large USB or FireWire drive connected and it's at 98%, the system will spend a lot of time managing that drive and to heck with your show.

Koz

[zen_city]

I figured that was the problem with the crashes.

So if I leave my 70 gigs of software on my 100 gig MacBook Pro and I direct all my audacity files to be saved to a USB back up drive which isn't really full, will there still be playback delays just because of having the data go through a USB connection?

In fact I just read somewhere that some USB devices (hopefully not my Korg unit) create unavoidable delays in recordings.

Also I figured out the dif. between speed and tempo and that I should use tempo so the pitch won't change. But surely I don't adjust the track speed just by trial and error?

[kozikowski]

<<<direct all my audacity files to be saved to a USB back up drive>>>

You can do that with a FireWire drive which was designed for that sort of thing, but not USB. USB external drives like to pause every few minutes and powder their nose. Blammo, there goes the show.

USB drives are good for long term (slow) storage and not operating, live, or real-time files. Two USB drives. For proper backup, all the valuable work should be on two different drives. Two partitions on one drive doesn't count because if the drive motor goes down, both "drives" hit the mud. You can use a drive stack with RAID5. We do that where I work.

This also assumes you have Exports of your shows because Audacity Projects (Save Project, Save Project As...) will not copy between drives.

Koz

[zen_city]

I have been trying to use this Korg Pandora box which is doing both audio in and out at the same time via a USB connector so that is most likely the problem. I have cleaned off my hard drive, reduced the quality of the audacity recordings down to 22000, recorded mono instrumental tracks, tried swithching between laptop usb ports in case one was faster and the problem persists.

But I still want to know what the latency correction controls in the audacity preferences actually do and what they should be set on.

Now I am going to get out the maudio fast track ultra and try recording guitar tracks through that instead of the Korg, but it has a USB interface too I believe. Hope that doesn't continue the problem.

Next fix is to buckle down and learn ableton live, which I was trying to avoid with audacity.

It's not about live shows by the way. Just recording my learning curve on the guitar and bass for myself and friends. So I don't have a raid computer or a firewire drive at this point.

But any advice on what other equipment or software to buy would be appreciated.

[kozikowski]

The analog world doesn't have latency. If you put audio into the biggest audio mixing console ever made, it would come out the other end essentially at the same time it went in. All tools are in real time except the ones specifically designed to make the audio late. This is what allowed Les Paul to do all his overdubbing and sound-on-sound guitar tracks and Mary Ford to sing quartet with herself in 1947.

The first hint of troubles was MIDI. Computer control of a live musical instrument. Did you know that it's impossible for MIDI to start every instrument in a "band" at the same time? Can't be done. Some instruments are going to be late compared to all the others, but as a musician once told me, have you ever met a band where everybody started at the same time? Well.....no. But that was foreshadowing. MIDI is a computer musical system with errors that happen to fit in a musician's sense of the real world.

Enter computers with audio processing. Every time you do audio in a computerized audio device, a computer somewhere has to take a second to think about it. All those seconds add up. What latency compensation tries to do is make the errors as small as possible.

In the case of recording, what the computer will normally do is play the internal bitstream back out to the speakers if you have it set up that way for monitoring. This is usually two computers late and can be very annoying. Operating Systems and Audio Programs try to catch this and ship the input sound directly back out to the speakers eliminating the computer stuff in the middle, but then they have to know when to stop doing that, because that screws up normal playback.

And speaking of normal playback, everything you play from hard drive is one computer late because of the digital to analog converter and anything else in the pathway. Everybody wants to play one track while they're recording another. Well! The playback is one computer late because of playback electronics, and the recording on the hard drive is one computer late because of recording electronics, but the sync playback to the artist is two computers late because that live sound has to pass through twice. You begin to see the problem. You can't play the artist's guitar before he actually touches the strings and you can't delay his headphone feed. Oh, and after it's all over, the tracks on the hard drive have to end up in perfect sync. And have a happy day.

There are ways out of this. You can't play the guitar strings before the artist does, but you can play the rhythm track from the hard drive too soon. Then you can take the live recorded material still in computer memory and put it in the sound system where it would have been had there not been any latency. Sometimes the system gets those delays and advances wrong and you can get some really odd results, but most time, it's remarkably accurate.

Complicate all of this with the hard drive system fragmented or filling up. "Record this now!" "Wait a second while I find room."

That's different from the poster whose live recordings drift slowly out of sync over time. He has a broken sound card.

Koz

[Gale Andrews]

I still want to know what the latency correction controls in the audacity preferences actually do and what they should be set on.

http://www.audacityteam.org/manual/inde ... es#Latency

Gale