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Audio Recording Basics / Monitoring Latency




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Subtitles of the Movie (Digidesign Pro Tools LE 6 / Audio Recording Basics / Monitoring Latency)

In this movie, I will explain the problem of monitoring latency and ways of reducing or eliminating it. When you record into Pro Tools or any other digital audio work station using microphones or instrument or line level inputs, the signal needs to be converted from analog to digital. This is what an analog-to-digital or A/D converter does. Unfortunately for us this conversion is not instantaneous. Instead there's a slight delay from when the input signal hits the analog side of the A/D converter until the output signal comes out of the digital side. This delay is referred to as converter latency and is present on all A/D converters like the ones in the Mbox, Digi001, the 002 and the 002Rack. Even expensive outboard converters have some latency. Once the signal is in digital form, it still has to pass through the USB cable as in the case of the Mbox or through the firewire cable as in the case of the 002 and the 002Rack. There's a slight bit of delay introduced there as well as the digital information is buffered, send out for the wire and then processed in your computer. This delay is called interface latency. Even the pci-based interface of the 001 introduces some interface latency. Once in the Pro Tools LE environment, the digital information representing your signals being processed not continuously but in segments using something called a hardware buffer. This buffering introduces yet another delay called mixing latency. In Pro Tools LE you can adjust the size of this hardware buffer by going to the setup menu and choosing playback engine. Because I have a Digi002Rack connected to my Macintosh, I have the option of choosing one of five different hardware buffer sizes from a 128 samples all the way up to 2048 samples. Depending on which hardware you are using, you will see different numbers here. Regardless of hardware, smaller buffer sizes give you smaller amounts of mixing latency. Check 1, check 1, isn't this a lot of fun. And larger buffers give you a larger amount of mixing latency. Check 1, check 1 isn't this a lot of fun. If you were to add some interesting plug-ins to the channel strip for the track you are recording, you might have plug-in latency to deal with as well. Check 1, check 1 isn't this a lot of fun. However, we are not finished. In order to monitor that new instrument you are recording into Pro Tools LE along with others that are playing back, the mixed audio needs to be sent back over the interface connection to the Mbox, the 001, the 002 or the 002Rack, more interface latency. Lastly we have the digital-to-analog or D/A conversion which introduces one last bit of converter latency. When all is said and done, that instrument you are playing might take a tenth of a second to make its round trip from the analog world to the digital world into your Pro Tools and back again. This overall delay called monitoring latency can be very disorienting when you record. What can be done to eliminate or at least reduce monitoring latency so that we are back in real time or at least much closer to it. For Mbox users, Digidesign has included a feature called Zero Latency Monitoring. As you see here on the front of the Mbox, there's a knob which lets you select between input and playback. In order to monitor the track that you are currently recording while listening to the tracks already recorded without any latency, set this knob halfway between input and playback. One more thing though, if you would like to eliminate the echo you hear in the headphones plugged into this headphone jack, you can mute the track that you are currently recording in Pro Tools. This keeps Pro Tools from sending the track that you are recording back out into the Mbox for monitoring and yet still allows the track to be recorded properly. Test 1, test 1 isn't this a lot of fun. Why yes it is. When you have finished recording, simply un-mute the new track to hear it played back with the other tracks. Test 1, test 1 isn't this a lot of fun. Why yes it is. It's almost as simple for the 001 and the 002 users. There is no zero latency monitoring for you but if you go to the operations menu and choose the low latency monitoring menu item, Pro Tools will put itself and your interface hardware into a special state where the signal you are recording is mixed with the other tracks with very little latency and send it back out to the monitoring system in the headphone output. Check 1, check 1 isn't this a lot of fun. Low latency monitoring works with the analog 1 and 2 outputs considered to be the main outputs of your interface hardware. The low latency monitoring option is not available to Mbox users because they have zero latency monitoring build into their interface. If you don't want to use zero latency or a low latency monitoring techniques for creative reasons, and yet still need quick feedback of your signal with a mix, try setting the playback edge to the least number of samples that your system will allow. Even though a small buffer setting here will reduce monitoring latency, you have to be careful not to tax your computer by setting it too small. A smaller buffer setting here makes your computer work harder to keep up with all the demands placed upon it by the mixing system in Pro Tools LE. If you have set this to a smaller size and you notice that Pro Tools keeps complaining that the computer doesn't have enough power to keep up, you can try the next larger buffer size. You can keep doing this until the computer can handle the load or the monitoring latency becomes too great for you. At that point you should revert back to either zero latency or low latency monitoring. Make sure to check out the movie using click tracks to see how the low latency monitoring modes affect the use of the click plug-in.

Tutorial Information

Course: Digidesign Pro Tools LE 6
Author: Nathan Dickson
SKU: 33599
ISBN: 1-932808-46-9
Release Date: 2005-02-25
Duration: 9 hrs / 101 lessons
Work Files: Yes
Captions: No
Compatibility: Vista/XP/2000, OS X, Linux
QuickTime 7, Flash 8

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