-- Hardware Overdubbing/Multitrack - Shure X2U Microphone Amplifier --



I'm going to describe a known, good, working method of creating a multiple sound track overdubbing session from scratch in Audacity 1.3. That is, you record one track and then play it back and add a second track against it. Drums, guitar, voice. Repeat as needed.

This process uses a special, required USB microphone amplifier with headphones plugged into its side and a plain XLR microphone.

You will hear a useful, theatrical mix of your live performance and the previous tracks in your headphones. This is the step often missing or damaged without purpose-built hardware.



The Shure X2U is an XLR microphone amplifier (preamp) with USB interface and headphone management. The X2U Volume and Monitor controls are used to provide a headphone mix of computer playback and live performance sound. This is the mix you need to sing or play to yourself with no latency, echoes or other impairment.

The Shure X2U will supply 48v phantom power to any microphone that needs it.

I'm using an ordinary Shure SM-58 "rock band" microphone. Any XLR microphone will work. The system is plugged into my Mac, but almost any PC, Mac or Linux machine with fast enough USB and good storage can be used.

I'm using my Mac earbuds for listening in this example, but nearly any good headphone or earbud is OK. If you require cranium-crusher headphone volume, you may need a headphone amplifier. You must get the headphone sound from the Shure X2U, not the computer sound card.

Headphones are good. Live microphones and live speakers in the same room do not get along.

I'm using Audacity 1.3.13. Audacity 1.2 is more difficult to use, is not well supported, and can be unstable.


The Shure X2U has three jobs:

-- It amplifies the tiny microphone signal, digitizes it and send it down the USB cable for recording.
-- It converts the USB digital show from the computer back to analog.
-- It has an internal headphone mixer. The X2U performs a mix of your existing track playback and your new, live performance so you can listen to both.



We will do a simple recording. No overdubbing or other fancy tricks. The system has to work correctly for simple recording and playback before we go further.

Connect the USB and headphones. The X2U must have a USB Home Run to the computer. USB audio will not go through a hub or long USB cables

The X2U must be connected and with its light on before you start Audacity.

There is no requirement to have a microphone cable between the microphone and the X2U, so for the test I didn't use one. Good quality XLR microphone cables can be as long as 100'/30M. One of the advantages of this system is the separation of the microphone and the computer.


This is not a tutorial on making a recording and playing it back, but you need to be able to do that before you do anything else. There are multiple tutorials and wikis on Recording.

Audacity Manual
First Recording
General Workflow

Set the computer control panels, preferences and Audacity drop-downs to recognize the X2U USB device for both recording and playback. The X2U's USB name is SHURE DIGITAL-USB.

Set Audacity Preferences:

Audacity Preferences > Quality > 44100, 32-bit Floating.
Audacity Preferences > Devices > Mono.
Audacity Preferences > Devices > Recording and Playback to the USB sound device.

Audacity Preferences > Recording (you may not have all of these settings)

[X] Overdub...
[  ] Hardware Playthrough...
[  ] Software playthrough...

OK > Restart Audacity.


When Audacity starts, expand the sound meters by clicking on the right edge and drag to the right. The rest of the tools and panel graphics will move out of the way.

Click once anywhere inside the red recording meters to put them in Monitor Mode. They will measure the microphone sound without sending Audacity into full record and wasting drive space. This may fail on certain linux machines using Jack.

Adjust the X2U Mic Gain for good level. Don't peak much over -10 to -6. You can fix funny levels later, but you cannot fix overloading, smashing, and clipping (meters too far to the right). You should be able to hear your performance in the headphones. Adjust the Volume control as needed for comfortable listening, and Monitor in the middle of its range.

Press Record. Audacity will take a second to configure itself and start recording. The blue waves will start to crawl left to right as you perform. Play or sing a simple song that you can use for rhythm and timing later.

Press Stop, Home. Play the track you just made. You should hear the track in your headphones. If you tap or scratch the microphone, you can hear that, too. They're both alive.

This is what you will hear during the overdub sessions. Any combination of existing tracks will play in your headphones in addition to your live voice allowing you to set a good theatrical or musical mix and timing.



Home the cursor and Press record again and you will get a new recording underneath the first one. Sing or perform in time to the first track. Press Stop, Home.

The show will have two tracks, one from each performance, but it may be seriously out of time or rhythm -- even though you were in perfect time when you recorded it. This is Recording Latency and you can adjust it to zero using Audacity 1.3 Latency tools.

Done properly, both the live recording session and the playback later will be in perfect time.

File > Close > Don't Save


File > New

Generate > Click Track > OK.

Audacity Preferences > Recording > Latency Correction [ 0 ] milliseconds . . . > OK.

Play the new track and set VOLUME for loud but not painful. Take off your earphones.

-- Turn the Mic Gain all the way up. The gain on the X2U is a little wimpy, so that control usually lives up there.

-- Roll the Monitor control toward the microphone. This causes only the click track to play into the headphones.

Hold the headphones against the microphone.

Press Record.

Track one's click track is now being recorded -- badly -- onto track two through the headphone and microphone. Good fidelity is irrelevant.

Do that for five or ten seconds and press Stop.

Select the new track and Effect > Amplify > OK.

Reduce Volume a bit and put your headphones back on.

Play and both tracks will probably play out of step.

Magnify the timeline around one of the pair of clicks (drag-select and Control-E or Command-E.

Drag-Select the distance between the start of the click on the top track and the start of the same click on the bottom track.

That's how much the rhythm misses and that's the latency. Keep magnifying until you can get a good shot at accuracy. Control-3 or Command-3 to back out slightly if you magnify too much by accident.

Bottom of the screen > Middle Time Window > Length

Change the format using the dropdown menu to:

hh:mm:ss: + miliseconds.

You're mostly interested in the milliseconds -- the last numbers on the right. My reading was 244msec.

Audacity Preferences > Recording > Latency Correction [-244] milliseconds . . . > OK.

Restart Audacity and go through generating Click Track, etc., again.

This time the two click tracks should look perfectly on or very close to it, and sound perfectly in time. If not, zoom in, measure the new difference and add that number to the latency value.

Before you get too obsessive about this, an orchestral musician once told me that the chances of any two instruments in the orchestra starting the same note at the same time is zero, so you don't need to adjust things down to the digital sample level. The latency values on home computers can wander in normal use.



Restart Audacity (you don't need to save any of your tests) and you're ready for the first theatrical session.

The first recording can be whatever you're planning to use as a base, guide or rhythm track. It can be anything including Generate > Click Track which can be adjusted for rhythm and composition. I used the music from my rhythm and chord machine playing to the microphone.


You should record a lead-in. That is, a non-musical rhythmical clue when to start before the music starts. In a live band, this would be the drummer or lead guitar count-in. I used the keyboard rhythm stops in my tests, but you can perform several rim shots into the microphone to establish the rhythm before the first note. Tapping on the table with a pencil works. Anything. You can sheer it off in post production later so nobody else will hear it.

Tick, tick, tick, tick, Music. Adjust as appropriate for music type and rhythm.

Stop > Home the cursor, press record and record track two using your live performance and track playback in your headphone mix as a guide. Repeat until bedtime.

The Mute and Solo buttons to the left of each track are valuable in overdubbing. Solo causes only that track to play and Mute turns that track off. Solo has options. The little volume control to the left of each timeline also controls the playback volume -- it affects the headphone mix.

When you get to a stopping point, press Stop and File > Save Project As. As you progress, you should save a new Project periodically with a slightly different filename. I don't use a kitchen timer, but I probably should. Being obsessive, I use date and time for the filenames.


That's today at 3PM. 2011 October First, 1500hrs.

I will never duplicate a file by accident short of a typo. Do Not Use Slash Marks or other punctuation marks in a filename.


New version of the song about every twenty minutes.

Do Not go weeks with one Project and filename, and never cover up or record over existing work. If anything happened to that one Project, you'd be dead and could take weeks of work with you. Think of what would happen if the lights went out right now, the computer ground to a halt and you were forced to use the last known good version of the show.

Audacity Projects are brittle, easily damaged and do not save UNDO.

Export to WAV for archive and Music CD, or MP3 for internet delivery or email. Do Not do production in MP3.

You may need to adjust the volumes and levels of the show so the Export doesn't overload.

All of the Audacity editing, filtering, and effects tools are available for each track or any combination of tracks before or after you sing/play. But not during. Audacity will not apply filters and effects in real time.



How much hard drive space do you have? If your only experience with computer files is with spreadsheets, email or Photoshop pictures, live audio (and video) production will stun you. Particularly with high quality overdubbing and UNDO, those project files and folders get big in a hurry, and with periodic saving, a project can get very seriously large.

Newer Windows machines in particular are openly hostile to entertainment production.
My music changes volume by itself and sounds tinkly/hollow

The headphones must be plugged into the Shure X2U for overdubbing, and not the computer. The computer sound card show will most likely be late (off rhythm) and you can't easily change it.


It's certainly possible to do a credible overdubbing job without specialty hardware -- in software only -- but it can take very serious programming effort and some knowledge of how the inside of your computer works. If compiling binary libraries is too much for you and the hardware devices are inappropriate, you could always write a check to buy a commercial sound mixing program.

If no matter what you do the show sounds terrible or doesn't work at all, do drop in to the Audacity Help Forum where the elves and I will try to dig you out of trouble. Registration is required and we may make you wait before your posting appears.

At minimum, you will need to tell us exactly which Audacity you're using, what kind of computer you have and which operating system. I use Audacity 1.3.13 on a Mac Mini with Snow Leopard, OS-X 10.6.8. Don't head straight to the details of the problem without telling us what you're producing and why.

Be prepared to tell us how the straight recording session went -- the one you did before you tried overdubbing.

Please don't post pages of diagnostic dump or error log unless we ask for it. If you do insist on posting it, use the [data] tags.