Digital Delay

When you hear the words "digital delay," you most often think about those little one rack high boxes that delay audio on purpose. You send the audio signal in one end, set the delay time to match the tempo, and one or more copies of the input audio comes out the other end. Well, all forms of digital audio incorporate delays, and if we don’t pay attention they will nip us in the butt.

Pesky Delays in MDMs

When you are working with one tape machine, the internal delays are compensated for so that you don’t have to worry about the recordings being in sync with the playback. For this purpose, a stack of machines synced to get you more tracks is effectively one many track machine. It doesn’t matter whether the machines are ADATs or DA-88s or Sony 48 track machines, they all work basically the same way.

Here is where the delay rears its ugly head. When you have to make a digital tape copy. I will use ADAT as an example. If you make a copy from one machine to another there is a delay. If the tape copy you made is never going to be referenced to the original tape, then it doesn’t matter how much delay there is. All of the copied tapes will only be referenced to each other and everything will be fine. If, however, you are making a backup copy of one tape and you are going to sync it up with one of the originals, you will be in big trouble. If you follow the directions in the owner’s manual, there is a specific mode that the machines must be placed in offsetting the source machine to compensate for the delay. A tape made correctly will be a clone and can be used in sync with the original tapes. Other digital multi-tracks have this same type of offset mode for making clone copies.

If you don’t make the clone tapes properly, the instruments on the copy will sound slightly out of sync, and if there is a stereo instrument whose left and right channels are on different tapes, there will be significant phasing in the stereo image.

Mixed MDMs

If you have a couple of ADATs and your friend has a couple of DA-88s, you can usually synchronize everything without a problem. There are a bunch of boxes that will take care of making sure everything is copasetic. The problem comes when you want to make a digital copy of some tracks from one format to the other, and then later decide that you want that guitar back on the original tape. There are boxes from many sources that will transfer your audio digitally back and forth between ADAT machines and DA-88 machines. What is not taken into account is the "round trip delay." This is the total amount of delay going from one format to another and then back again digitally.

Remember, I said that for recording on one machine all of the delays are taken care of internally so that what you record is in sync with what is playing back. This still holds true with mixed formats synchronized together, but only while you are going in and out of the machines analog. If you want to make your copies via analog, then everything will line up. If you do that, however, you can never again read one of my columns! You know who you are! We only make digital copies here at Digi-Land.

You can compensate for the digital delay manually during these digital copies by advancing the tracks on the source machine. You can measure the exact delay in your system by recording a click to all of the tracks of the source machine. Bounce them digitally to the destination machine. Bounce some of the tracks digitally back to the original source machine, leaving at least one original track for reference. Now take one of the original tracks and one of the tracks that made the round trip bounce, and put them into a hard disk editor. It doesn’t matter which one. Zoom in to the sample level and measure the difference between the two signals in samples. This is now the amount of advance that you will have to enter in to the source machine before making the digital copy.

DAW Delay

Most of the time I record onto digital tape, either MDM or Sony 48 track. I then transfer the instruments to Pro Tools for editing, flying choruses around, and noise clean up. After all of the juggling, I transfer the tracks back to the digital tape. What is on the tape is the absolute master. The tracks can’t accidentally slip around, and the wrong guitar solo can’t accidentally play.

After this round trip to the workstation, I want the tracks to line up exactly where they were before they left the digital tape. The interface boxes or digital console that gets the audio into the workstation delays the digital audio, the processing of signals through plug-ins delays the digital audio, and the interface box or digital console that passes the digital audio back out to the digital tape delays the digital audio. The easiest place to compensate for this delay is inside Pro Tools, or whatever workstation you are using. Here is how to do it.

1) Identify a percussive track to use for alignment purposes. A click, a snare, guitar chinks, or any other track that contains percussive attacks.
2) Make sure the workstation is chasing the tape machine in sync.
3) Record the track into the workstation though the chain of boxes you plan to use.
4) Without sliding the audio track, assign it to an empty track on the digital tape machine.
5) Record the track in sync from the workstation to the digital tape machine.
6) Delete the audio track in the workstation to avoid confusion during the next step.
7) Make sure the workstation is chasing the tape machine in sync.
8) Set up to transfer the original percussive track AND the track you just transferred to tape. They MUST be transferred together.
9) Record both tracks into the workstation.
10) Zoom in to the sample level and measure the distance between the attacks of the two different tracks. This is the round trip delay in samples.

In my setup this turns out to be about 42 samples, which is about one millisecond. I make the compensations right after recording into Pro Tools. I record the audio, then slide the tracks over 42 samples earlier. I can then cut up tracks, copy them from place to place, and when it comes time to copy the tracks back to digital tape, all of the compensation is already done.

The amount of delay will vary from setup to setup. ADAT to Pro Tools and back will be different from DA-88 to Logic Audio and back. If you are going through a digital console, the delay will be different than through an Apogee AD-8000. Most digital consoles have a delay of around three milliseconds.

Because time code is the usual synchronizing reference, the resolution of the lockup can be off by a few samples here and there. When I transfer additional tracks from the digital tape to Pro Tools, I always digitally bounce a second of the click track (or some other percussive reference) onto the track I am going to transfer. After the track is in Pro Tools, I slide the track earlier until the click piece lines up with the master click reference that is already in Pro Tools (from the initial transfer). Because of sync slippage, the amount of delay may wander from 42 to 48 samples.

The one time I didn’t take time to put some click on the track to be transferred was during a horn session with six drooling horn players getting triple scale weekend overtime (the equivalent of about 1 Porsche per three hours of session). I needed to transfer the three passes (18 tracks) into Pro Tools to make room on the Sony 48 track for three more passes (we then take the best parts of the six passes to make the master horn track). Everything had been going fine in transfer land, so I just transferred the 18 tracks, slid them earlier by 42 samples, and went back to recording.

We recorded two more passes and then recorded just the tag of the song one more time on the third set of tracks. "Great, I think we’ve got it in there somewhere." The remaining 18 tracks were transferred to Pro Tools, but this time the horn players were gone (they were out of the studio before I let go of the talkback button). I transferred some click to one of the 18 tracks, transferred the horns to Pro Tools, and slid the tracks over until the clicks lined up--- exactly 42 samples.

While listening back to the horns, I found that the first 18 tracks were horribly out of sync by about 27 milliseconds. It turned out that there was some sync burp during the transfer, the horns were way off, and there was no reference click. The client asked, "Everything’s going to be alright, isn’t it?" I smiled and said, "Uh, sure!"

Luckily, I didn’t record the whole song the second time on the third set of tracks, so there were some horns left from pass three. Pass three had been transferred into the computer on the first pass of 18 tracks. I now had a common piece of audio on both the first set of 18 tracks and the second set of 18 tracks. I used a horn stab in the second chorus to line up the first set of 18 tracks. Saved by the bell (of the trumpet.)

I just got a new Mexican Digital Audio Workstation that has so much internal delay that the audio signal won’t come out until tomorrow. It’s made by a company called Mañana Digital Audio. Cool.