Analog Tape Property I Forgot About!

[dbbubba]

I have been transferring a lot of my analog mixes from 2-track to CD for archive purposes and I have been really surprised in how much analog tape machines vary in speed!

I have cases where pulled four or five mixes with slight changes, but when I look at the times of the trimmed soundfiles the get progressively shorter as they occur later in the reel!

The particular mixes that really show this where recorded to an Ampex AG440 which IS NOT known for accurate speed at the beginning or end of a reel. I am playing back on a perfectly calibratred Studer A810, so I know it's speed is as accurate from beginning to end as any analog deck is going to be.

As an experiment I was going to line up the different mixes in my DAW (DP5) and each take was shorter than the next.

This is stuff we take for granted now with digital and I guess we just dismissed it as part of the "deal" back in analog days.

I do recall that my ATR102 would hold almost "perfect" sync when running along side my early '80s MCI JH-24. I used to fly in parts and I always expected to hear timing issues, but never did if the start time was right. I eventually played a 1/2" mix along with the 2" master and I expected to hear flanging, but it just stayed in one spot!

Thought I'd share this for fun.

Danny Brown

[mixerguy]

I've used 2" machines many, many, many times ovr the last twenty years

I would ALWAYS record 29.97 timecode as the rest of the tracks were recorded..... (NOT striping it later as is the common approach).... then simply resolve the machine when playing it back. Thus I was always guaranteed that playback would be EXACTLY on speed.



Not a common technique in music production.... but I always did it - regardless of if it was a record project or film score. It has proven to be a life saver time and time again!

[MJGreene Audio]

Yes. I don't miss those days at all. Try recording on a Studer A-80 Mk 1 with 14" reels and see what happens when you get to the end of a reel. Wee let all speed up now.

[Jamz]

After all, analog machines do use a motor to operate. They're not quartz locked.
Too some degree (no matter how slight) they fluctuate.
Computers are quartz locked and are not motor driven. When I first started to use computers locked up to 2" machines I realized just how sensitive the syncronization was. Without a proper sync box the audio would drift at some point into the song or sometimes you'd get pitch variation as the machine fluctuated.

[mixerguy]

Originally Posted by Jamz

After all, analog machines do use a motor to operate. They're not quartz locked.
Too some degree (no matter how slight) they fluctuate.... (snip).....

I wanted to clarify for the benefit of people who may not know...

if, during playback, you resolve (to the correct video synced time code) a properly maintained 2" machine using a good resolving system (with a recording that was made with time-code put down as the music was recorded) ..... it’ll playback on speed exactly - even if for some reason it was running off speed when the recording happened.

[Jamz]

Absolutely!
PROVIDING

Originally Posted by mixerguy

you resolve (to the correct video synced time code) a properly maintained 2" machine using a good resolving system (with a recording that was made with time-code put down as the music was recorded) ..... it’ll playback on speed exactly - even if for some reason it was running off speed when the recording happened.

[mixerguy]

Originally Posted by Jamz

Absolutely!
PROVIDING

yes. If the recording was done as I explained.

[Jamz]

Originally Posted by mixerguy

yes. If the recording was done as I explained.

Perhaps you noticed I quoted your exact post.

[dbbubba]

Most all pro analog decks after the late '60s were crystal controlled which is a QUARTZ crystal. ALL MCIs were, ALL Otaris were, almost all Ampex decks after the AG440 were, but AG440s were available with a crystal controlled capstan motor. I had one in the mid '80s, but obviously the one I used for the mixes I mentioned was a DC capstan motor model. The AC or sync capstan motors on Ampexes had an extra circuitboard. On AG-440s it was usually located behind the transport on the back of the roll-around case.

I'm not sure when Studers switched to crystal control. Obviously the J39 four track decks that Abbey Road used weren't crystal controlled because there is plenty of stuff about how they used an oscilator to vary the tape speed. This wasn't that difficult really. All you needed was an variable oscilator driving an audio amp that would put out 120VAC. I guess in England it required an amp capable of putting out 220V. In fact, if the AC capstan motor requites less voltage than the full 120V (220V) the amp doesn't have to supply 120V or 220V. You can't do this method of VSO on an AC or sync (quartz chrystal) capstan motor.

I have an AG-440 manual, so I am going to look and see if the capstan motor required less than 120 V. I don't know why I wanna' know, but I do.

Quartz crysral control is not a technology that came from computer technology.
It is used extensively in many industrial applications.
It is how electrical generators maintain a constant 60 hz state.

The reason that tape decks like the AG-440s and such ran at a different speed near the ends of the reel was mostly due to the supply and take up reel motors and their velocity. In fact, the constant velocity tape path was one of Jeep Harned's inovations at MCI. The was a counter that knew whwre you were on a reel of tape. You could fool it if you used a reel that was not a full 2500' reel.

I just thought it was funny to experience the speed change anamoly after years away from old analog decks. I have used JH-110s, ATR-102s, Studer A810 and a few Fostex decks mostly in the last twenty years. The are all crystal controlled.

Danny Brown