Audio engineering help needed

Factual Questions
21

scotandrsn

You are almost right in your code illustration but not quite.

Its almost certain that the tapes mentioned in the OP are of your last format, but that this is configured as two mono tracks A & B in one direction only, but the replay head is also a two track head so you have a single direction stereo track. This is also known as half track recording.

The machine that is being used to replay is highly likely to be your first diagram, which is two tracks of stereo, one in each direction.

You middle diagram is what you would find on a cassette player.

I will be specific about the problems that will definately result in trying to botch the head alignment on the machine.

First, the head azimuth will probably end up out ( head azimuth means that the replay head must be at perfect right angles to the tape), this will cause a reduction in the bandwidth, and will sound like the top frequencies, sibilant sounds such as cymbals, spoken esses etc, will become muffled.

Next, the yaw of the head ( this means that the head it not vertically aligned with the tape) will be put out, this will cause one track to sound quieter than the others and it if its gross misalignement, you could score or crease the tape.

Next the tape height, the problem here is that the tape guides are set to get the tape in line with the head height, if you move the head up or down then you tape guides will be wrong, and it you try to correct the guides, there is a reasonable chance you will crumple one side of the tape.
The effect of head misalignment is such that it can sound very similar to head yaw misalignment, and this is what can make it so difficult to correct.

I have not even begun to mention the effect it will have on recording.

Reel to reel machines almost always have 3 seperate heads, one is to erase, one is to record and one is to replay. This has the huge advantage that you can listen to the stuff being laid down on tape, in real time (well apart from a small time delay)
(Yes I know some machines have six heads, the few auto reverse machines have 3 heads for one direction and another 3 heads for the other direction, but these are quite rare)

These must be in perfect alignment with each other as well as in respect to the tape, or the erase will not remove the unwanted signal before the tape reaches the record head, and if the replay head is not at the same alignment as the record head, it becomes impossible to monitor what you are actually laying down on tape, which means you will make adjustments to the mixed in signal.
It will result in tapes that will sound terrible when replayed on another machine, you will not only get poor recordings, you will lose compatability with all other reel to reel machines.

Amateur adjustments have the potential to misalign the head in so many differant ways that it really isnt worth the hassle.

Proffessionals not only have alignment tapes, they also have model specific alignment jigs, and the necessary test equipment to ensure consistant performance across all channels, which is vital when you are talking of serious recording.

You have to remember that reel to reel quality can be nothing short of phenomenal, to such an extent that the vast majority of music recordings were laid down on reeel to reel masters, you want to hear the best quality sound of almost anything done before the late 1980’s ? You will find that the CDs in the shops were mastered on reel to reel machines, and trust me, CDs in general are not as good as you might imagine, SACDs sound significantly better, and most of those were mastered from recordings made on reel to reel machines.

Today we are all digital, but to be honest, the majority of chart music is poo anyway, very high quality sound, but rubbish music (actually the sound quality isn’t all that good either for other technical reasons such as limiting, gating and compression)

22

That’s exactly right. The campus radio station, where the tapes were originally recorded, migrated from R-T-R years ago. I don’t know where this machine came from; for all I know, it’s the original machine. But it’s what we have.

As I said upthread, I have access to a certified engineer, who checked the machine thoroughly when it was pulled out of storage and made whatever necessary repairs before he hooked it up, which I assume included aligning the heads.

I understand that I can’t fix it in post. Digital production has limits, which is why I know I need to find a way to get it right before it’s digitized. I can isolate left and right channels and will try that. I’ll post again on Monday with the results.

Robin

23

Unfortunately if it is in the format I expect, you can’t get the backwards sounding artifact out from your current machine, this is because two track tape tracks are laid out in such a way that they come across about one and a half of the 4 channel playback heads.

Each 2 track channel is somewhat wider than one 4 track head and its just laid out differantly, each 2 track channel takes up around two thirds of each half of the tape and is centred around the middle of that half of the tape - if that makes sense to you.

This means that track one on the 4 track machine is ok-ish, but will be low volume and pretty muffled but track 3 on the 4 track machine will pick up track two of your 2 track tape at even lower volume but backwards.

2 track machines are fairly easy to come by, and as I say, there are quite a few machines that can playback both 2 and 4 track, because what those machines have is an extra head with differant spacings, and you have a selector mechanism to switch.
There are programs that can reverse the sound, but in this case it just won’t work, because to do that you have to be able to completely isolate the backwards part, and you cannot on this machine format.

Since this is a campus tape machine, I would expect it was of a good quality, the very best reel to reel machines for this sort of use were 2 track and ran at fairly high speed, I’m thinking of the broadcast standard Revox A77 which had many differant configurations.

This will give you some idea of what you are up against.

http://www.reeltoreel.de/worldwide/Tips09.htm

24

scotandrsn: As I said in a previous post, and as casdave implied, there are no R-T-R recorders that record in the configuration of your second example. (This is apparently a very common misunderstanding.) Cassettes are recorded in that pattern, but not R-T-R.

casdave: First, thanks for explaining the some of the intricacies and risks of adjusting heads that others here seem not to appreciate fully.

I’ve been assuming the OP’s situation is the opposite of what you are thinking: that the tape is a four-track, bi-directional recording, and the playback machine is a two track. Let’s hope for MsRobyn’s sake that you are right. Although you’re right that playback from the left track will be somewhat noisier than if it was played on a two-track machine, I think she’ll find it acceptable for her purposes.

In any case, it will be much preferable to my scenario, in which case she will not be able to get a clear digital recording without finding another whole machine. We agree about this, yes?

BTW, some half-track machines were configured so that their heads perfectly overlaid that tracks laid down by quarter track recorders. There was a term that distinguished this from the conventional half-track configuration, but I don’t recall it at the moment.

MsRobyn: Do you know how many hours of recording you have? How many tapes are there, what size (ordinary reel sizes are 10", 7", and 5") and what length? (Should be printed by the manufacturer on the box.) What recording speed was used? (Should be hand-written by the engineer on the box.) Also, what are they recordings of? Music, spoken word?

25

Most likely, as others have said, there is a mismatch between the head alignment of the machine used to record the tape and the one used to play it back.

Actually, there is a scenario where scotandrs,'s second scenario is possible, thiough it is very unlikely. A 4-track machine might record any two tracks in the same direction, but under normal circumstances would never be flipped around and recorded in the opposite direction after the first recording. And if it was reversed, the erase heads would normally wipe clean all the channels before the tape gets to the record head for the second time. But the erase heads could have been disabled, and it’s even possible that different machines with different formats were used to record the tapes when they passed through one direction as opposed to the other.

My best advice is to use a 4-track machine for playback. You can isolate whichever tracks are recorded in a sigle direction, and turn the others down.

26

I’ve got five reels of what are basically news broadcasts. The alumnus did a show during his college days, and that’s what I’m digitizing. The tapes weren’t marked for speed, but it sounds like they were done at 3.75 ips. I’m not worried about that; I can definitely fix that in post.

Also, a HUGE thank you to everyone for the information. One of the problems with, ah, legacy technology is that novices like me have no idea what’s involved with using them.

Robin

27

I’ll be interested in hearing the outcome. Your in-house engineer should be able to figure out the issue in no time, and with any luck you should be able to get something off these tapes. Unfortunately, if they really were recorded at 3.75 ips, the quality will be only mediocre at best.

I’m about to embark on a similar program of digitizing a lot of old stuff from RTR and LPs. I’m expecting it to be time consuming and tedious, but I want to get rid of all my old analog tapes and disks while I still have machines that can play them.

When you have a chance, could you please tell us the make and model of your playback machine? That will probably answer a lot of questions.

28

Well, I tried it again. I managed to isolate the right and left tracks, which helped, but I still got that reverse signal.

I did contact the alumnus, who told me he used a Crown recorder and (he thinks) four-track tape. Unfortunately, he doesn’t remember much more than that. I sent him one of the mp3s from one of the tapes so he can hear what I’m talking about and to see if that jogs his memory a bit. Since we don’t have a lot to work with, we’re probably going to give the tapes back so he can have them digitized at a professional lab.

Robin

29

What kind of track configuration is your playback machine? Are you saying that when you play either track in isolation, you hear things backward on both?

30

All I know about the playback machine is that it’s a Revox.

I had to isolate the left and right tracks because I was getting two separate signals going forward and (probably) two separate signals in reverse. When I isolated the tracks, I was getting one going forward and (probably) one in reverse.

Now that I think about it, I can send anyone who’s interested a sample of what I’m talking about. E-mail me and I’ll send it.

Robin

31

Unfortunately, “Revox” is not sufficient information.

Are there 1, 2, or 4 Vu meters (the needles that swing with the sound level)? Sometimes, rather than meters, there are lines of LEDs, and as the volume goes up, more lights go on.

32

The machine has two VU meters. I’m just afraid that I’m trying to play a four-track tape on a two-track machine.

Robin

33

Okay. Now it’s completely clear: hearing a forward signal and a backward signal on both channels can only mean that the tapes are quarter-track, and the playback machine is half-track. This is the only situation that explains the circumstances you’ve described.

And as I’ve said, there’s no way you’ll get a clear recording from this machine.

There’s only one faint ray of hope. I believe (but don’t know for sure) that some Revox half-track decks also had quarter-track playback heads. As casdave and I have mentioned, you’d have to throw a switch or switch some jumpers to get quarter-track playback. (I never used a Revox, so I don’t know the details.)

Look up underneath the head assembly. There will be at least three heads, from left to right, Erase, Record, Playback. If there are only three, I think you’ll find that the rightmost one looks like the first example below.




Half-track
    -------------------------------
   |     ---------------------    |
   |    |                     |   |
   |    |                     |   |
   |    |                     |   |
   |    |                     |   |
   |      --------------------    |
   |     ---------------------    |
   |    |                     |   |
   |    |                     |   |
   |    |                     |   |
   |    |                     |   |
   |      --------------------    |
    -------------------------------
Quarter-track
    -------------------------------
   |     ---------------------    |
   |    |                     |   |
   |    |                     |   |
   |     ---------------------    |
   |                              |
   |                              |
   |                              |
   |     ---------------------    |
   |    |                     |   |
   |    |                     |   |
   |     ---------------------    |
   |                              |
   |                              |
   |                              |
   |                              |
    -----------------------------

But if there are four, and the furthest one on the right looks like the bottom one, you should be able to play the tapes okay. It’s just a matter of figuring out why you’re not getting the quarter-track output.

But I’m sorry to say that I think it’s a long shot.

Does the radio station have any other reel-to-reel machines?

34

Note: the playback head will be farthest to the right, though all the heads should look the same.

The problem is that the playback head of you machine is physically wider than the original record head, so it is actually playing back two tracks at once.

35

So there is good news and bad news.

The good news:
Almost certainly, your recorded material can be recovered and properly re-recorded without hearing “backwards” interfering sound.

The bad news:
you need a different playback machine to do it, or an engineer to install a different set of heads on your machine.

Does your machine look like this? I used to use that exact model. I have never seen one with two playback heads with different configurations, tho.

36

The controls look identical. Our machine works in 7.5 and 15 ips, but otherwise, it looks pretty much the same. I don’t think it’s exactly the same, though.

Robin

37

If you can’t get another machine, I suggest contacting your local public radio station and talking to the chief engineer or the production manager. Tell them you’re looking to copy this tape and having track configuration issues. You have a 1/4 inch, 1/2 track tape player, and a 1/4 inch, 1/4 track recorded tape. Preferably you want to use a 1/4 track playback machine, but a 4-track* should also do. Maybe they can help, and a public station is more likely to have old gear around.

  • A 4-track machine records and plays all 4 tracks in the same direction, but for playback purposes you should be able to isolate a single track and get decent results.
38

I’ve never worked in studio with Revox machines (Studer, Ampex, Tascam, Teac, etc., but not Revox), which is why I said I wasn’t certain. I have seen pro machines with two playback heads, but a little quick Googling hasn’t brought up any examples of Revox decks set up like that.

Bad news for MsRobyn, unless the station has a quarter-track machine lying around somewhere.

39

It’s been a while since I’ve been there (I interned there), but I’m reasonably sure they don’t have any RTR equipment. I’m also not willing to go much further with this.

I did speak to the alumnus, and he’s understanding of the situation. It would have been nice to have this finished, but if we can’t do it, we can’t do it, and he’ll have the tapes digitized at a professional lab. If nothing else, he knows what he’s got, so he’ll have that information on hand.

Thanks to everyone for all the help and advice. It was helpful even though I couldn’t do much with it.

Robin

40

I can think of several machines that can play back half track as well as quarter track tapes.

Most of the later Bang & Olusen ones will, Saba will so will Uher.

The earlier Akai ones might depending upon the model.

Your best bet is likely to be one of the tascam 4 track machines, certain ones can record all 4 tracks in one direction and have 4 vu meters along with a simple mixer arrangment.

If you are attempting to playback on a Revox and it is half track, it could be almost any machine, as they had literally dozens upon dozens of variations of the same model, and several differant models to choose from.

Your problem isn’t as bad as it seems, if we are dealing with quarter track tapes, then almost any of the later reel to reel machines will be fine, except for the proffessional Revox/Studer machines.