There is a recurring question as to what is the best way to set azimuth for playing a tape. Many people assume that using the test-tape alignment is best. Well, that makes another big assumption: The recorder used a proper test tape alignment. While that can be the case, it usually is not.
If the tape has tones on it (a big if in many collections), you align to that, right? Better, but consider this: When a 1963 album was being remastered, the mastering engineer aligned to the tones and after doing all his magic found that the tones were not recorded at the same azimuth as the program. Big redo!
Aligning to the track material itself is the only way to properly adjust azimuth in the absence of tones and it is still necessary to check the track material even if there are tones. The wider the track and the slower the speed, the more critical the azimuth adjustment is. Stereo makes it more critical and often times easier to adjust.
What is azimuth? It is simply the angle of the recording or reproducing head gap relative to the motion of tape travel. It should be perpendicular. 90 degrees. 90.000 degrees. But there are limits to the accuracy of even the best test tapes. And there are limits to the operator’s ability to adjust the machine. And the machine is subject to vibration and shock in its life. Here is an excellent paper by Jay McKnight of Magnetic Reference Lab about how they do azimuth and the limitations. Figure 6 of this MRL paper shows the frequency response when the azimuth is varied from optimum. What happens is that there is a combing effect and that can not fully be removed by re-equalizing after the transfer. Therefore, despite hardware and software “azimuth correctors” that are available, it is best to align the playback azimuth as best you can prior to transferring the tape — and do it with program.
Properly adjusting azimuth to program takes practice. If you have a mono tape, that’s all you have, but if you have a stereo or multi-track tape, you can sum channels that have similar program on them that should be in phase and get a wider baseline for tweaking the azimuth. Of course, gap scatter in a multi-track head, especially, can corrupt this approach. Gap scatter is the linear distance along the direction of tape motion between gaps in a multi-track head. It should be 0 which means all tracks’ gaps are in perfect vertical alignment. With stereo or multi-track recordings, you can use a phase scope to narrow the line of the mono component. Dale Manquen commented that he thought gap scatter was more of a problem than I made it out to be. He offered the following suggestion:
If you use a dual-trace chopped display triggered off one track, you can simultaneously see azimuth peaking in the amplitude, and time errors in the time offset. If the amplitude and phase adjustments don’t coincide, then you would have a starting point for the time offset due to gap scatter.
Adjusting azimuth is very similar to manually focusing a camera lens. You rock through the point of best azimuth and see how far out it is on either side and try to manually find the centre between two equally bad side points. You tune for MAXIMUM high end. Don’t worry about making a tape screachy or annoying — all you’re doing is capturing what’s on the tape. There is no point of azimuth adjustment that will make the tape brighter than the original recording. Anything but the proper azimuth will produce a tape that is duller than the original recording. The point of proper azimuth is very narrow. Often, the best performance will be within +/- 5 degrees adjustment of the azimuth setting screw. Make sure you’re listening to the highs as you do this. If the cymbals stop, go back to where the cymbals are. If you don’t have cymbals, listen for whatever high end you have. If you don’t have good speakers, use headphones. Remember to listen to stereo programs in mono.
One word of caution, especially with tones: You can set the azimuth to a false peak. This happens most often on narrow tracks. Checking with the voice announcements on a test tape will tell you if you’re close. It’s possible to have 8 kHz in phase on a stereo machine, appearing to be proper azimuth, and still have 4 kHz out of phase. If that happens, go back and tweak the 4 kHz and then proceed up the scale. This error is actually less likely on program material than on tones. The one time this happened to me was the first time I aligned an 8-track 1/4″ head. I wasn’t used to very small difference between the main peak and the peaks on either side of the main peak due to the very narrow (21 mil) track width.
[2013-06-27] Another word of caution. I have one butterfly DIN stereo head (1/4-inch) that actually shows HF tone peaking at different settings on each of the two channels/tracks. This is separate from gap scatter which is a longitudinal displacement of the head gaps. In the case of this head, the gap alignment/azimuth is different for the two different tracks. It is somewhat easy to envision how this could happen for the butterfly heads. I have not seen it with the more conventional stacked head arrangements that common for most other head configurations. In this case there is no one correct azimuth and the correct solution is to discontinue use of that head.
Here is an example of a 1.88 in/s mono two-track tape that was recorded with, I believe, test-tape setup followed by the same section after I adjusted azimuth to the program. In the piano section of the demo, you can actually hear the “combing” effect in the first section. There is some digi-junk even though it’s a 128 kb/s MP3, but you should be able to hear through that.
It is critical to play the tape with the proper azimuth. Remember, at this point in the tape’s life, this might be the last playing as you digitize it. Make sure it’s the best playing you can give it.
Bertrand Navarro commented:
I\’ve leave a comment here because i\’ve been working for the past two years on a new system that basically remove the burden of azimtuh adjustment.
I work in a R&D data storage company and we developped a system based on our previous magneto optical data reader.
We demonstrated that with magneto optical reading of the tape we can reach pretty good performances (SNR, Bandwidth, and THD). Plus, instead of using a single head, we\’re generating a laser line across the tape which is then imaged onto a photodiode linear array. The main idea is to reduced the indivual height of each optical head and then lower the constraint on the azimuth alignment. The azimtuh is seen as phase shift between optical signals and can be corrected by simple signal processing. So bassically you just have to set the head close to 0 ° azimuth but this setup can be done with less constraint. Furthermore, is the azimuth changes during playback (Tape written on two different decks or LTM), it can also be corrected by post processing.
I hope this technologies will be push forward to help in the preservation process that is going on.
Posted Apr 3, 2007 7:53 AM