<html><head><title>Re: [ProAudio] FeralA - Recordings released encoded with Dolby A</title>
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<span style=" font-family:'Times New Roman'; font-size: 12pt;">Hello James,<br>
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I'm not really a head fabrication expert, but it's approximate like this: There were at least two ways of making multi-track heads: one used milled metal side pieces into which the cores were epoxied, and the whole thing either bolted or epoxied together. The other used a jig to hold the cores together, then epoxy was poured to hold it together without using any actual side pieces.<br>
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Glue together is a nice idea, but subject to the cores moving around from the intended positions. So the former is subject to less "drifting" of the cores, which causes gap scatter, but is more expensive to produce. Ampex professional heads were usually the former, consumer heads usually the latter. Other vendors often used the latter. <br>
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So if you want to know what the gap scatter is on your recorder, you really need to measure the time delay between the several tracks. And remember that you are measuring the sum of the delays in recording and reproducing the tracks.<br>
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The gap scatter is much more a time-delay problem than an azimuth problem in the individual tracks.<br>
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On Tuesday, February 11, 2020 at 10:00, you (James Perrett via ProAudio) wrote:<br>
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<td><span style=" font-family:'times new roman'; font-size: 12pt;">With regard to setting the azimuth by summing to mono and adjusting for maximum hf - I've heard people say that there could be a problem if the head gaps on each track are not in line with each other. I've not experienced this myself as I've found that setting the azimuth with a 15kHz tone using just one channel matches the mono sum method well on the machines that I use but I guess some manufacturers made their heads more precisely than others. Have others experienced head gaps being out of line with each other?<br>
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On Tue, 11 Feb 2020 at 17:21, Bob Katz via ProAudio <</span><a style=" font-family:'times new roman'; font-size: 12pt;" href="mailto:proaudio@bach.pgm.com">proaudio@bach.pgm.com</a><span style=" font-family:'times new roman'; font-size: 12pt;">> wrote:<br>
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<td><span style=" font-family:'times new roman'; font-size: 12pt;">Here are three responses to three different subjects that have expanded from the original topic! <br>
I. There are so many ways that test tones at the head of a tape could be different from the audio on the tape. In the old days in some studios I saw some visiting engineers lay down test tones after the fact, on a different machine than was used for the mixdown, as if that would help the situation. <br>
We transfer engineers have developed a spidey sense and often check and recheck the content to ensure it has not changed from cut to cut. On most machines, before making the transfer for a new cut, you can put your finger on the side of the tape to tilt it slightly, while listening in mono to ensure the azimuth has not drifted, and then transfer the new cut. </td>
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<span style=" font-family:'Times New Roman'; font-size: 12pt;">Agreed.<br>
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<td><span style=" font-family:'times new roman'; font-size: 12pt;">II. As for Bob O's comment against the practice of doing elevated dolby level, I agree there was a standard for 185, but as people started to use elevated levels as high as 6 dB over 185, I was seriously concerned about running out of headroom in the Dolby gear if standard dolby level was used, and so as a practice, I see less harm in using an elevated dolby level than to overload the Dolby processor with too hot audio. I always recorded dolby tone as well as 1 kHz @ VU. The Dolby 361 meters, as Richard mentioned, were notoriously inaccurate, I would put a sharpie mark on the real dolby level on the meter, for what it was worth. There was a Dolby tester that could be used for accuracy of the dolby tone or I believe a test point that could be checked. </td>
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<span style=" font-family:'Times New Roman'; font-size: 12pt;">Remember that the Dolby process does NOT increase the level of the high-level passages, only that of the LOW passages. And if you record at higher level then 185 nWb/m, that increase should happen AFTER the Dolby processor, NOT before it. So distortion of the Dolby processor should not be a problem.<br>
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<td><span style=" font-family:'times new roman'; font-size: 12pt;">III. Dear John: <br>
So you set azimuth by looking at the bias. Are you looking at two channels of bias? And how is this superior to the tried and true method of mono-summing the left and right audio channel and adjusting for maximum high frequency response, also checking by inverting the polarity of one and going for a minimum as a cross check? </td>
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<span style=" font-family:'Times New Roman'; font-size: 12pt;">In my experience, the limitation on the accuracy of setting azimuth is not the measurement accuracy (how high a frequency you use), but the azimuth wander caused by tape slitting wander, and wander of the tape in the transport guides.<br>
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<span style=" font-family:'times new roman'; font-size: 12pt;">Best wishes,<br>
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Bob<br>
On 2/9/20 4:32 PM, John Chester via ProAudio wrote:<br>
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<td><span style=" font-family:'times new roman'; font-size: 12pt;">On 2/9/20 3:56 PM, Richard L. Hess via ProAudio wrote: <br>
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<td><span style=" font-family:'times new roman'; font-size: 12pt;">Also, a funny story, the tones at the head of that master tape caused Alan a bit of a challenge...the azimuth of the tone didn't match the azimuth of the audio! </td>
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<span style=" font-family:'times new roman'; font-size: 12pt;">Been there, seen that. An assembled album master may contain cuts that were recorded on different machines, and sometimes even in different studios. This becomes really obvious when I'm doing Plangent transfers. I can see when the bias frequency and flutter profile change, and I set azimuth on each cut by looking at the bias. I have seen a master where the head tones were recorded on a different machine which didn't match *any* of the music. <br>
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When the album master is Dolby encoded, and several different machines were used for mixing, I can't believe that the Dolby setup on all of those different machines perfectly matches the head tones. Fortunately the tape with the head tones recorded on a completely different machine wasn't Dolby..... <br>
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-- John Chester <br>
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