[ProAudio] FeralA - Recordings released encoded with Dolby A

Richard L. Hess lists at richardhess.com
Tue Feb 11 16:40:47 EST 2020

Hello, Jay,

As you may recall, Azimuth has become an issue with me. I hope you're 
happy with my two slides on pages 27 and 28 of my Powerpoint on the 
subject at the Culpeper Archiving Conference in June 2018.
The two azimuth audible demos are at the following URLs in the order 

As to the levels, of course you are correct that the tape reference 
levels should be set in the machine not ahead of it (rec level cal and 
play level cal). My comment elsewhere is that you do run into gain 
staging issues with dbx, but you don't see those with Dolby for 
precisely the reason you mention.

As to azimuth wander, I had two tapes and the best copies of the program 
that could be found were the half-inch four-track loop-bin running 
copies. There were two. One of them was reasonable, the second had the 
worst wander I think I've ever seen on a tape. Of course, the loop-bin 
masters were probably run to death before hand, and the 1,3 and 4/2 
track pairings didn't help. On this project I tried to keep up with the 
gross changes as the tape went, but I found I got a much better mono sum 
(I'm 99% certain they were intended to be mono tapes) with running the 
"Azimuth correction" function in iZotope RX. The Stereotool.com azimuth 
"strip chart" display (page 24 of the Culpeper Powerpoint) went from 
full extremes to barely moving. Of course, the azimuth correction 
doesn't and really can't do much about the HF loss in the single track, 
but if it can consistently time-align the tracks, then the mono sum will 
have as good HF response as an individual channel (or almost). See chart 
on page 21 of the Culpeper Powerpoint.



On 2020-02-11 5:29 p.m., Jay at MRL Support via ProAudio wrote:
> Re: [ProAudio] FeralA - Recordings released encoded with Dolby A Hello 
> James,
> 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.
> 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.
> 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.
> The gap scatter is much more a time-delay problem than an azimuth 
> problem in the individual tracks.
> On Tuesday, February 11, 2020 at 10:00, you (James Perrett via 
> ProAudio) wrote:
> 	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?
> On Tue, 11 Feb 2020 at 17:21, Bob Katz via ProAudio 
> <proaudio at bach.pgm.com <mailto:proaudio at bach.pgm.com>> wrote:
> 	Here are three responses to three different subjects that have 
> expanded from the original topic!
> 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.
> 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.
> Agreed.
> 	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.
> 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.
> 	III. Dear John:
> 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?
> 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.
> Best wishes,
> Bob
> On 2/9/20 4:32 PM, John Chester via ProAudio wrote:
> 	On 2/9/20 3:56 PM, Richard L. Hess via ProAudio wrote:
> 	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!
> 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.
> 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.....
> -- John Chester
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> *       JRP Music Services, Hampshire, U.K.
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Richard L. Hess                   email: richard at richardhess.com
Aurora, Ontario, Canada           http://www.richardhess.com/
Quality tape transfers -- even from hard-to-play tapes.

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