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Signal chain in 'the old days' of tape

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Re: Signal chain in 'the old days' of tape

Postby ManFromGlass » Sat Jun 01, 2019 3:05 pm

the OP probably fell asleep as the old ones(!) reminisced. Or maybe just put in the earbuds and is checking out a new hot band or whatever they do under earbuds. Back in my day we sometimes used to put on the small headphones just to disengage while listening to nothing, or eavesdropping.
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Re: Signal chain in 'the old days' of tape

Postby Hugh Robjohns » Sat Jun 01, 2019 6:34 pm

MOF wrote:...the problem came with ENG cameras that (to save money ?) didn’t have dolby tone generators.

I think you're probably talking about Betacam ENG video cameras which used the domestic Dolby C noise-reduction system, and - frustratingly - that didn't incorporate a lineup tone facility at all (reflecting its domestic intentions). The problem was that Dolby C was much more powerful than Dolby B, and so even quite small rec/rep level misalignments caused very audible filter mistracking effects -- exactly as you describe.

Only the professional noise reduction systems -- Dolby A and Dolby SR -- used calibration tone systems to ensure accurate alignment, and when used correctly they worked very well.

However, because professional stereo mastering and multitrack tape machines were so stable, and so regularly calibrated, they remained accurately aligned to the associated studio Dolby systems... So lazy, ignorant, or time-pushed tape ops often simply didn't bother to stripe the Dolby tones at the top of the tape.

And as long as the tapes never left that studio, and as long as no one decided to mess with the machine calibrations (such as recording at a higher flux level etc), it would all work fine.

But the moment those tapes left the building no one else could (easily) align their own Dolby's properly, because there was no reference on tape... And probably no documentation of the relationship between the Dolby Level and the tape's reference fluxivity.
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Re: Signal chain in 'the old days' of tape

Postby ef37a » Sat Jun 01, 2019 11:37 pm

MOF wrote:
Ha! You'd be amazed and appalled at how many professional multitrack reels (many from big name studios) exist without Dolby tones on the front! And how much misunderstanding of the technology and its calibration there was (and still is) amongst tape ops and engineers in pro studios. The broadcast world was a lot better -- but then in 'dem dayz' they had proper training and rigid lineup procedures were the stock-in-trade.

We did know how to line up dolby tones in the studios but the problem came with ENG cameras that (to save money ?) didn’t have dolby tone generators. The edit suite playback machines had the requisite dolby cards in them but no tones to line up to, hence the higher frequencies pumping effect.
The worst dolby artefact I ever heard was a non dolby tape being dolby decoded: muffled a lot of the time and then really bright for short periods.

Seems odd to me that they chose a level sensitive system like Dolby for a relatively non critical application like ENG? DBX was reckoned to be inferior to Dolby A for most demanding music recording but DBX was surely good enough for the guy or gal reporting from the airport? (standing outside in the obligatory force eight of course with a tailplane in shot just to make sure us pea brained proles KNEW it was an airport!)

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Re: Signal chain in 'the old days' of tape

Postby Hugh Robjohns » Sat Jun 01, 2019 11:45 pm

ef37a wrote:Seems odd to me that they chose a level sensitive system like Dolby for a relatively non critical application like ENG?

Non-critical? We're talking broadcast sound here! :-) The requirement was to improve the signal-noise ratio of a pair of narrow linear tracks on slow-moving video tape... At relatively low cost and with minimal circuitry. Dolby A and SR were too complex and expensive, Dolby B not effective enough, and Dolby S not yet invented... So Dolby C was the least worst option.

DBX was reckoned to be inferior to Dolby A for most demanding music recording but DBX was surely good enough for the guy or gal reporting from the airport?

It also suffered severe mistracking issues...
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Re: Signal chain in 'the old days' of tape

Postby ef37a » Sat Jun 01, 2019 11:54 pm

Did not know about the miss tracking Hugh.

Was this also too early for digital "hi fi" video sound? I had one of the first JVC VHS machines, did not have that blasted AVG but a switchable limiter. Revelation in noise performance!

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Re: Signal chain in 'the old days' of tape

Postby Hugh Robjohns » Sun Jun 02, 2019 12:04 am

It was all a long time ago, but AFAIK both the original Betacam and the later BetacamSP formats used only linear audio tracks and not the (entirely analogue* FM carrier-based) 'hifi' tracks embedded within the video stripe that became popular with the later developmental stages of the VHS format.

*The digital label on VHS machines of that era related to the introduction of NICAM digital broadcast sound... But the audio recording on the machine was e tirely analogue, albeit on an FM carrier allowing much higher quality than the narrow linear tracks.
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Re: Signal chain in 'the old days' of tape

Postby MOF » Sun Jun 02, 2019 12:08 am

I think you're probably talking about Betacam ENG video cameras which used the domestic Dolby C noise-reduction system, and - frustratingly - that didn't incorporate a lineup tone facility at all (reflecting its domestic intentions).
Yes it was Betacam ENG. As a side note these broadcast cameras were always £60k excluding the zoom lens. When Betacam, Betacam SP, Digibeta and then Beta HD were released they always cost that amount. Now the cameras are much cheaper e.g. Red, Arri etc.
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Re: Signal chain in 'the old days' of tape

Postby Tim Gillett » Sun Jun 02, 2019 12:25 am

Hugh Robjohns wrote:
It also suffered severe mistracking issues...
I think the more common issue with dbx was pumping of tape noise.
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Re: Signal chain in 'the old days' of tape

Postby Tim Gillett » Sun Jun 02, 2019 12:32 am

John Willett wrote:
John Egan wrote:...8 track 1/4" machine

Same track-width as a cassette ;)
Narrower as I recall. The Dolby C kept the noise at bay but being only a 2 head design, it was a pain to recal all 8 tracks. At least on these Fostex machines the cal trimmers were accessible without tearing the machine apart, unlike Portastudios.
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Re: Signal chain in 'the old days' of tape

Postby Tim Gillett » Sun Jun 02, 2019 12:44 am

MOF wrote:
Ha! You'd be amazed and appalled at how many professional multitrack reels (many from big name studios) exist without Dolby tones on the front! And how much misunderstanding of the technology and its calibration there was (and still is) amongst tape ops and engineers in pro studios. The broadcast world was a lot better -- but then in 'dem dayz' they had proper training and rigid lineup procedures were the stock-in-trade.

We did know how to line up dolby tones in the studios but the problem came with ENG cameras that (to save money ?) didn’t have dolby tone generators. The edit suite playback machines had the requisite dolby cards in them but no tones to line up to, hence the higher frequencies pumping effect.
The worst dolby artefact I ever heard was a non dolby tape being dolby decoded: muffled a lot of the time and then really bright for short periods.
It's still a problem today transferring older encoded tapes. Cassettes can be the worst.
The tape might have been nominally B or C encoded but with the deck's bias or EQ switch set incorrectly . So without special tricks in playback, which can be educated guesses at best, decoding can be tricky if not impossible.
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Re: Signal chain in 'the old days' of tape

Postby Hugh Robjohns » Sun Jun 02, 2019 12:49 am

Tim Gillett wrote:
Hugh Robjohns wrote:
It also suffered severe mistracking issues...
I think the more common issue with dbx was pumping of tape noise.

Decoder mistracking the encoder... But yes, the symptom was noise pumping, as opposed to the HF modulation of Dolby C codec mistracking.
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Re: Signal chain in 'the old days' of tape

Postby Tim Gillett » Sun Jun 02, 2019 1:01 am

Hugh Robjohns wrote:
Tim Gillett wrote:
Hugh Robjohns wrote:
It also suffered severe mistracking issues...
I think the more common issue with dbx was pumping of tape noise.

Decoder mistracking the encoder... But yes, the symptom was noise pumping, as opposed to the HF modulation of Dolby C codec mistracking.
I meant the pumping of hf tape noise being now audible when a lf source like bass only couldn't mask it.
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Re: Signal chain in 'the old days' of tape

Postby Hugh Robjohns » Sun Jun 02, 2019 1:09 am

Yes indeed. One of the inherent limitations of DBX's broadband noise reduction design, and something the various band-splitting technologies of the Dolby systems were intended to avoid, of course.

But either way, the point was that processing artefacts of one form or another are always made much worse when rec/rep alignment issues cause the decoder to mistrack the encoder.
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Re: Signal chain in 'the old days' of tape

Postby ConcertinaChap » Sun Jun 02, 2019 6:38 am

Hugh Robjohns wrote:I note that while we are all happy reminiscing, the OP is nowhere to be seen...

Who cares, something I've seen several times before on the forum is when a thread is started in this way but then turns to gold in the hands of the forum's alchemists.

I'm especially enjoying this one as it's a sort of analogue recording Matter of Britain for us digital Johnny-come-latelies :)

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Re: Signal chain in 'the old days' of tape

Postby ef37a » Sun Jun 02, 2019 7:26 am

Tim Gillett wrote:
Hugh Robjohns wrote:
It also suffered severe mistracking issues...
I think the more common issue with dbx was pumping of tape noise.

Indeed and this I think was the main reason DBX was not used for highest quality music recording. I would have thought however that it was good enough for news interviews and such? Surely not going to record a prom with an ENG camera?

Of course, any technology will fail if a monkey is holding the spanner!

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Re: Signal chain in 'the old days' of tape

Postby Mike Stranks » Sun Jun 02, 2019 8:02 am

I had a cassette deck in the mid 70s - Toshiba - that was allegedly 'state of the art', although it never challenged the mighty TEAC A450 or the Nakamichis in my estimation.

Anyway... This thing was all satin-silver metal work and about the size and dimensions of one of the shoe-box digital mixers of today. It had decent sized VU meters and had a Dolby B line-up facility. It produced superb recordings, but was let down by it's mechanical drive system. Twin-capstan which had a tendency to mangle tape at the drop of a hat.

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Re: Signal chain in 'the old days' of tape

Postby ef37a » Sun Jun 02, 2019 8:22 am

Hi Mike, I "acquired" a top of the range Denon cassette deck.
Many years ago I did the hi fi repairs for a local TV service chap and picked up the Denon which the customer admitted had been dropped on one corner.

There was a crack across the main PCB and one across the dual loop capstan servo board.

I belled Roger and said "this thing will take hours to fix and link out all the broken print and that servo board will cost at least one arm!"

Rog' came back a few days later to tell me the punter did not want to spend FA and did I want to keep the deck for spares? Ok by me and over the course of a few weeks, on an off I fixed it. The servo board was just a fiver and Denon service spares dpt a joy to deal with (Oh! THOSE were the days!)

So, I had dual loop capstans, impeccable electronics, 3 heads and a CPU based bias and EQ tape test/optimizer. Superb machine. I later did a deal for it with another mate. Now have a very good Sony Dolby S machine.

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Re: Signal chain in 'the old days' of tape

Postby Tim Gillett » Sun Jun 02, 2019 8:50 am

Hugh Robjohns wrote:Yes indeed. One of the inherent limitations of DBX's broadband noise reduction design, and something the various band-splitting technologies of the Dolby systems were intended to avoid, of course.

Quite, Dolby did a far better job at using programme to mask tape noise.

Hugh Robjohns wrote: But either way, the point was that processing artefacts of one form or another are always made much worse when rec/rep alignment issues cause the decoder to mistrack the encoder.

Tape noise pumping/breathing (with some programme), is not mistracking. They are two different things. Noise pumping can occur with dbx even with perfect tracking.
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Re: Signal chain in 'the old days' of tape

Postby Tim Gillett » Sun Jun 02, 2019 9:06 am

Mike Stranks wrote:I had a cassette deck in the mid 70s - Toshiba - that was allegedly 'state of the art', although it never challenged the mighty TEAC A450 or the Nakamichis in my estimation.

Anyway... This thing was all satin-silver metal work and about the size and dimensions of one of the shoe-box digital mixers of today. It had decent sized VU meters and had a Dolby B line-up facility. It produced superb recordings, but was let down by it's mechanical drive system. Twin-capstan which had a tendency to mangle tape at the drop of a hat.


Dual capstan was more complex than single, with more to go wrong. The two capstans had to work at slightly different speeds, the tape had to have no slack when play was pressed, and the left pinch roller could not engage the tape earlier than the right one, or there could be tape chews. Possibly your machine just needed a good service.

Classic Naks are dual capstan and they too need regular service attention to make sure they don't chew tapes.
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Re: Signal chain in 'the old days' of tape

Postby ef37a » Sun Jun 02, 2019 10:31 am

Ah now TIm! This was explained to me MANY years ago at a two day service tech's training course* at Sony UK (those WERE the days!) .

The capstan flywheels are driven by a belt around a single motor pulley. The feed side of the belt is in compression and the leave side in tension. If you get the 3 shaft centres right and the belt's characteristics correct the system gives just the right speed differential to get the right tape tension across the heads.

I certainly have never had any tape damage problems with my machine and have never had any back in the day. What would "servicing" consist of? Keeping things clean is a no brainer with any tape machine. Change the oil after 2,000 cassettes?

*Travel cost probably tax deductible for the SE?

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