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Analogue versus Digital - Radio 4

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Re: Analogue versus Digital - Radio 4

Postby Hugh Robjohns » Thu Jun 27, 2019 10:05 am

CS70 wrote:Very cool graph, Hugh!

It is, and I use it a lot! But it's not one of mine I'm afraid; I borrowed it from Elliott Sound Products.
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Re: Analogue versus Digital - Radio 4

Postby Arpangel » Thu Jun 27, 2019 10:11 am

Hugh Robjohns wrote:The biggest practical difference, though, is that loud sounds on tape are progressively distorted, whereas they aren't in a digital system. H

That's what I miss most about tape, you can save what would be unusable on digital.
There were a couple of over loads on our last Marimba session, completely ruined the piece, on tape it would have just got thicker.
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Re: Analogue versus Digital - Radio 4

Postby Hugh Robjohns » Thu Jun 27, 2019 10:11 am

The Elf wrote:
Arpangel wrote:High bit rates are a bit like.....well, it's there so let's use it.
Bit rates and word lengths are significantly different subjects.

:D Weeelllll.... the word length will inherently affect the bit rate.... :ugeek:

But I think this is a terminology issue because what Arpangel meant -- and the Elf automatically interpreted -- was Sample Rates and Word-length.

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Re: Analogue versus Digital - Radio 4

Postby Hugh Robjohns » Thu Jun 27, 2019 10:16 am

Arpangel wrote:
Hugh Robjohns wrote:The biggest practical difference, though, is that loud sounds on tape are progressively distorted, whereas they aren't in a digital system. H

That's what I miss most about tape, you can save what would be unusable on digital.

Kind of... But there really is no excuse for anything to become 'usable' on a modern digital system because with 24 bit systems you can set up a massive amount of headroom if needed, quite happily, and without compromising the working dynamic range. So overload shouldn't ever be a risk.

And/or, if you like your meters glowing red, many source recorders have limiters of one form, or another that replicate the saturation effects of analogue tape to some degree.

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Re: Analogue versus Digital - Radio 4

Postby CS70 » Thu Jun 27, 2019 10:25 am

Arpangel wrote:
Hugh Robjohns wrote:The biggest practical difference, though, is that loud sounds on tape are progressively distorted, whereas they aren't in a digital system. H

That's what I miss most about tape, you can save what would be unusable on digital.
There were a couple of over loads on our last Marimba session, completely ruined the piece, on tape it would have just got thicker.

Yep, the digital moon is a harsher mistress than the analogue one if you make a mistake...
But with 24 bits, you really need to work hard to make mistakes and overload!

Recording to 16 bits is harder but nowadays basically all gear is 24 bit, even the little handheld recorders. Heck, you have to make an effort to find something 16 bit.

And one of the unsung beauties of DAWs is the internal 64bit computations.. you simply cannot overload internal channels and busses - only the physical outpus (and inputs at recording of course).

Not that I advocate to do it on purpose, but if you finally have your perfect sounding mix and you made a mistake and say the drum bus is overloading... you can just leave like it is and not worry about a thing, rather than risking to fiddle with the gain staging of all the 40 other tracks and busses. Try that with a console!
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Re: Analogue versus Digital - Radio 4

Postby The Elf » Thu Jun 27, 2019 10:35 am

Hugh Robjohns wrote:
The Elf wrote:
Arpangel wrote:High bit rates are a bit like.....well, it's there so let's use it.
Bit rates and word lengths are significantly different subjects.
:D Weeelllll.... the word length will inherently affect the bit rate.... :ugeek:
Hmmm... Tenuous? :lol:

Hugh Robjohns wrote:But I think this is a terminology issue because what Arpangel meant -- and the Elf automatically interpreted -- was Sample Rates and Word-length.
As you keep telling us, Hugh - it's important that we all understand what we're discussing!
:lol:

When you get into lossy technology then 'bit rate' becomes the standard currency.
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Re: Analogue versus Digital - Radio 4

Postby Hugh Robjohns » Thu Jun 27, 2019 10:38 am

The Elf wrote:As you keep telling us, Hugh - it's important that we all understand what we're discussing!
:lol:

:ugeek: Precissement!

When you get into lossy technology then 'bit rate' becomes the standard currency.

Yep, because there is no fixed word-length anymore...

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Re: Analogue versus Digital - Radio 4

Postby The Elf » Thu Jun 27, 2019 10:39 am

When you get into lossy technology then 'bit rate' becomes the standard currency.
Yep, because there is no fixed word-length anymore...
:thumbup: :beamup:
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Re: Analogue versus Digital - Radio 4

Postby The Elf » Thu Jun 27, 2019 10:43 am

Arpangel wrote:That's what I miss most about tape, you can save what would be unusable on digital.
There were a couple of over loads on our last Marimba session, completely ruined the piece, on tape it would have just got thicker.
There's absolutely no reason to 'miss tape' once you've grasped that there's no need to push the levels any more like the old days.

Lower recording levels at 24-bit and there's no reason you should have been anywhere even close to overloading.
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Re: Analogue versus Digital - Radio 4

Postby Hugh Robjohns » Thu Jun 27, 2019 10:48 am

Arpangel wrote:There were a couple of over loads on our last Marimba session, completely ruined the piece, on tape it would have just got thicker.

Sorry... no sympathy from me! :wave: There's absolutely no excuse for getting caught out with overloads these days. Headroom and gain structuring... the foundations of any recording session.. is all you need.

I did baulk when you mentioned only using 16-bit formats the other day -- why would you go out of your way to constrain yourself so unnecessarily. Silly idea! :lol:

I had to record a choir concert last weekend without a sound check or rehearsal before hand. So I just made sure I set up the recorder with a generous headroom margin.

The loudest singing peak ended up around -15dBFS with the average down around -25dBFS, and the loudest clapping from someone right under one of the outrigger mics hit -6dBFS. No limiters engaged. Very few red lights on the meters. No nervous concerns or fingers hovering over the gain controls during the recording...

The ambient acoustic noise of the church and audience was the defining factor for the recording's noise floor, so stripping out the no-longer necessary headroom in post production resulted in a perfectly acceptable recording level for the CD.

Did the same a few weeks ago when recording the engine start-up sequence of a Lancaster bomber. Never got anywhere close to clipping, even when there were backfires...

The recording engineer's life has never been easier than it is today with 24-bit word-lengths ! :thumbup:

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Re: Analogue versus Digital - Radio 4

Postby N i g e l » Tue Jul 02, 2019 1:16 pm

Hugh Robjohns wrote:The recording engineer's life has never been easier than it is today with 24-bit word-lengths

.. also, presumeably for long recordings using digital, the recording engineer doesn’t have to keep checking the recording medias “time remaining” and be poised ready to start a second tape machine or even swap to a fresh tape ?

Perhaps the disappearance of media time limitations is another by product of todays digital recordings as there is no longer an “A” side / ”B” side half time split as per vinyl and tracks don’t have to be ordered or composed to fit within the 20 (?) min /side limit.

I am reminded of the rear cover of an early Beatles album (CD artwork was reused from the LP which made it even more ironic) which said somthing like "Once again the beatles have given excellent value in providing a full 24 minutes of music on this long player".
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Re: Analogue versus Digital - Radio 4

Postby Elephone » Thu Jul 04, 2019 5:53 pm

I have it in my notes that cassettes can approach about 9 bit quality, though 4 or 5 bits was more typical on most tape-decks, whereas professional studio analogue tape recordings were only about 13 bit.
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Re: Analogue versus Digital - Radio 4

Postby Hugh Robjohns » Thu Jul 04, 2019 6:18 pm

To calculate the dynamic range, it's roughly 6dB per bit, minus 3dB (to account for the dither noise).

So a CD is (16x6)-3 = 93dB.

13 bits gives about 75dB which is extremely optimistic for a straight quarter-inch analogue tape machine -- although a high speed half-inch mastering machine might get closer to that. I'd suggest 11 bits as a more reasonable estimate, or 11.5 with Dolby A noise reduction.

9 bits sounds about right for standard cassette... 4 or 5 bits is rather pessimistic -- I don't think any of the cheap cassette recorders I abused through my early teenage years were even that bad (<27dB dynamic range?).

In our modern 24-bit era, it's worth remembering that most supposedly 24-bit converters actually only deliver a meaningful 20 bits or so of actual audio data. The last four bits are generally carrying preamp/converter self-noise. (There are some new converter topologies which can supposedly improve on that, but I've not had the opportunity to test any as yet).

Of all the D-A converters I've measured so far, the very best* managed the equivalent of 21.9 bits -- from an AES17 dynamic range measurement of 129dB(A) -- and the average is the equivalent of 19.9 bits (116.8dB).

For the A-Ds -- which is the more critical end of the signal chain, of course -- the average figure is 19.8 bits (which is 116.3dB(A) for the AES17 dynamic range), with the very best** managing 21.1 bits (124dB).

And if you need to know...

* Apogee Symphony
**RME ADI-2 Pro

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Re: Analogue versus Digital - Radio 4

Postby N i g e l » Thu Jul 04, 2019 6:49 pm

Elephone wrote:cassettes can approach about 9 bit quality, though 4 or 5 bits was more typical

Those figures would be for what is technically possible: In reality the pre-recorded casettes were a mixed bag, the worst ones were nth generation high speed copies = abysmal. Towards the end of the casette era, duplication quality picked up when they started using a digital technique.

The quality limitations of casette tape were well known and in the 70s, a 2xspeed, 2xwidth
Large cassette was developed, called the Elcaset. I think it was meant to take on HiFi reel-reel machines.
But it was expensive and CDs appeared on the market. The public embraced CD, home duplication to chrome cassette and walkmen (walkmans ?).
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Re: Analogue versus Digital - Radio 4

Postby James Perrett » Thu Jul 04, 2019 8:56 pm

One thing that I would point out with all those bit depths mentioned above is that you would need an instrumentation grade ADC/DAC to obtain the equivalent performance. Audio convertors tend to have a more relaxed performance spec - often the step sizes are not constant across the whole range of inputs so greater bit depths would be required to allow for this.
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Re: Analogue versus Digital - Radio 4

Postby Tim Gillett » Thu Jul 04, 2019 9:14 pm

N i g e l wrote:
Elephone wrote:cassettes can approach about 9 bit quality, though 4 or 5 bits was more typical

Those figures would be for what is technically possible: In reality the pre-recorded casettes were a mixed bag, the worst ones were nth generation high speed copies = abysmal. Towards the end of the casette era, duplication quality picked up when they started using a digital technique.

.
. High speed cassette duplication was the norm even with the top record labels. Tape didn't care what speed it was played or recorded at but of course real time monitoring was tricky.
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Re: Analogue versus Digital - Radio 4

Postby Tim Gillett » Thu Jul 04, 2019 9:21 pm

Hugh Robjohns wrote:To calculate the dynamic range, it's roughly 6dB per bit, minus 3dB (to account for the dither noise).

So a CD is (16x6)-3 = 93dB.

13 bits gives about 75dB which is extremely optimistic for a straight quarter-inch analogue tape machine -- although a high speed half-inch mastering machine might get closer to that. I'd suggest 11 bits as a more reasonable estimate, or 11.5 with Dolby A noise reduction.
Dolby A gave 10db reduction , rising to 15db at 15 kHz, so closer to a 2 bit reduction.
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Re: Analogue versus Digital - Radio 4

Postby Hugh Robjohns » Thu Jul 04, 2019 11:40 pm

I wouldn't get too bogged down in the numbers because there are lots of variables involved so it's all a bit ball-park-ish...

However... ;) ...the manufacturer's specs for my Studer A807 stereo quarter-inch machine state it should deliver a signal-noise ratio of 62dB (CCIR468-unweighted, 510 nWb/m) which is about 10.75 bits.

Dolby A is indeed promoted as providing 10dB of noise reduction overall (with a bit more at the very high end) but it's a dynamic psychoacoustic system that's relies on noise masking so it's a perceived noise reduction of 10dB rather than a flat, uniform, continuously measurable one.

If it was uniform, though, we should add about another 1.6 bits of dynamic range which takes it up to 12.25 bits or so ...on a good day, with everything lined up accurately.

...but I'm not entirely convinced a quarter-inch machine with Dolby A was ever that capable in practice. I still remember some trials I was involved in with the BBC back in the late-80s, making studio recordings to an A807 with Dolby A and simultaneously to a Sony PCM-701 in 14-bit mode. As I recall, the Sony was undoubtedly substantially quieter -- far more so than the theoretical 10dB difference should have provided (14-bit providing a dynamic range of around 81dB verses a theoretical 72dB for the tape with dobley).

So I reckon 11.5 bits is still about the right area... but I'll be pushed to 12 if you insist! :D
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Re: Analogue versus Digital - Radio 4

Postby Tim Gillett » Fri Jul 05, 2019 6:44 am

Hugh Robjohns wrote:...Dolby A is indeed promoted as providing 10dB of noise reduction overall (with a bit more at the very high end) but it's a dynamic psychoacoustic system that's relies on noise masking so it's a perceived noise reduction of 10dB rather than a flat, uniform, continuously measurable one.

If it was uniform, though, we should add about another 1.6 bits of dynamic range which takes it up to 12.25 bits or so ...on a good day, with everything lined up accurately...



Yes Dolby's noise reduction wasnt uniform. Where the input signal was strong enough to mask the tape noise, Dolby A did nothing, rightly assuming that masking would do its job, as it does.

This I think is a good example of recordings being made for people listening, not merely instrument measurements. Sure we can measure the noise underneath the wanted signal, but it's the (audible) result that counts.

I agree the non linearities of tape machines could be exaggerated by NR, and could introduce NR related artifacts. Digital was a great advance.
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Re: Analogue versus Digital - Radio 4

Postby N i g e l » Fri Jul 05, 2019 10:01 am

Hugh Robjohns wrote: ...on a good day, with everything lined up accurately.

....another item to put on the analogue maintainance schedule thats not required for (non mechanical) digital.

I was involved in the manufacture of a digital instrumentation tape recorder (mainly operating the kettle & teapot) on which the head was aligned by recording 2 tracks ( 1 forward, 1 reverse with the tape flipped) and then measuring the mid point/head center position, under a microscope so that the tape head could then be adjusted appropriately.

The small section of tape to be measured was cut off and “developed” it in some kind of liquid so that the magnetic pattern could be seen under the microscope.

I have often wondered what the developing fluid was, does anyone here know ?

The reels were about 14" diameter of 1" tape but only lasted for 12minutes recording time.
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