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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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1st Jan 2024, 11:11 am | #21 |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
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Re: BBC FM latency experiment
I remember years ago visiting a microwave link repeater station that carried the Eastern Radar (Watton) "pictures" to Midland Radar at North Luffenham and being told that the digital network was so slow 2 jet aircraft would have probably collided before they saw the danger on screen and had time to warn them!
Given what was said earlier in this thread and how slow digital was decades ago, it was probably true? Rob
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1st Jan 2024, 11:58 am | #22 | |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,682
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Re: BBC FM latency experiment
Quote:
Audio compression doesn't generally use this sort of technique, concentrating more on what's audible to the human ear within a frame of audio data which may be as little as 1ms long (in NICAM). So there's little or nothing to be gained in terms of compression or transmission quality by taking more time over it. Chris
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1st Jan 2024, 1:32 pm | #23 |
Octode
Join Date: Feb 2009
Location: Bletchley, Buckinghamshire, UK.
Posts: 1,223
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Re: BBC FM latency experiment
I have noticed how the situation of audio and video latency has changed over the years, having spent a lot of time working on conferences and large-scale entertainment events that used video displays and audio reinforcement or amplification.
Back in the late 1980s / early 90s at pop festivals the video was all analogue and almost instantaneous. Although it is natural to see things before we hear them, 40 or 50m from the stage the big screens used to make the performers look as though they were miming, due to the propagation delay of the sound. I remember once suggesting to a video production crew that a delay would be useful, to sync things up in the middle of the audience. They laughed, and told me what a video framestore cost - which in those days was more than enough to make the idea economically unviable. These days the situation is reversed. Digital audio has low latency but digital video is very slow, the delay being in multiples of frames. The video delay is often much more than that which would usefully synchronise for the audience who are far enough away to rely on the screens for a good view of the show. Hearing sound in advance of picture is not natural, and consequently far more noticeable than the other way around. This can cause difficulties at conference events where the presenters are all on radio (or top table) mics and appear on screens along with the videos, powerpoints, etc. We would get asked to delay the sound to match the picture, which of course is technically simple now that the audio desks are all digital. But the required delay can be surprisingly large, resulting in the presenters hearing an echo of themselves via the sound reinforcement system (and the room) which can make it very difficult to keep speaking coherently. Being corporate execs, rather than musicians, they don't want to be wearing in-ear monitors. In the worst cases there has to be a compromise between having them look silly on the screens, and having them become hesitant or confused when speaking. And it gets even worse when mics get handed around the audience for the Q&A session... |
1st Jan 2024, 6:39 pm | #24 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,015
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Re: BBC FM latency experiment
Audio latency can be fascinating: it is a big issue in 'communications' applications too; if the audio sidetone you hear in your headset is significantly delayed it can lead to a degree of additional mental-processing overhead you really don't want in a combat situation.
OTOH it can be a useful diagnostic tool; one of my relatives worked as an assessor for claims of hearing-damage in the mining industry; the test kit was a tape-based delay loop, you got the claimant to wear headphones and, talking into a microphone, read a page from a newspaper while the degree of audio-delay was varied. If you've got good hearing a small amount of delay is tolerable, but lots will soon cause you to go to pieces. If you've got compensation-worthy hearing damage you don't notice the delay even when it's half a second or more. He found lots of 'supposed' claims of hearing damage were spurious.
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1st Jan 2024, 7:01 pm | #25 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,244
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Re: BBC FM latency experiment
This is a big deal for presenters wearing earpieces or headphones. They are generally given a "clean feed", which should not include their voice, but sometimes that isn't the case, often by accident, sometimes by design.
Hearing yourself can be distracting, but with the right delay it can be disastrous. Rarely a problem in the analogue era, but with digital circuits and data rate reduction codecs, delay is inevitable today. A real problem for systems based on AoIP over WANs, which is why the ViLoR (local radi) project uses a locally generated analogue mix for the presenters. With this in mind, we test every student who comes through our training. Of course, it's an informal test - just a bit of fun, really, but the learning point is to demonstrate that latency can be a real problem for the presenters and guests that we have to look after. The test is really simple - we put them in a studio with headphones on and ask them to read something. Meanwhile, we gradually increase the delay to their headphones and listen carefully to how they get on. We find that people start to notice delay at 10-20ms, but aren't bothered by it. It just sounds like echo. As humans we're used to that - think about the speed of sound and the size of rooms we tend to inhabit. Long delays - 200ms or more - are also fine. Distracting, but most people can ignore it. That's because the brain is able to rationalise the situation - it's just a delay. There is quite a range in reported annoyance, but it rarely stops people from being able to talk. However, there is a region in-between these where most people find it incredibly hard to speak. Some just have to stop - others can keep going, but their words are heavily slurred, as if they are seriously intoxicated. This can be anywhere between 80 and 160ms, but it does depend on the individual, because it's related to the way the brain processes hearing and speech particularly. As you increase the delay into this zone, you notice some minor slurring of words before it gets really bad. But some people are totally unfazed by this, whatever delay you do, and no matter how carefully you repeat the experiment. It's a definite minority - perhaps only 1-2% in our tests. But they exist. The test subjects are generally young, and have normal hearing. Yet despite this, there is an absolutely huge variation in results. Our test pool might not be massive, but it's more than 100 by now. The variation in our tests is caused by brains, not ears. As a result of this direct experience, I seriously call into question the validity of the methodology described in #25, and would appreciate references containing more information about the rational behind it, and the weighting given to that test (presumably one of several tests forming the overall conclusion). |
1st Jan 2024, 7:59 pm | #26 | ||
Heptode
Join Date: Jul 2006
Location: Winchester, Hampshire, UK.
Posts: 639
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Re: BBC FM latency experiment
Quote:
Quote:
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2nd Jan 2024, 8:56 am | #27 | |
Heptode
Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
Posts: 648
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Re: BBC FM latency experiment
Quote:
This effect was demonstrated during my professional training, using exactly the same sort of tape based system described in post #25. Given that this was 1975 and we were in our early 20’s (with hearing that met or exceeded armed forces entry standards) it seems improbable that it’s anything other than a psychological (psycho acoustic?) effect. The only way in which I can see it being used to assess hearing loss would be to induce the effect in the test subject and then reduce the volume fed to the headphones until normal speech returns. What advantage that might have over the usual methods of assessing hearing I don’t know. Hugh |
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2nd Jan 2024, 11:40 am | #28 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,015
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Re: BBC FM latency experiment
I can't provide any references about the logic of the delayed hearing test, it was back in the early 70s, my relative is long since dead but worked for what was then the National Coal Board in Nottinghamshire.
The effect definitely existed though, when tried on me. I am assuming it was in part because of the combination of through-bone sound conduction and that through the air. Get the respective levels of the two components right and add some delay, then resolve the needed amplification to cause the speech dysfunction at different degrees of delay..
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2nd Jan 2024, 12:40 pm | #29 |
Nonode
Join Date: Dec 2012
Location: Stockport, Greater Manchester, UK.
Posts: 2,122
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Re: BBC FM latency experiment
I can't see any argument against this cunning test being used to evaluate hearing loss during employment. If only 1-2% of those tested can continue to read under that challenge, that means 1-2% of the claims might be fraudulent, while correctly assessing all those with impaired hearing.
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2nd Jan 2024, 1:39 pm | #30 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,244
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Re: BBC FM latency experiment
Other than the fact it's deeply flawed.
If you've set the level in the headphones at a sufficient level to overcome the worst of the hearing impairment, then you're just testing brain processing, not hearing. They are two very different things. I just feel sorry for those who were wrongly denied support and compensation as a result of this. Hearing loss is a terrible condition to live with. |
2nd Jan 2024, 2:54 pm | #31 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 28,003
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Re: BBC FM latency experiment
Back on topic please.
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9th Jan 2024, 4:16 pm | #32 |
Heptode
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 669
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Re: BBC FM latency experiment
Interesting numbers from Mark Hennessy in post #8 about the speed of sound.
Many years ago, working with fast logic devices, since light travels 30cm in 1ns, we sometimes called a nanosecond a “light-foot”. I shall have to start calling a millisecond a “sound-foot”! Stuart |
9th Jan 2024, 4:19 pm | #33 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: BBC FM latency experiment
For PAL colour TV, on flesh tones, the human eye can spot one degree of phase error which is roughly 1ns which is roughly 6" of pcb track on FR4
Rough but useful DAvid
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9th Jan 2024, 6:52 pm | #34 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,726
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Re: BBC FM latency experiment
Received wisdom (see what I did there?) has it that the timing of the BBC time signal is most accurate when received 100 miles from the Droitwich station.
It just so happens that I live "Only, one hundred miles from Wychbold". Could be the title of a song? Perhaps not.
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