BBC FM latency experiment

[GMB]

Am I the only one that heard a BBC Radio4 programme this morning where they tested the hypothesis that a person standing at the base of the Big Ben clock tower would hear the bongs on an FM radio before they heard it "live" ?

It seemed to work thus proving that however the FM signal is distributed these days it still has very low latency.

They mentioned in passing that it would not work with anything digital and did not mention AM. I wonder how many people appreciate how un-live modern media have become?

[fetteler]

That's really good that is! To be honest I'm amazed that the FM broadcast chain is so fast. There are definite delays through the yards of coax and electronics that the signal has to travel through.

As for modern broadcasting (especially video) the layers of coding, processing and decoding that are involved give some huge delays. Those who have sat in a studio or OB truck and watch their own output off air know how long it is!

Steve.

[paulsherwin]

I'm a little suspicious of this. While the FM lags are lower than digital systems like DAB and Freeview, there is still Nicam encoding taking place at the playout centre and decoding being done at the transmitter.

[SiriusHardware]

I could be showing my ignorance, but I thought encoding to / from stereo on the FM brodcast band was a purely analogue process involving a subcarrier? I honestly thought Nicam was TV-only.

(The OP was talking about FM radio, not analogue TV with Nicam sound).

Back in the late 1970s we tried a similar experiment instigated by my secondary school physics teacher who noticed I had brought a pair of walkie-talkies in to school. His immediate response was to incorporate them into a physics lesson, to demonstrate that a shout emanating from the holder of one radio would arrive at the other radio faster than the direct sound across 200m or so. It worked really well.

[paulsherwin]

No, the BBC FM transmitter distribution system was completely re-engineered using Nicam technology in the 80s.

Note that this is a professional distribution technology, not a consumer broadcast standard like TV sound. FM transmissions themselves are obviously analogue.

[SiriusHardware]

Sorry, I had never heard of that before. Nicam must be quite quick though, I think the 'N.I.' in Nicam stands for 'Near Instantaneous' and it really needs to be, given that it is transmitted along with real time analogue video and real time analogue sound for non-Nicam equipped receivers? If the delay was significant then both the analogue video and the analogue (non-Nicam) audio would need to be delayed by the same amount in order to keep everything in sync? (Maybe they were).

[paulsherwin]

See https://audiomisc.co.uk/BBC/PCMandNICAM/History.html

[mhennessy]

NICAM deals with audio in 1ms blocks, so the latency is very low indeed compared to modern methods.

As a rule of thumb, sound travels at 1 foot per millisecond. Or 34cm/ms if you need to be more precise.

How high up are the bells? If we guess at 300 foot, then that's 300ms for the sound to reach the pavement. Which is likely to be considerably longer than the broadcast latency, even with the digital infrastructure we've had for around 2 decades. It'll be interesting to see how this changes as things migrate to AoIP within BH, but for now, with no need to delay radio signals to maintain lipsync with video as that gets processed, it's not surprising that FM radio remains low latency.

From Wikipedia:

Londoners who live an appropriate distance from the tower and Big Ben can, by means of listening to the chimes both live and on analogue radio, hear the bell strike thirteen times. This is possible because the electronically transmitted chimes arrive virtually instantaneously, while the "live" sound is delayed travelling through the air since the speed of sound is relatively slow.

[paulsherwin]

Fair enough, I stand corrected.

[dave walsh]

Yes I heard the Radio 4 "Experiment". Interesting and apparently coinciding with the Anniversary of Big Ben's Chimes being Broadcast for the first time in 1923. I'm not sure that [despite an attempt at clarity and avoiding too much tech info] there weren't still assumptions about audience knowledge. I'm not that bright so I thought it was just going to be the old question about why there's a delay between different audio sources at first but DAB was thrown out immediately.

I suspect it doesn't help that terms like VHF or FM can mean the same thing [or not] like Analogue and AM. I suspect though that, ironically, nobody actually cares very much about where the new technologies come from... or are now going... as long as the cell phone is still working! Will streaming turn into screaming in the end I wonder The best and most succinct phrase in the BBC piece was that "One is at the speed of light and the other the speed of sound". I was, for some reason, reminded of that other demo from a Medieval tower involving a feather and a cannon ball. I'm watching now, on BBC2, the recent production of the Matrix, at Manchester's fantastically expensive new record breaking venue, It starts with a Simulacrum of Alan Turin lecturing from inside a very large TV of a type familiar to Forum members. It is fairly impressive and I bet that most of the, in house, audience will be up to speed at least!

Dave W

[Aub]

I had to look up Simulacrum. Never heard of the word.

Aub

[emeritus]

Anyone seeing in the New Year at midnight tonight will almost certainly find that the chimes on FM are heard several seconds before they are heard on TV or digital radio.

[paulsherwin]

Yes, that's what you'd expect. There is much more buffering and processing involved with consumer digital video and audio codecs, which are effectively streamed rather than 'broadcast' in the traditional sense. You can expect a delay of several seconds, though the professional encoders used by the broadcasters are much faster than the decoders in domestic equipment.

[paulsherwin]

It may well have started out as a well meaning attempt to demonstrate the difference between the speeds of sound and light, but it's ended up being quite perpleximg for anyone with a basic understanding of radio transmission and distribution. BBC popular science stuff is often like this.

[emeritus]

At school we did the experiment on the sports field, using a starting pistol and a stop watch to measure the delay between seeing the flash & smoke, and hearing the sharp crack.

[GMB]

I think it was interesting that it worked. I am glad that the FM distribution is not adding noticable delay to FM

And at new year I timed the delay between the bong on TV and my Rugby-synched clock as about 11.5secs.

[emeritus]

That's similar to the delay I got between Radio 4 FM and the ITV news live coverage.

[Radio Wrangler]

In digital signal processing, a lot of the processes amount to "transversal filters", where a digital signal is passed down a delay line of many samples length (Shift registers or circular memory buffers) and the versions of different delayed taps are combined to realise filters and different sorts of encoders and decoders.

This results in delay to the signal which cannot be won back. The delays are typically related to the lowest frequency being acted upon. These delays would be equivalent to significant delays at the speed of light. All this happens before data is collected up into packets for fancy transmission.

Nicam for tellies had to be engineered for low latency so it would sync up with the picture at the receiver, while simpler sets with FM sound still lip sync'd.

David

[Station X]

Two posts relating to the TV Licence Fee deleted. The forums are not a place for politics. Please stay on topic.

[PaulR]

There is a entry about this in my early 1930s copy of Arthur Mee's Children's Encyclopaedia. It talks about hearing an extra bong if you live close enough to hear it "live" and also listen on the wireless.

Regarding FM latency I wake up to an AM valve radio alarm but also leave an FM alarm set for 1/2 hour later just in case the AM set doesn't come on for any reason. There is a tiny delay in the FM signal but it is just heard as a slight echo.

I can remember watching Wimbledon on the TV and listening to the radio commentary at the same time in the analogue days, You certainly couldn't do that now!