FM, DAB and internet broadcasting delays

The chimes are always live (unless Big Ben is broken, which happens occasionally!). There’s a few small delays for microphone-to-FM delay: a BBC friend tells me it is “in the order of a couple of tens of milliseconds”, since there are some small delays in this complex broadcast chain. As a former FM radio presenter, it was possible to listen to your voice “off-air”, and there was no discernable delay.

Some stations are delayed more than that. 5Live or Absolute 1215AM, for example, are satellite-delayed; 5Live deliberately so, while Absolute 1215AM is fed via satellite to the transmitters. Satellite delay (down and up) is roughly 250ms (depending on where you are and where the satellite is), i.e. a quarter of a second.

DAB is delayed further than this: the encoding/decoding for the multiplex adds an additional delay to the output. The national commercial multiplex is also distributed via satellite, adding a further 250ms delay to the broadcast. DAB decoder chipsets differ, and some take longer to decode than others.

Internet radio has a delay of 20 seconds or above, due to the way that works. Two internet radio sets in the same room will almost never decode the audio at the same time.

Hope that’s interesting and helps.

I’m not entirely sure about this, James:

Internet radio has a delay of 20 seconds or above, due to the way that works. Two internet radio sets in the same room will almost never decode the audio at the same time.

There’s definitely a delay and a ‘lag’ from device to device, but I’ve certainly known it to be under 10 seconds, and definitely under 20.

There’s so many variables to it, though, that it’s hard to pin point the broadcast delay online unless you do it on a case-by-case basis.

There is also a varying delay caused by the codec and transport. For Windows Media streams using FastStart technology, I measured over 30 seconds once. Shoutcast streams are rather less.

there are also deliberate delays at the likes of talkSPORT (used to be seven seconds) and a lot of American talk stations just in case a caller swears or says something libelous this can be dumped before broadcast.

Thanks, that’s answered my questions nicely!

At talkSPORT we now run 14 seconds delay, except for live football coverage.

The sound delay from Big Ben to the ground is about 250ms, so the theory is reasonably sound (oops, sorry!).
However, leaving aside profanity delays, only the very simplest of stations will now broadcast FM with a delay of ‘only a few ms’.
Many (most?) stations use digital signal processors/compressors in the signal chain that introduce a delay of typically 20-50ms, even for direct FM.
That short delay is deliberate and beneficial, in that it allows the compressors and limiters to ‘look ahead’ – ie to be able to turn the wick down smoothly just ahead of any excessive peaks.
I’ve found that at about 20ms look-ahead buffer off-air monitoring is fine. At 50ms presenters start to find the slight echo effect offputting.
Re streamed outputs, quite apart from any encoding/decoding delay, there are buffering delays introduced deliberately at various stages in the chain to smooth out delivery of data first to the encoder, and then over the somewhat variable Internet, and finally in the decoding machine.
It’s typically a total of around 20s at 64kbps, but often less on higher bit rates, and more on lower ones, because a lot of software uses a default buffer size set in number of bytes rather than seconds.
We have a private 20kbps stream used for some remote listeners on dial-up and for station monitoring on non-3G mobile. On the default encoding buffer that has a delay of about 1 minute, which has proved quite handy as a quick “What did he just say?” action replay tool for managers!
And finally, again even for a regular AM or FM broadcaster, it is very common nowadays, especially on remote or tight budget stations, to use an IP feed to the transmitter (such as with Barix boxes) and these systems generally settle on a delay of around 8s for reliable buffering and encoding/decoding, so they have an 8s delay even on the main transmitter.
Sorry you asked yet?
Alex

PS

There are lots of sources of delay in the typical FM Broadcast chain. Firstly, there will be a very small delay introduced by use of a digital mixer (which is the norm in larger stations). There then is usually a small delay from the method of delivery to the transmitter. Finally, as mentioned already, the programme audio processor will add another small amount of delay. The end result tends to be a few tens of miliseconds, which is usually low enough that the presenter can monitor themselves off-air, all be it with a slightly odd effect in their ears from the delay.

Occasionally the delay can be pushed high enough that off-air monitoring during links is impossible (or at least quite off putting). This would usually be due the addition of an extra source of delay… e.g. if the programme was being broadcast from a remote studio. Delay would be introduced between the remote studio and the main studio.

Wherever I’ve worked on an FM station, I’ve been able to monitor off-TX.

Generally the total delay is around 20ms on-average on FM, but this can vary.

I’m pretty sure Global’s Leicester Square stations have separate processors just for the studio so that presenters can monitor a processed feed.

Just to throw in my few cents – we’re a community station and we run a processor (which adds about 10ms of delay) in the chain, along with our digital studio-transmitter link (over an IP network) which adds another 10ms (5ms getting in and out of sound cards, 5ms jitter buffer). That’s the lot. The main thing which creates delay in systems these days is intentional delays (time stretching tools or profanity delays); digital mixers usually add something on the order of 50us delay at most for a complete chain. STLs can induce a lot of delay depending on what technology is used, but most are on the order of sub-10ms.